A self-collimating and thermally-buffered exhaust 3D stacked printed circuit board assembly method

Abstract

The present application relates to the technical field of circuit board, especially to a 3D stacked printed circuit board assembly method with self-alignment and thermal buffer exhaust, comprising the following steps: S1, phase change medium filling step; S2, constant temperature welding step; S3, ultrasonic self-alignment step; S4, exhaust solidification step. The present application fills phase change thermal buffer medium with shell wrapped phase change working medium in the three-dimensional slit gap, creates a temperature safety window for ultrasonic self-alignment operation by using the phase change heat absorption constant temperature effect, and releases the phase change working medium vaporization expansion to actively exhaust the residual gas and volatile substances in the gap through shell rupture, thereby realizing high-precision self-alignment and low-defect-rate welding. The present application solves the problems of insufficient alignment time window and bottom layer welding point remelting risk caused by lack of temperature buffer control during welding in the assembly process of the existing 3D stacked printed circuit board, and the problem of welding point porosity defects caused by the residual gas in the three-dimensional slit gap.

Description

Technical Field

[0001] This invention relates to the field of circuit board technology, and in particular to a method for assembling 3D stacked printed circuit boards with self-collimation and thermal buffer venting. Background Technology

[0002] 3D stacked printed circuit boards (PCBs) are high-density assembly structures that vertically stack multiple PCB assemblies and interconnect them with solder. This is a key process for achieving miniaturization and high performance in advanced electronic packaging. During 3D stacking assembly, the alignment accuracy between the top and bottom PCB assemblies directly affects the electrical performance and mechanical reliability of the interconnect solder joints. Simultaneously, residual gases and volatile substances within the three-dimensional gaps during soldering can form pores or voids when the solder joints solidify, reducing the connection strength and long-term reliability of the solder joints.

[0003] However, existing 3D stacked printed circuit board assembly methods rely solely on external visual alignment equipment or mechanical positioning fixtures for interlayer alignment during the soldering process. When the second-melting-point solder melts, the top-layer printed circuit board assembly shifts under the surface tension of the solder. Furthermore, residual air and volatile substances within the three-dimensional slit gaps cannot escape during the soldering heating process, becoming trapped inside the solder joints after solidification, forming porosity defects. This assembly method has a significant drawback: the lack of buffering control over the temperature within the three-dimensional slit gaps and an active gas venting mechanism during the soldering process. The rapid temperature rise after the second-melting-point solder melts results in insufficient time for alignment operations, and the bottom-layer solder joints face the risk of remelting. Simultaneously, the inability to expel residual gas within the gaps leads to porosity defects inside the solder joints, severely impacting the interlayer alignment accuracy and solder joint connection reliability of the 3D stacked printed circuit board.

[0004] Therefore, how to solve the problems of insufficient alignment window and risk of remelting of bottom solder joints due to lack of temperature buffer control during the assembly of 3D stacked printed circuit boards, and the problem of solder joint porosity defects caused by residual gas in the gaps, are the technical problems that urgently need to be solved in this field. Summary of the Invention

[0005] The purpose of this invention is to address the shortcomings of existing technologies by providing a self-aligning and thermally buffered venting method for assembling 3D stacked printed circuit boards. This method involves filling the three-dimensional slit gap with a phase-change thermal buffer medium encased in a shell, utilizing the phase-change endothermic isothermal effect to create a safe temperature window for ultrasonic self-alignment. Furthermore, the shell ruptures to release the vaporized phase-change working medium, which actively expels residual gases and volatile substances from the gap, achieving high-precision self-alignment and low-defect welding. This solves the problems of insufficient alignment time window and the risk of remelting of bottom-layer solder joints due to the lack of temperature buffer control during welding in existing 3D stacked printed circuit board assembly processes, as well as the inability to expel residual gases from the three-dimensional slit gap, leading to solder joint porosity defects.

[0006] To achieve the above objectives, the present invention provides a method for assembling a self-collimating and thermally buffered venting 3D stacked printed circuit board, comprising the following steps:

[0007] S1. Apply a first melting point solder to the substrate pads of the bottom printed circuit board assembly, and apply a second melting point solder to the connection part of the top printed circuit board assembly, forming a three-dimensional slit gap between the two; fill the three-dimensional slit gap with a phase change heat buffer medium to form the assembly to be soldered. The melting point of the first melting point solder is higher than that of the second melting point solder; the phase change heat buffer medium includes multiple heat storage units, each heat storage unit includes a shell and a phase change working medium enclosed by the shell; the shell has temperature-sensitive cracking characteristics, and the cracking threshold temperature is not lower than the upper limit of the phase change temperature of the phase change working medium; S2. Heat the component to be soldered. When the temperature reaches the liquidus temperature range of the second melting point solder, use the isothermal effect of the phase change heat buffer medium to keep the second melting point solder in a liquid state and the temperature of the bottom printed circuit board component is lower than the remelting threshold of the first melting point solder. S3. During the window period when the second melting point solder is in a liquid state, the physical inflection point of the phase change endothermic stage is identified by monitoring the temperature rise rate change trend in the three-dimensional slit gap in situ. Accordingly, a horizontal ultrasonic excitation force parallel to the substrate plane of the bottom printed circuit board assembly is applied to the component to be soldered, driving the top printed circuit board assembly to slide laterally relative to the bottom printed circuit board assembly to complete self-alignment. S4. After self-alignment, the temperature continues to rise. When the temperature exceeds the rupture threshold temperature of the shell, the shell ruptures and releases the phase change working medium. The phase change working medium vaporizes and expands in the three-dimensional slit gap, expelling residual air and volatile substances generated during the welding process. Then, the temperature drops to solidify the second melting point solder.

[0008] The beneficial effects of this invention are as follows: by filling the three-dimensional slit gap with a phase change heat buffer medium containing a shell encapsulating the phase change working fluid, a temperature safety window is created using the phase change heat absorption isothermal effect for ultrasonic self-alignment operation, and the residual gas and volatile substances in the gap are actively discharged by releasing the vaporization and expansion of the phase change working fluid through shell rupture, thus achieving high-precision self-alignment and low-defect welding. Attached Figure Description

[0009] Figure 1 This is a schematic diagram of the structure of the 3D stacked printed circuit board of the present invention.

[0010] The reference numerals in the figures include: 1. Bottom printed circuit board assembly; 11. Substrate pads; 12. First melting point solder; 2. Top printed circuit board assembly; 21. Connection part; 22. Second melting point solder; 3. Phase change heat buffer medium. Detailed Implementation

[0011] The present invention will now be described in detail with reference to the accompanying drawings.

[0012] like Figure 1 As shown, the present invention provides a method for assembling a 3D stacked printed circuit board with self-collimation and thermal buffer venting, comprising the following steps: S1. Apply a first melting point solder 12 to the substrate pad 11 of the bottom printed circuit board assembly 1, and apply a second melting point solder 22 to the connection part 21 of the top printed circuit board assembly 2, forming a three-dimensional slit gap between the two; fill the three-dimensional slit gap with a phase change heat buffer medium 3 to form the assembly to be soldered. The melting point of the first melting point solder 12 is higher than that of the second melting point solder 22; the phase change heat buffer medium 3 contains multiple heat storage units, each heat storage unit contains a shell and a phase change working medium enclosed by the shell; the shell has temperature-sensitive cracking characteristics, and the cracking threshold temperature is not lower than the upper limit of the phase change temperature of the phase change working medium. S2. Heat the component to be soldered. When the temperature reaches the liquidus temperature range of the second melting point solder 22, use the isothermal effect of the phase change heat buffer medium 3 to keep the second melting point solder 22 in a liquid state and the temperature of the bottom printed circuit board component 1 is lower than the remelting threshold of the first melting point solder 12. S3. During the window period when the second melting point solder 22 is in a liquid state, the physical inflection point of the phase change endothermic stage is identified by monitoring the temperature rise rate change trend in the three-dimensional slit gap in situ. Based on this, a horizontal ultrasonic excitation force parallel to the substrate plane of the bottom printed circuit board assembly 1 is applied to the assembly to be welded, driving the top printed circuit board assembly 2 to slide laterally relative to the bottom printed circuit board assembly 1 to complete self-alignment. S4. After self-alignment, the temperature continues to rise. When the temperature exceeds the shell's rupture threshold temperature, the shell ruptures and releases the phase change working medium. The phase change working medium vaporizes and expands in the three-dimensional slit gap, expelling residual air and volatile substances generated during the welding process. Subsequently, the temperature drops to solidify the second melting point solder 22.

[0013] By filling the three-dimensional slit gap with phase change heat buffer medium 3, the phase change heat buffer medium 3 fills the three-dimensional slit gap. During the subsequent heating process, the phase change heat buffer medium 3 plays a temperature buffering function through phase change heat absorption, thereby providing a heat source reserve for the temperature plateau period after the second melting point solder 22 melts, and forming a controllable temperature buffer zone in the three-dimensional slit gap.

[0014] By setting the melting point of the first melting point solder 12 to be higher than that of the second melting point solder 22, the second melting point solder 22 reaches the melting temperature before the first melting point solder 12 during the heating process. The first melting point solder 12 remains solid at the melting temperature of the second melting point solder 22. This achieves the goal of ensuring that the solder joints already completed on the bottom printed circuit board assembly 1 will not remelt and fail when the second melting point solder 22 melts, thus providing a safe upper limit for temperature for self-alignment operation.

[0015] The phase change heat buffer medium 3 contains multiple heat storage units. The shell of each heat storage unit encloses the phase change working medium. When the phase change working medium reaches the phase change temperature, it undergoes a phase change and absorbs a large amount of latent heat under isothermal conditions. The phase change heat absorption effect of multiple heat storage units is superimposed, realizing the formation of a heat buffer layer composed of dispersed heat storage units in the three-dimensional slit gap. The isothermal heat absorption effect of the latent heat of phase change is used to lock the temperature near the phase change temperature, so that the temperature rise rate is significantly reduced and a temperature plateau period is formed.

[0016] By setting the shell to have temperature-sensitive rupture characteristics and a rupture threshold temperature not lower than the upper limit of the phase change temperature of the phase change working fluid, the shell only begins to rupture after the phase change working fluid has completed the phase change heat absorption process. This achieves the timing control of the thermal buffering function of the thermal storage unit completing the phase change heat absorption first, followed by the shell rupturing to release the phase change working fluid. This ensures the orderly connection between the thermal buffering stage and the exhaust stage, and avoids premature rupture of the shell leading to the premature release of the phase change working fluid and interruption of the thermal buffering effect.

[0017] By heating the components to be soldered to the liquidus temperature range of the second melting point solder 22, the phase change working medium 3 reaches the phase change temperature and begins to absorb heat during phase change. Under constant temperature conditions, the phase change working medium absorbs a large amount of latent heat, temporarily locking the temperature in the three-dimensional slit gap near the phase change temperature and preventing it from rising rapidly. The phase change temperature is within the liquidus temperature range of the second melting point solder 22 and is lower than the remelting threshold of the first melting point solder 12, thus creating a time window for temperature stability. During the temperature window period, the second melting point solder 22 remains liquid and meets the self-alignment flow conditions. At the same time, the temperature of the underlying printed circuit board assembly 1 is lower than the remelting threshold of the first melting point solder 12, ensuring that the soldered joints do not remelt. This solves the problem of insufficient alignment time window and the risk of remelting of the underlying solder joints coexisting during the soldering process.

[0018] By monitoring the temperature rise rate in the three-dimensional slit gap in situ during the window period when the second melting point solder 22 is in a liquid state, the temperature rise rate of the phase change heat buffer medium 3 decreases significantly during the phase change heat absorption stage. When the phase change heat absorption is about to end, the temperature rise rate rises from a low speed and reaches an inflection point. By identifying the physical inflection point in the temperature rise rate trend, the optimal time for the end of the phase change heat absorption stage and the start of the self-alignment operation is accurately determined. This achieves accurate online determination of the start time of the self-alignment operation, avoiding the problem of the self-alignment operation being executed too early or too late due to inaccurate timing judgment.

[0019] By applying a horizontal ultrasonic excitation force parallel to the substrate plane of the bottom printed circuit board assembly 1 to the assembly to be soldered at the physical inflection point, the horizontal ultrasonic excitation force generates lateral ultrasonic vibration within the three-dimensional slit gap. At this time, the second melting point solder 22 is in a liquid state. Under the action of ultrasonic vibration, the wettability of the liquid second melting point solder 22 is enhanced and the surface tension is reduced. The horizontal ultrasonic excitation force drives the top printed circuit board assembly 2 to undergo lateral micro-slippage relative to the bottom printed circuit board assembly 1 in the horizontal plane under the combined action of the surface tension of the solder pad and ultrasonic vibration. The connection part 21 of the top printed circuit board assembly 2 slides towards the alignment position to achieve self-alignment. This realizes accurate automatic alignment of 3D stacking without the need for external visual alignment equipment or mechanical positioning fixtures, improving alignment accuracy and alignment efficiency.

[0020] After self-alignment, the temperature is further increased to exceed the rupture threshold temperature of the thermal storage unit shell. Due to its temperature-sensitive rupture characteristics, the shell ruptures. After the shell ruptures, the phase change working fluid enclosed by the shell is released into the three-dimensional slit gap, realizing the orderly release of the phase change working fluid from the thermal storage unit. This transforms the phase change working fluid from a thermal storage state into an exhaust working fluid, providing a material basis for the subsequent exhaust process.

[0021] The phase change working fluid released by the shell rupture is heated and vaporized in the three-dimensional slit gap. After the phase change working fluid changes from liquid to gas, its volume expands rapidly. The expanded phase change working fluid gas forms a high-pressure airflow in the three-dimensional slit gap. The high-pressure airflow squeezes out the residual air in the three-dimensional slit gap and the volatile substances generated during the welding process towards the opening of the three-dimensional slit gap. This achieves the removal of gas inclusions and volatile residues in the three-dimensional slit gap before the weld solidifies, and eliminates porosity and void defects inside the weld joint.

[0022] After self-alignment and venting are completed, the components to be soldered are cooled down. When the temperature drops below the solidus temperature of the second melting point solder 22, the second melting point solder 22 solidifies from a liquid state to a solid state. After the second melting point solder 22 solidifies, it solidifies the connection part 21 between the top printed circuit board assembly 2 and the bottom printed circuit board assembly 1 into a whole, realizing the permanent solidification of the 3D stacked printed circuit board and ensuring the interlayer connection strength and structural stability.

[0023] During operation, in step S1, a first melting point solder 12 is applied to the substrate pad 11 of the bottom printed circuit board assembly 1, and a second melting point solder 22 is applied to the connection part 21 of the top printed circuit board assembly 2. The melting point of the first melting point solder 12 is higher than that of the second melting point solder 22. A three-dimensional slit gap is formed between the bottom printed circuit board assembly 1 and the top printed circuit board assembly 2. The three-dimensional slit gap is filled with a phase change heat buffer medium 3. The phase change heat buffer medium 3 contains multiple heat storage units. Each heat storage unit contains a shell and a phase change working medium wrapped by the shell. The shell has temperature-sensitive cracking characteristics and the cracking threshold temperature is not lower than the upper limit of the phase change temperature of the phase change working medium, thus forming the component to be soldered. Step S2: When the component to be soldered is heated to the liquidus temperature range of the second melting point solder 22, the second melting point solder 22 transforms into a liquid state. The isothermal effect of the phase change working fluid's endothermic phase change keeps the temperature locked near the phase change temperature. The second melting point solder 22 remains liquid, and the temperature of the bottom printed circuit board assembly 1 is lower than the remelting threshold of the first melting point solder 12. Step S3: The temperature rise rate is monitored in situ to identify the physical inflection point. At the physical inflection point, a horizontal ultrasonic excitation force is applied to drive the top printed circuit board assembly 2 to slide laterally and complete self-alignment. Step S4: When the temperature continues to rise above the shell rupture threshold temperature, the shell ruptures and releases the phase change working fluid. The phase change working fluid vaporizes and expands, expelling residual air and volatile substances. Subsequently, the temperature drops, causing the second melting point solder 22 to solidify and complete the assembly.

[0024] Specifically, the height of the three-dimensional slit gap is 50 μm to 300 μm.

[0025] When the height of the three-dimensional slit gap is less than 50μm, the space inside the three-dimensional slit gap is insufficient to accommodate a sufficient amount of phase change heat buffer medium 3. The phase change heat absorption of the phase change heat buffer medium 3 cannot meet the minimum heat capacity requirement required for the isothermal effect, resulting in the temperature rise of the second melting point solder 22 being too fast after melting and insufficient alignment operation time window. At the same time, the three-dimensional slit gap is too narrow, and the lateral sliding stroke of the top printed circuit board assembly 2 driven by the horizontal ultrasonic excitation force in step S3 is limited, resulting in a decrease in self-alignment accuracy.

[0026] When the height of the three-dimensional slit gap is higher than 300μm, the exhaust path of residual air and volatile substances in the three-dimensional slit gap increases, the energy dissipation of the vaporization exhaust wavefront in step S4 increases during propagation, the exhaust effect in the gap edge area is significantly weakened, and the risk of residual porosity defects inside the solder joint increases; at the same time, the filling uniformity of the phase change heat buffer medium 3 in the three-dimensional slit gap decreases, and filling gaps may appear in local areas, resulting in uneven isothermal effect, premature solidification of some areas of the second melting point solder 22, or the local temperature of the bottom printed circuit board assembly 1 exceeding the remelting threshold of the first melting point solder 12.

[0027] When the height of the three-dimensional slit gap is 50μm to 300μm, the phase change heat buffer medium 3 forms a uniform filling layer in the three-dimensional slit gap. The isothermal effect generated by the phase change heat absorption covers the entire welding area, ensuring that the temperature fluctuation of the second melting point solder 22 does not exceed ±5℃ during the liquid window period. In step S3, the lateral sliding stroke of the top printed circuit board assembly 2 is sufficient, and the self-alignment accuracy can reach ±10μm. In step S4, the propagation distance of the vaporization exhaust wavefront matches the height of the three-dimensional slit gap, and the exhaust efficiency of the residual gas in the gap is not less than 95%.

[0028] In this embodiment, the phase change heat buffer medium 3 is a microcapsule phase change heat buffer medium, which contains multiple microcapsule units as heat storage units; the shell is the capsule wall of the microcapsule unit. By setting the phase change heat buffer medium 3 as a microcapsule phase change heat buffer medium and using the microcapsule unit as the heat storage unit and the capsule wall of the microcapsule unit as the shell, the capsule wall size of the microcapsule unit is in the micrometer range. The microcapsule units can be uniformly dispersed and filled in the three-dimensional slit gap. The gaps between the microcapsule units facilitate the penetration of liquid solder under capillary action. This achieves the goal of maintaining the heat buffering function while ensuring that the microcapsule phase change heat buffer medium is uniformly distributed in the three-dimensional slit gap, ensuring the uniform absorption and buffering of heat in the three-dimensional slit gap. At the same time, the small size of the microcapsule unit ensures that the solder can smoothly penetrate the microcapsule gap to complete the welding during subsequent self-alignment sliding.

[0029] The phase change temperature range of the phase change working medium is between the liquidus temperature of the second melting point solder 22 and the solidus temperature of the first melting point solder 12. By setting the phase change temperature range of the phase change working medium between the liquidus temperature of the second melting point solder 22 and the solidus temperature of the first melting point solder 12, the phase change temperature of the phase change working medium being higher than the liquidus temperature of the second melting point solder 22 ensures that the second melting point solder 22 has been fully liquefied, and the phase change temperature of the phase change working medium being lower than the solidus temperature of the first melting point solder 12 ensures that the first melting point solder 12 will not remelt. This achieves a temperature locking range of the phase change endothermic temperature platform that is precisely within a safe working window where the second melting point solder 22 is liquid and the first melting point solder 12 is solid, thereby improving the temperature control accuracy.

[0030] During the phase change endothermic stage, the microcapsule phase change heat buffer medium significantly reduces the temperature rise rate due to phase change endothermic heat, forming a plateau characteristic segment on the temperature rise curve. During the phase change heat buffering stage, the temperature rise rate is significantly reduced due to the phase change heat absorption of the phase change working medium. The temperature rise curve shows a plateau characteristic segment where the temperature no longer rises rapidly. The duration of the temperature plateau characteristic segment is the time required for the latent heat of the phase change working medium to be completely absorbed. This plateau characteristic segment provides a stable temperature maintenance time for the liquid window period of the second melting point solder 22, providing a sufficient temperature safety operating window for self-alignment operation.

[0031] The rupture threshold temperature of the microcapsule unit is not lower than the upper limit of the phase change temperature of the phase change working fluid, and not higher than 1.2 times the upper limit of the phase change temperature.

[0032] By setting the capsule wall rupture threshold temperature of the microcapsule unit to be no less than the upper limit of the phase change temperature of the phase change working medium and no more than 1.2 times the upper limit of the phase change temperature, the capsule wall begins to rupture after the phase change working medium completes phase change heat absorption. The temperature difference between the capsule wall rupture threshold temperature and the upper limit of the phase change temperature does not exceed 20% of the upper limit of the phase change temperature. This minimizes the temperature interval between capsule wall rupture and the completion of phase change heat absorption, shortens the transition time from the thermal buffer stage to the exhaust stage, and improves the time efficiency of the welding process. At the same time, the capsule wall rupture threshold temperature is strictly not lower than the upper limit of the phase change temperature to ensure that the phase change working medium will not be released before the phase change heat absorption is completed.

[0033] Specifically, in S1, the microcapsule phase change heat buffer medium is arranged in the three-dimensional slit gap around the outer region of the substrate pad 11 connection area, and a flow-blocking interface is formed between the outer region and the substrate pad 11 connection area. The flow-blocking interface is maintained by the yield stress of the microcapsule phase change heat buffer medium, preventing the microcapsule phase change heat buffer medium from migrating to the substrate pad 11 connection area.

[0034] By arranging the microcapsule phase change heat buffer medium in the outer region of the substrate pad 11 connection area within the three-dimensional slit gap, rather than within the substrate pad 11 connection area, the microcapsule phase change heat buffer medium forms a flow-blocking interface maintained by the yield stress of the microcapsule phase change heat buffer medium between the outer region and the substrate pad 11 connection area. The flow-blocking interface prevents the microcapsule phase change heat buffer medium from migrating to the substrate pad 11 connection area, thus confining the microcapsule phase change heat buffer medium to the outer region of the substrate pad 11 connection area. This avoids the microcapsule phase change heat buffer medium from entering the solder fusion area and affecting the solder joint quality, ensuring the cleanliness of the welding area and the reliability of the solder joint.

[0035] In S2, the microcapsule phase change heat buffer medium laterally conducts the heat from the top printed circuit board assembly 2 to the outside of the substrate pad 11 connection area during the phase change heat absorption stage. With the help of the longitudinal thermal resistance of the microcapsule phase change heat buffer medium, the temperature of the bottom printed circuit board assembly 1 is lower than the remelting threshold of the first melting point solder 12. The microcapsule units are arranged in a transverse orientation in the dispersion matrix, which makes the microcapsule phase change heat buffer medium exhibit anisotropy with a transverse thermal conductivity greater than a longitudinal thermal conductivity, and the ratio of transverse thermal conductivity to longitudinal thermal conductivity is not less than 3.

[0036] During the phase change heat absorption stage, the microcapsule phase change heat buffer medium conducts heat from the top printed circuit board assembly 2 laterally to areas outside the connection area of ​​the substrate pad 11. At the same time, the thermal resistance of the microcapsule phase change heat buffer medium in the longitudinal direction makes it difficult for heat to be transferred longitudinally to the bottom printed circuit board assembly 1. This achieves heat conduction from the top printed circuit board assembly 2 to the lateral direction rather than the longitudinal direction, keeping the temperature of the bottom printed circuit board assembly 1 always below the remelting threshold of the first melting point solder 12, thus eliminating the possibility of remelting of the bottom solder joints from the heat conduction path.

[0037] By arranging microcapsule units in the dispersion matrix along the lateral orientation, the microcapsule phase change heat buffer medium exhibits anisotropic thermal conductivity with a lateral thermal conductivity greater than the longitudinal thermal conductivity. The ratio of the lateral thermal conductivity to the longitudinal thermal conductivity is not less than 3, which enables heat to be preferentially conducted laterally away from the connection area of ​​the substrate pad 11 rather than to the longitudinal bottom layer, further ensuring the temperature control safety of the bottom printed circuit board assembly 1. At the same time, the efficient lateral heat conduction makes the heat distribution in the three-dimensional slit gap more uniform.

[0038] In S4, the temperature continues to rise, causing the capsule wall of the microcapsule unit to rupture and release the phase change working fluid. The released phase change working fluid continues to heat up and vaporize, generating a pressure expansion wave from the inside out within the three-dimensional slit gap. The pressure expansion wave is reflected at the gap boundary and superimposed with the vaporization pressure wave of the subsequently released phase change working fluid to form a transient pressure oscillation field.

[0039] By continuing to raise the temperature, the capsule wall of the microcapsule unit ruptures, releasing the phase change working fluid. The phase change working fluid continues to heat up and vaporize, generating a pressure expansion wave from the inside out within the three-dimensional slit gap. After reaching the boundary of the three-dimensional slit gap, the pressure expansion wave is reflected. The reflected wave superimposes with the vaporization pressure wave generated by the subsequently released phase change working fluid to form a transient pressure oscillation field. The transient pressure oscillation field generates alternating pressure within the three-dimensional slit gap. The alternating pressure repeatedly impacts the residual gas and volatile substances, peeling off and expelling the bubbles and residues adhering to the gap wall. This achieves enhanced exhaust effect by utilizing the wave effect of the transient pressure oscillation field, eliminating residual gas in dead zones, and providing a more thorough cleaning effect than static pressure exhaust.

[0040] The microcapsule phase change heat buffer medium in this embodiment also includes a dispersion matrix. Multiple microcapsule units are core-shell structured microcapsules, with organic phase change materials as the core and inorganic materials doped with nano-metal oxides as the capsule walls, and are dispersed in the dispersion matrix. By using organic phase change materials as the core to provide latent heat storage capacity, and inorganic materials doped with nano-metal oxides as the capsule wall to improve the high temperature resistance and thermal conductivity of the capsule wall, the thermal conductivity of the capsule wall is enhanced by the doping of nano-metal oxides, making the endothermic response speed of the phase change working medium faster. The core-shell structure microcapsules are supported and dispersed by a dispersed matrix to ensure the uniform distribution of the microcapsules. This achieves the dual capabilities of efficient phase change heat storage and rapid thermal response of the microcapsule phase change heat buffer medium, improving the response speed and stability of temperature buffering.

[0041] Nanoscale metal oxides are arranged in a transverse orientation in the capsule wall to form an anisotropic thermal conductive network. The transverse thermal conductivity of the anisotropic thermal conductive network increases in a stepwise manner when the volume fraction of the microcapsule phase change heat buffer medium reaches the percolation threshold. The percolation threshold is the critical value at which the volume fraction of microcapsule units in the microcapsule phase change heat buffer medium reaches the formation of a continuous thermal conductive path, and the critical value is between 15% and 30%.

[0042] By arranging nano-metal oxides in a lateral orientation within the capsule wall, an anisotropic thermal conductivity network is formed inside the capsule wall. This anisotropic thermal conductivity network makes the heat conduction capacity of the capsule wall in the lateral direction significantly higher than that in the longitudinal direction, thus enabling heat to be preferentially conducted laterally rather than longitudinally to the bottom layer. This further enhances the temperature protection of the bottom printed circuit board assembly 1. Combined with the anisotropic thermal conductivity effect of the lateral orientation of the microcapsules, a dual anisotropic thermal conductivity system at both the capsule wall level and the microcapsule level is formed.

[0043] When the volume fraction of microcapsule units reaches the percolation threshold, the anisotropic thermal conductive networks within each capsule wall are interconnected to form a continuous thermal conductive path, resulting in a step increase in the lateral thermal conductivity. The critical value of the percolation threshold is between 15% and 30%, which realizes that when the microcapsule concentration reaches the critical value, the lateral thermal conductivity increases rapidly instead of slowly increasing. This significantly improves the lateral heat dissipation efficiency above the percolation threshold. At the same time, the volume fraction range of 15% to 30% balances the microcapsule concentration and the phase change heat storage, avoiding insufficient available solder space in the three-dimensional slit gap due to excessively high microcapsule concentration.

[0044] Specifically, in S3, the frequency of the horizontal ultrasonic excitation force is no higher than 0.5 times the first natural frequency of the inorganic capsule wall of the core-shell structure microcapsule, so as to retain sufficient safety margin to avoid the capsule wall resonance fatigue failure caused by ultrasonic excitation; the first natural frequency is determined by the elastic modulus, thickness and average radius of the inorganic capsule wall and the microcapsule unit. By setting the frequency of the horizontal ultrasonic excitation force to no more than 0.5 times the first natural frequency of the inorganic capsule wall, and keeping the frequency of the ultrasonic excitation force far away from the first natural frequency of the inorganic capsule wall, sufficient safety margin is maintained to avoid resonant fatigue damage to the inorganic capsule wall caused by ultrasonic excitation. This achieves the protection of the inorganic capsule wall from premature rupture due to resonance while applying ultrasonic excitation force to drive self-alignment, thus ensuring the structural integrity of the microcapsule during the self-alignment stage.

[0045] The rupture of the capsule wall in S4 is subject to the following conditions: The ratio of the duration τ of the phase transition plateau to the square of the inorganic capsule wall thickness δ and the thermal diffusivity α of the inorganic capsule wall material satisfies: τ=k·δ 2 / α, Where k is a safety factor between 1 and 5; The duration of the phase transition plateau was determined by differential scanning calorimetry.

[0046] By setting the relationship between the duration τ of the phase transition plateau and the inorganic capsule wall thickness δ and the material's thermal diffusivity α as τ = k·δ 2 / α, by adjusting the capsule wall thickness δ and selecting the thermal diffusivity α of the capsule wall material, the duration of the phase transition plateau can be controlled. The k value between 1 and 5 provides a safety margin, which realizes the precise control of the duration of the phase transition plateau through the design of the capsule wall thickness. This ensures that the phase transition plateau covers the time window required for self-alignment operation, guaranteeing that the self-alignment operation has sufficient and not excessively long operation time within the phase transition plateau.

[0047] In this embodiment, the edge of the three-dimensional slit gap is also provided with at least one thermally conductive connection terminal, one end of which is embedded in the gap and in thermal contact with the microcapsule phase change heat buffer medium, and the other end extends to the outer surface of the top printed circuit board assembly 2 or the bottom printed circuit board assembly 1. The thermally conductive connection terminals are made of insulating and thermally conductive material; In S2, the heat conducted to the gap edge by the anisotropic heat-conducting network is discharged through the heat-conducting connection terminal.

[0048] By setting thermally conductive connection terminals at the edge of the three-dimensional slit gap, one end of the thermally conductive connection terminal is embedded in the three-dimensional slit gap and directly contacts the microcapsule phase change heat buffer medium, while the other end extends to the outer surface of the outer printed circuit board assembly. The thermally conductive connection terminal acts as a thermally conductive bridge between the three-dimensional slit gap and the external environment, realizing an active heat dissipation channel for the heat conducted to the edge of the gap by the anisotropic thermal network to be discharged outward.

[0049] By using thermally conductive connectors made of insulating thermally conductive material, the thermally conductive connectors can conduct heat while also having electrical insulation properties. This ensures that when the thermally conductive connectors are embedded in the three-dimensional slit gap and come into thermal contact with the microcapsule phase change heat buffer medium, no electrical short circuit will be formed between the top printed circuit board assembly 2 and the bottom printed circuit board assembly 1, thus guaranteeing the safety of interlayer electrical isolation.

[0050] In step S2, the anisotropic heat-conducting network conducts heat laterally to the edge of the three-dimensional slit gap. The heat conducted to the edge of the gap is then conducted outward to the outer surface of the printed circuit board assembly via the thermally conductive connection terminal. The thermally conductive connection terminal efficiently transfers the heat from the edge of the gap to the external environment, thus enabling the continuous discharge of heat conducted laterally by the anisotropic heat-conducting network through the thermally conductive connection terminal. This prevents heat from accumulating at the edge of the gap and then being transferred back into the three-dimensional slit gap, ensuring the continuous efficiency of the lateral heat dissipation path.

[0051] Specifically, the insulating and thermally conductive material may be selected from at least one of the following: (1) Aluminum nitride (AlN) ceramics with a thermal conductivity of 170 W / (m·K) to 200 W / (m·K) and a volume resistivity of not less than 10 Ω·m. 14 With a thermal conductivity of Ω·cm, it combines high thermal conductivity with excellent electrical insulation, enabling it to quickly conduct heat from the edge of the three-dimensional slit gap to the outer surface of the top printed circuit board assembly 2 or the bottom printed circuit board assembly 1, while ensuring that no electrical short circuit occurs between the top printed circuit board assembly 2 and the bottom printed circuit board assembly 1. (2) Hexagonal boron nitride (h-BN) ceramic with a thermal conductivity of 30 W / (m·K) to 60 W / (m·K) and a volume resistivity of not less than 10 Ω·m. 14 With an Ω·cm, it has in-plane anisotropic thermal conductivity and its layered structure is compatible with the inorganic capsule wall material of the microcapsule unit. It does not undergo chemical reaction during high-temperature welding and is suitable for assembly scenarios with higher dielectric performance requirements. (3) Alumina (Al2O3) ceramics with a thermal conductivity of 25 W / (m·K) to 35 W / (m·K) and a volume resistivity of not less than 10 Ω·m. 13 With a low cost and mature manufacturing process, it is suitable for assembly scenarios where thermal conductivity requirements are relatively low but economic efficiency must be considered.

[0052] In S3 of this embodiment, the physical inflection point is detected in the following way: Monitor the temperature rise curve within the three-dimensional slit gap. When the temperature rise rate drops to less than 10% of the initial temperature rise rate during the phase change endothermic stage and remains so for at least 2 seconds, the physical inflection point is confirmed. Horizontal ultrasonic excitation is triggered by a physical inflection point.

[0053] By monitoring the temperature rise curve within the three-dimensional slit gap, when the temperature rise rate drops to below 10% of the initial temperature rise rate in the phase change endothermic stage, it indicates that the phase change working fluid has entered a highly efficient phase change endothermic state. Furthermore, the low rate lasting for at least 2 seconds eliminates the interference of brief fluctuations. This allows for reliable confirmation of the physical inflection point by measuring the temperature rise rate below the threshold and maintaining a stable state for an extended period. This avoids misjudgments caused by instantaneous temperature fluctuations and improves the accuracy and reliability of physical inflection point detection.

[0054] By triggering horizontal ultrasonic excitation after confirming the physical inflection point, the physical inflection point indicates that the phase change endothermic stage has been fully established, the second melting point solder 22 has been completely liquefied, and the temperature has entered a stable plateau period. This enables the self-alignment operation to be started at the optimal time, ensuring that the self-alignment operation is performed under the conditions of temperature safety and full liquefaction of solder.

[0055] In this embodiment, the activation of horizontal ultrasonic excitation in step S3 and the triggering of cyst wall rupture in step S4 are achieved through the following adaptive temperature control method: The heating temperature rise curve is obtained, the change in surface temperature rise rate is detected, and the inflection point of the phase change plateau period is confirmed as the physical inflection point after a preset lag time. The preset lag time is the heat penetration time required for heat to be conducted from the gap surface to the phase change working medium inside the microcapsule unit. The heat penetration time is not less than the ratio of the square of the capsule wall thickness to the thermal diffusivity of the capsule wall material. Upon confirmation, horizontal ultrasonic excitation is triggered. After ultrasonic excitation, the temperature is controlled to rise to a preset threshold exceeding the upper limit of the phase change temperature, so that the temperature exceeds the rupture threshold temperature of the capsule wall, causing the capsule wall to rupture and release the phase change working fluid, and then the temperature is cooled and solidified.

[0056] By acquiring the heating temperature rise curve and detecting the change in surface temperature rise rate, the time difference between surface detection and actual phase change is compensated by a preset delay time. Since there is a thermal conduction delay between the temperature rise rate change detected by the surface temperature sensor and the actual start of phase change of the working medium inside the microcapsule unit, the inflection point of the phase change plateau period confirmed after the preset delay time is closer to the actual start of phase change of the working medium inside the microcapsule unit. This eliminates the time deviation between the surface detection signal and the actual internal phase change, and improves the correspondence accuracy between the physical inflection point confirmation time and the actual phase change state.

[0057] By setting the preset lag time as the heat penetration time required for heat to be conducted from the gap surface to the phase change working medium inside the microcapsule unit, and the heat penetration time being no less than the ratio of the square of the capsule wall thickness to the thermal diffusivity of the capsule wall material, the heat penetration time is accurately calculated using the physical parameters of the capsule wall. This achieves precise setting of the preset lag time, ensuring that the physical inflection point of delayed confirmation closely matches the actual phase change time of the phase change working medium inside the microcapsule.

[0058] After confirmation, horizontal ultrasonic excitation is triggered. After ultrasonic excitation, the temperature is controlled to rise to a preset threshold exceeding the upper limit of the phase change temperature so that the temperature exceeds the rupture threshold temperature of the capsule wall. The capsule wall ruptures, releasing the phase change working medium to complete the exhaust. Then, the temperature is cooled and cured to complete the assembly. This achieves an orderly process connection of self-alignment → heating and exhaust → cooling and curing. The temperature control of the preset threshold ensures that the capsule wall ruptures only after the phase change working medium has completed the phase change and absorbed heat, thus ensuring the reliability of the process sequence.

[0059] The detection of physical inflection points in this embodiment also includes a multimodal sensor fusion criterion, specifically: During the transition from S2 to S3, the temperature rise rate signal of the component to be welded, the acoustic emission signal of the microcapsule unit in the three-dimensional slit gap, and the equivalent complex impedance signal are simultaneously acquired. The zero-crossing point of the second derivative of the temperature rise rate signal is used as the first inflection point precursor index, the inversion characteristic of the acoustic emission event rate slope of the acoustic emission signal is used as the second inflection point precursor index, and the rate of change of the real component and the extreme value characteristics of the imaginary component of the equivalent complex impedance signal are used as the third inflection point precursor index. When at least two inflection point indicators simultaneously meet their respective preset conditions, the physical inflection point is confirmed and the horizontal ultrasonic excitation force is triggered.

[0060] By simultaneously acquiring signals from three different physical modes during the transition from step S2 to S3—temperature rise rate signal, acoustic emission signal, and equivalent complex impedance signal—the temperature rise rate signal reflects the macroscopic temperature change trend, the acoustic emission signal reflects the microscopic mechanical response during the phase transition process inside the microcapsule unit wall, and the equivalent complex impedance signal reflects the electromagnetic property changes caused by the change in the state of the working fluid during the phase transition inside the microcapsule unit. These three signals capture the characteristic information of the phase transition endothermic stage from three independent physical dimensions: temperature, mechanics, and electromagnetics. This achieves improved accuracy and robustness of physical inflection point detection through cross-validation of multi-modal signals, avoiding misjudgment due to noise or anomalies caused by a single signal mode.

[0061] Specifically, the temperature rise rate signal reflects the changing trend of the macroscopic temperature field within the three-dimensional slit gap. During the phase change endothermic stage, the phase change working fluid absorbs latent heat, causing a significant decrease in the temperature rise rate. The zero-crossing point of the second derivative of the temperature rise rate corresponds to the turning point where the temperature rise rate changes from accelerating to decelerating, i.e., the moment when the phase change endothermic rate changes from increasing to decreasing. The advantages of the temperature rise rate signal are its simplicity of measurement and stable response.

[0062] Acoustic emission (AE) signals reflect the microscopic mechanical response within the microcapsule unit's wall during phase transitions. During a phase transition, the working fluid undergoes a solid-liquid or liquid-vapor phase change. The volume change caused by this phase change generates periodic stress on the capsule wall, which in turn produces micro-strain and emits elastic waves—the acoustic emission signal. In the early stages of the endothermic phase transition, a large number of microcapsule units undergo phase transitions simultaneously, leading to a continuously increasing acoustic emission event rate. As the endothermic phase transition nears completion, the amount of working fluid available for phase transition decreases, and the acoustic emission event rate begins to decline, with the slope changing from positive to negative. The advantages of acoustic emission signals lie in their extreme sensitivity to the microscopic response of phase transition processes, enabling the detection of phase transition events in individual microcapsule units; their high propagation speed (approximately 3000 m / s to 6000 m / s in solids) provides high temporal resolution; and their independence from the thermal inertia of temperature sensors allows for the detection of subtle phase transition features that temperature signals cannot distinguish.

[0063] The equivalent complex impedance signal reflects the changes in electromagnetic properties caused by the phase change working fluid state changes within the microcapsule unit. The core-shell structure of the microcapsule unit behaves as an equivalent circuit model in an alternating electromagnetic field. The real component, i.e., the resistive component, reflects the energy dissipation within the microcapsule unit, including the dielectric loss of the phase change working fluid and the ohmic loss of the capsule wall material. The imaginary component, i.e., the reactive component, reflects the energy storage characteristics within the microcapsule unit, including the polarization response of the phase change working fluid and the capacitive effect of the capsule wall structure. When the phase change working fluid undergoes a phase change, its dielectric constant and dielectric loss factor change significantly. During a solid-liquid phase change, the polarization degrees of freedom of the working fluid molecules increase, the dielectric constant increases, the rate of change of the real component decreases, and energy dissipation tends to saturate. During a liquid-vapor phase change, the working fluid density decreases sharply, the imaginary component exhibits an extreme value, and the energy storage characteristics undergo a step change. The advantages of equivalent complex impedance signals are that they are non-contact measurements and do not affect the phase transition process; they can reflect the true state of the phase transition working fluid inside the microcapsule unit, rather than an indirect inference from the external temperature field; and the real and imaginary components provide two independent criteria dimensions, enhancing the reliability of the criteria.

[0064] The three signal modes capture characteristic information of the phase transition endothermic stage from three independent physical dimensions: temperature, mechanics, and electromagnetics. Each mode provides an independent information dimension, and the failure or abnormality of any mode does not affect the independent judgment of the other modes, thus achieving information redundancy and complementarity at the physical dimension level.

[0065] By using the zero-crossing point of the second derivative of the temperature rise rate signal as the first inflection point indicator, the zero-crossing point of the second derivative marks the inflection point when the temperature rise rate changes from an accelerating decrease to a decelerating decrease, corresponding to the turning point when the phase change heat absorption rate of the phase change working fluid changes from increasing to decreasing. This allows for the accurate capture of the dynamic transition of the phase change heat absorption stage through the second derivative characteristics of the temperature signal.

[0066] By using the slope reversal characteristic of the acoustic emission event rate of the acoustic emission signal as a second inflection point indicator, the acoustic emission event rate continues to increase in the early stage of the phase change endothermic stage as the phase change working medium absorbs a large amount of heat. When the phase change endothermic stage approaches saturation, the acoustic emission event rate begins to decrease. The sign that the slope changes from positive to negative indicates that the phase change endothermic process is nearing completion. This allows us to capture the completion trend of the phase change endothermic stage from a microscopic level through the mechanical response characteristics of the acoustic emission signal.

[0067] By reducing the rate of change of the real component of the equivalent complex impedance signal to below a preset proportion of the initial value of the phase change endothermic stage, and using the current sampling point value of the imaginary component to be greater than the values ​​of the preceding and subsequent sampling points as the third inflection point precursor indicator, the real component of the equivalent complex impedance signal reflects the change in energy dissipation inside the microcapsule unit, and the imaginary component reflects the change in dielectric properties inside the microcapsule unit. The simultaneous appearance of the decrease in the rate of change of the real component and the extreme value of the imaginary component indicates that the phase change state of the phase change working fluid has changed from rapid phase change to phase change completion. This enables independent verification of the completion of the phase change endothermic stage from the electromagnetic dimension through changes in electromagnetic properties.

[0068] By confirming a physical inflection point only when the inflection point precursor indicators of at least two signals simultaneously meet their respective preset conditions, and requiring at least two independent criteria from different physical dimensions to be met simultaneously, the reliability of physical inflection point confirmation is greatly improved through a cross-validation mechanism of multimodal signals. This avoids false triggering caused by noise interference or sensor anomalies of a single signal. At the same time, the requirement of at least two signals ensures that there are still redundant criteria available when a certain signal mode fails.

[0069] The second inflection point precursor indicator in this embodiment is extracted in the following way: The acoustic emission signal envelope is detected to obtain the acoustic emission energy envelope sequence; the acoustic emission energy envelope sequence is counted by unit time window events to obtain the acoustic emission event rate time series. When the slope of the acoustic emission event rate time series changes from positive to negative and the absolute value continues to increase for at least three consecutive time windows, the second inflection point precursor index is determined to meet the preset conditions. The third inflection point precursor indicator is extracted using the following method: Obtain the time series of the real and imaginary components respectively; When the rate of change of the real component drops below the preset proportion of the initial value of the phase change endothermic stage, and the current sampling point value of the imaginary component is greater than the values ​​of the preceding and subsequent sampling points, the third inflection point precursor index is determined to meet the preset conditions.

[0070] By extracting the acoustic emission energy envelope sequence from the acoustic emission signal through envelope detection, and then obtaining the acoustic emission event rate time series by counting events within a unit time window from the acoustic emission energy envelope sequence, high-frequency noise is removed by envelope detection to retain the energy change trend of the acoustic emission signal. The continuous acoustic emission energy envelope is transformed into a discrete event rate time series by counting events within a unit time window. This enables the extraction of the acoustic emission event rate time series reflecting the micromechanical response of the phase transition process from the original acoustic emission signal, providing a clear data foundation for the subsequent identification of slope reversal features.

[0071] When the slope of the acoustic emission event rate time series changes from positive to negative, it indicates that the acoustic emission event rate has changed from rising to falling. Furthermore, if the absolute value continues to increase for at least three consecutive time windows, the influence of random fluctuations is eliminated. This determines that the second inflection point precursor indicator meets the preset conditions. This improves the reliability of the second inflection point precursor indicator by combining the slope reversal feature with the duration window constraint, and avoids misjudgment caused by random fluctuations in a single time window.

[0072] By acquiring the time series of the real and imaginary components of the equivalent complex impedance signal respectively, and tracking the dynamic changes in energy dissipation and dielectric properties, a dual-component time series reflecting the state changes of the phase-change working fluid can be independently acquired from the electromagnetic dimension.

[0073] When the rate of change of the real component drops below a preset proportion of the initial value of the phase transition endothermic stage, it indicates that the energy dissipation tends to be stable. When the current sampling point value of the imaginary component is greater than the previous and subsequent sampling point values, it indicates that the dielectric properties have local extrema. When both conditions are met, the third inflection point precursor index is determined to meet the preset conditions. This achieves the precise capture of the completion moment of the phase transition endothermic stage from the electromagnetic dimension through the joint determination of the real and imaginary components.

[0074] The multimodal sensor fusion criterion in this embodiment also includes a mode priority and time window coupling rule, specifically: During the transition from S2 to S3, the expected inflection point time window is first determined based on the second inflection point precursor index. The length of the expected inflection point time window is determined by the duration of the acoustic emission event rate time series decreasing from its own peak value to a preset proportion of that peak value. Temperature data were collected at an enhanced sampling rate higher than the conventional sampling frequency within the expected inflection point time window. A physical inflection point is confirmed when at least one of the first and third inflection point indicators meets the preset conditions. If only the second inflection point precursor indicator is met, while the other inflection point precursor indicators are not met within the expected inflection point time window, then the expected inflection point time window will be extended by a preset expansion coefficient.

[0075] By first determining the expected inflection point time window based on the second inflection point precursor index of the acoustic emission signal, the acoustic emission signal is most sensitive to the micromechanical response of the phase transition process. The time window of the acoustic emission signal is used as the a priori range of the expected inflection point. The length of the expected inflection point time window is determined by the duration of the acoustic emission event rate time series decreasing from its own peak value to a preset proportion. This realizes the ability to pre-define the possible time range of the physical inflection point through the most sensitive acoustic emission signal, providing a time constraint for the subsequent joint determination of multimodal signals.

[0076] By acquiring temperature data at an enhanced sampling rate within the expected inflection point time window, maintaining a normal sampling frequency outside the expected inflection point time window to save system resources, and increasing the sampling rate within the expected inflection point time window to capture more refined temperature change characteristics, the system achieves improved temperature data acquisition accuracy within the expected inflection point time window while ensuring system resource efficiency, providing high-resolution temperature data for accurate determination of the first inflection point precursor indicators.

[0077] Modal priority mechanism solves the following technical problems: When one mode of the three signals completely fails due to sensor malfunction or environmental interference, the physical inflection point of the other two modes can still be confirmed by at least one satisfied rule, avoiding complete detection failure. Acoustic emission signals are most sensitive to phase transition processes; using them to determine the expected time window maximizes the utilization of time constraints and improves detection efficiency. The enhanced sampling rate is only activated within the expected time window, avoiding the waste of system resources caused by a consistently high sampling rate.

[0078] By confirming the physical inflection point within the expected inflection point time window when at least one of the first inflection point precursor indicators (second derivative of temperature rise rate) and the third inflection point precursor indicator (equivalent complex impedance) meets the preset conditions, and based on the existing second inflection point precursor indicator (acoustic emission) confirming the expected time window, independent verification of at least one dimension of temperature or electromagnetic dimension is required to pass, thus realizing a multi-layer confirmation mechanism that couples modal priority with time window to further improve the reliability of physical inflection point confirmation.

[0079] If only the second inflection point precursor indicator is met, while the first and third inflection point precursor indicators are not met within the expected time window, the expected time window is extended by a preset expansion coefficient to continue waiting. The expected time window may be too narrow due to thermal conduction delay or other factors, which may cause the first and third inflection point precursor indicators to fail to be met in time. By extending the window, sufficient confirmation time is given to other modal signals, thus maintaining the multimodal verification mechanism while avoiding missed detections due to an excessively narrow window, thereby improving the completeness of inflection point detection.

[0080] Specifically, the multimodal sensor fusion criterion is executed by a processor integrated into the heating equipment controller. The processor stores historical batch process data, including the deviation statistics between the trigger time corresponding to each inflection point precursor index and the actual inflection point time verified offline. The confidence weight of each inflection point precursor index is assigned according to the mean and variance of the historical trigger deviation of each inflection point precursor index. The smaller the mean and variance of the deviation, the higher the confidence weight of the inflection point precursor index. When the inflection point precursor judgments of the three inflection point precursor indices are inconsistent in the current batch, the prediction results are weighted by the confidence weight of each inflection point precursor index to obtain a weighted confidence score as the final criterion, so as to improve the accuracy of physical inflection point detection and the repeatability of the assembly method.

[0081] By entrusting the multimodal sensor fusion criteria to a processor integrated in the heating equipment controller, the processor stores historical batch process data and statistically analyzes the historical trigger deviations of each inflection point precursor index. Confidence weights are assigned based on the mean and variance of the deviations. The smaller the mean and variance of the deviation, the higher the confidence weight of the index. In case of inconsistency, a weighted confidence score is obtained through weighted calculation using the confidence weights as the final criterion. This achieves adaptive weight adjustment based on historical process data, enabling the detection results to continuously self-optimize with the accumulation of production batches. This improves the accuracy of physical inflection point detection and the repeatability of the assembly method during long-term operation.

[0082] Specifically, the confidence weight adaptive adjustment mechanism is essentially an online learning strategy. Let the historical trigger deviation mean be μi and the variance be σi for the i-th inflection point precursor indicator. Then, the confidence weight wi is defined as the product of the reciprocal of μi and the reciprocal of σi, divided by the sum of the corresponding products of all modes. The confidence weight wi reflects the historical reliability of the i-th indicator; the smaller the mean deviation and the smaller the variance, the higher the confidence weight.

[0083] The confidence weight adaptive adjustment mechanism has the following advantages: Regarding inter-batch self-learning, after each production batch is completed, the system automatically updates the deviation statistics of each indicator and adjusts the confidence weights, allowing detection accuracy to continuously improve with accumulated production experience. For environmental adaptation, when changes in the production environment cause a systematic increase in the deviation of a certain indicator, the confidence weight of that indicator automatically decreases, and the system automatically switches to relying on other more reliable indicators. Regarding fault tolerance, when a sensor completely fails, causing the deviation of the corresponding indicator to approach infinity, the confidence weight of that indicator automatically approaches zero, and the system automatically excludes the failed indicator in the weighted calculation, avoiding detection of complete failure.

[0084] In practical implementation, the temperature rise rate sensor employs a thermocouple or infrared temperature sensor, positioned at at least two symmetrical locations along the edge of the three-dimensional slit gap, with the average value taken to eliminate local temperature non-uniformity. The acoustic emission sensor uses a piezoelectric sensor, positioned on the outer surface of the top printed circuit board assembly 2 or the bottom printed circuit board assembly 1, close to the central region of the three-dimensional slit gap, to maximize the reception of acoustic emission signals generated by the phase transition of the microcapsule units. A pair of electrodes for the equivalent complex impedance measurement are respectively positioned on the outer surfaces of the top printed circuit board assembly 2 and the bottom printed circuit board assembly 1, forming an electric field across the three-dimensional slit gap to measure the overall equivalent complex impedance.

[0085] The sampling clocks for the three signal modes must be strictly synchronized, using a unified sampling clock source. The sampling frequency for the temperature rise rate should be no less than 1 kHz, the sampling frequency for acoustic emission should be no less than 100 kHz, and the sampling frequency for the equivalent complex impedance should be no less than 100 Hz.

[0086] The signal preprocessing process includes the following steps: The preprocessing procedure for the temperature rise rate signal is as follows: the original temperature signal is filtered by moving average, the temperature rise rate is calculated by difference, and then the second derivative is calculated by second-order difference.

[0087] The preprocessing procedure for acoustic emission signals is as follows: the original acoustic emission signal is subjected to bandpass filtering from 50kHz to 500kHz, followed by envelope detection, event rate time series is obtained by counting events within a unit time window, and then the slope is calculated.

[0088] The preprocessing procedure for the equivalent complex impedance signal is as follows: the original voltage and current signals are subjected to fast Fourier transform to extract the fundamental component, the real and imaginary parts are calculated, and then the rate of change and extreme value detection are performed.

[0089] In practical engineering implementation, the following priority determination logic is adopted: First, check the acoustic emission signal, i.e., the second inflection point precursor indicator, to determine the expected inflection point time window.

[0090] Check the temperature rise rate signal, i.e. the first inflection point precursor, within the expected inflection point time window. If it is satisfied, immediately confirm the physical inflection point and trigger horizontal ultrasonic excitation.

[0091] If the temperature rise rate signal is not met, check the equivalent complex impedance signal, which is the third inflection point indicator. If it is met, the physical inflection point is confirmed.

[0092] If only the acoustic emission signal is satisfied, extend the expected time window and continue waiting.

[0093] The multiple microcapsule units in this embodiment have spatial gradient rupture characteristics, specifically: The thickness of the capsule wall of the microcapsule unit is distributed in a gradient from the center to the edge within the three-dimensional slit gap, and correspondingly, the capsule wall rupture threshold temperature is distributed in a gradient from the center to the edge. As the temperature continues to rise in S4, when the temperature exceeds the rupture threshold temperature of the microcapsule unit in the central region, the microcapsule unit in the central region ruptures preferentially over the edge region, forming a directional and ordered rupture sequence and vaporization exhaust wavefront that propagates from the center to the edge.

[0094] By setting the capsule wall thickness of the microcapsule unit to be distributed in a gradient from the center to the edge within the three-dimensional slit gap, the greater the capsule wall thickness, the higher the rupture threshold temperature. Therefore, the capsule wall rupture threshold temperature is also distributed in a gradient from the center to the edge, enabling microcapsule units at different spatial locations to rupture sequentially at different temperatures, forming a directional and ordered rupture sequence that propagates from the center to the edge.

[0095] The formation mechanism of the directional ordered fracturing sequence is as follows: The rupture of the microcapsule unit's wall is essentially a thermal stress failure process of the shell material under temperature load. When the temperature reaches the rupture threshold temperature of the shell material, the accumulated thermal stress inside the shell exceeds the material's ultimate strength, causing brittle or ductile fracture and releasing the internal phase-change working fluid. The spatial gradient rupture characteristic is achieved by setting an increasing gradient distribution of the shell thickness from the center to the edge within the three-dimensional slit gap, resulting in different rupture threshold temperatures for microcapsule units at different spatial locations. The central region has the thinnest shell and the lowest rupture threshold temperature, while the edge region has the thickest shell and the highest rupture threshold temperature. During the continued temperature rise in step S4, the temperature in the central region rises first and reaches the rupture threshold temperature of the microcapsule units in that region first, leading to preferential rupture of the central region's microcapsule units. As the temperature continues to rise, the rupture front gradually advances towards the edge region, with the edge region's microcapsule units rupturing last. The timing and spatial location of each microcapsule unit's rupture strictly correspond, with the center rupturing first and the edge rupturing later, exhibiting spatial order rather than random disorder in the rupture process.

[0096] The phase change propellant released after each microcapsule unit ruptures continues to heat up and vaporize within the three-dimensional slit gap, expanding in volume as it changes from a liquid to a gaseous state. The microcapsule units in the central region rupture first, releasing the propellant. The high-pressure gas generated by vaporization in the central region expands and diffuses towards the edges, forming a vaporization exhaust wavefront originating from the center. This vaporization exhaust wavefront is a pressure front driven by gas expansion; the gas pressure at the wavefront's leading edge is higher than the pressure in the already exhausted area behind it. As the wavefront advances, it pushes outwards, expelling residual air and volatile substances from the three-dimensional slit gap. The directional and ordered rupture sequence ensures a clear wavefront interface: the inner side of the wavefront represents the ruptured region, where the microcapsule units have ruptured and completed exhaust; the outer side represents the unruptured region, where the microcapsule units have not yet ruptured, and the gas within the three-dimensional slit gap is systematically expelled primarily along the wavefront's direction of propagation.

[0097] By utilizing the lowest rupture threshold temperature of the microcapsule unit in the central region during the continued temperature rise, the central region microcapsule unit reaches the rupture condition first and ruptures preferentially. As the temperature continues to rise, the microcapsule units in the peripheral region rupture sequentially, forming a directional and orderly rupture sequence that gradually advances from the center to the edge. The phase change working fluid released after each microcapsule unit ruptures vaporizes and expands to form a vaporized exhaust wavefront. The vaporized exhaust wavefront advances orderly from the center to the edge, realizing the transformation of the exhaust process from disordered, simultaneous rupture to orderly, directional advancement. The directional and orderly vaporized exhaust wavefront allows the gas to be systematically squeezed out in the direction from the inside to the outside, avoiding gas collision and dead zone residue caused by simultaneous rupture in multiple directions, and significantly improving exhaust efficiency and the thoroughness of gas removal in the gap.

[0098] Specifically, the increasing gradient distribution has at least three discrete levels, as follows: Central level, intermediate level, and peripheral level; By setting the gradient distribution of increasing capsule wall thickness to at least three discrete levels (center level, middle level, and edge level), the rupture of microcapsule units forms a clear three-level ordered sequence. The center level ruptures first, followed by the middle level, and the edge level ruptures last. The three discrete levels ensure that the vaporization exhaust wavefront has a clear graded propulsion structure, realize the graded controllability of the exhaust process, and avoid the fuzziness of the exhaust wavefront boundary caused by too many continuous gradients, which would reduce the efficiency of directional exhaust.

[0099] Specifically, the rupture threshold temperature of the capsule wall in the central grade is not less than 1.0 times and not more than 1.05 times the upper limit of the phase transition temperature, the temperature in the intermediate grade is not less than 1.05 times and not more than 1.10 times the upper limit of the phase transition temperature, and the temperature in the edge grade is not less than 1.10 times and not more than 1.20 times the upper limit of the phase transition temperature. This ensures that as the temperature continues to rise in S4, the central level breaks first, followed by the intermediate level, and finally the edge level, forming a directional, orderly breakup sequence and a vaporization exhaust wavefront that propagates from the center to the edge.

[0100] By setting the rupture threshold temperatures of the three levels to specific multiples of the upper limit of the phase transition temperature, the central level (1.0-1.05 times) is closest to the upper limit of the phase transition temperature, ensuring that exhaust begins immediately after the phase transition heat absorption is completed. The intermediate level (1.05-1.10 times) and the edge level (1.10-1.20 times) increase sequentially. The uniform distribution of the three temperature levels ensures that the propagation speed of the exhaust wavefront from the center to the edge is stable and controllable, thus achieving precise temperature timing control of the exhaust process.

[0101] In S1, the spatial gradient breaking property is constructed in the following way: Prepare at least two sets of microcapsule units with different capsule wall thicknesses: The first group has a first cyst wall thickness, corresponding to a lower rupture threshold temperature; The second group has a second bladder wall thickness, corresponding to a higher rupture threshold temperature, and the second bladder wall thickness is greater than the first bladder wall thickness. During filling, the first group is injected into the central region, and the second group is injected into the edge region surrounding the central region, forming a stepped transition in the thickness of the capsule wall between the two.

[0102] By preparing at least two sets of microcapsule units with different wall thicknesses and injecting them into different regions during filling, the first set with a lower wall thickness was injected into the central region and the second set with a higher wall thickness was injected into the edge region. This method of spatially partitioned filling allows for the engineering-feasible construction of spatial gradient rupture characteristics, avoiding the high cost and difficulty of achieving continuous gradient manufacturing on a single microcapsule unit.

[0103] There is a thermo-mechanical coupling synergistic relationship between the spatial gradient fracturing characteristics and the anisotropic thermal conduction network, specifically: In S2, the anisotropic heat conduction network conducts heat from the center to the edge in the transverse direction, so that the temperature field at the end of S2 is distributed with high temperature at the center and low temperature at the edge. As the temperature continues to rise in S4, the central region, due to its higher initial temperature, reaches the rupture threshold temperature of the capsule wall first. The temperature field distribution and the gradient distribution of the capsule wall rupture threshold work together to enhance the directional and orderly rupture from the center to the edge.

[0104] In step S2, the anisotropic heat conduction network conducts heat from the center to the edge laterally, resulting in a temperature field distribution that is high at the center and low at the edge at the end of step S2. In step S4, as the temperature continues to rise, the central region reaches the capsule wall rupture threshold temperature first due to its higher initial temperature. The temperature field distribution and the capsule wall rupture threshold gradient distribution work together to enhance the directional and orderly rupture from the center to the edge, realizing the thermo-mechanical coupling synergy between the anisotropic heat conduction and the rupture gradient distribution. The anisotropic heat conduction makes the temperature in the central region higher, and the microcapsule units in the central region reach the rupture condition faster at a lower nominal rupture temperature. The superposition of these two factors makes the directional and orderly rupture progress faster and more controllable.

[0105] Specifically, there is a wavefront-pressure oscillation synergistic effect between the directional ordered rupture sequence and the vaporization exhaust wavefront and the transient pressure oscillation field formed in S4, specifically: The vaporized exhaust wave front reflects after reaching the gap boundary, and the reflected wave superimposes with the subsequent incident wave to form a transient pressure oscillation field. The antinodes of the transient pressure oscillation field coincide with or are adjacent to the transition regions of adjacent gradient levels. The enhanced pressure oscillations at the antinodes, combined with thermal stress, cause pre-damage to the capsule walls of the microcapsule units in the transition region, reducing their actual rupture threshold and causing pre-rupture before reaching the nominal rupture threshold temperature. The released phase change working fluid is added to the vaporization exhaust wavefront, forming a positive feedback loop of rupture-exhaust-pressure oscillation.

[0106] The formation mechanism of transient pressure oscillation field is as follows: When the vaporized exhaust wavefront propagates to the physical boundary of the three-dimensional slit gap, it is reflected at the boundary, and the reflected wave propagates towards the center. Simultaneously, subsequent ruptured microcapsule units continue to release phase-change working fluid and vaporize, generating new incident waves that propagate towards the edges. The reflected wave and the subsequent incident wave meet and superimpose within the three-dimensional slit gap, forming a transient pressure oscillation field.

[0107] The spatial characteristics of the transient pressure oscillation field are manifested by the presence of pressure antinodes and nodes within the oscillation field, with the position of the antinodes determined by the gap geometry and wavelength. The temporal characteristics of the transient pressure oscillation field are manifested as follows: the oscillation field is transient rather than steady-state, its amplitude decays with time, and its duration is determined by the replenishment rate of energy released by subsequent microcapsule unit rupture.

[0108] The frequency characteristics of the transient pressure oscillation field are that the oscillation frequency is determined by the rupture rate of the microcapsule unit and the sound velocity in the gap.

[0109] For a centrally heated, edge-closed 3D stacked structure, the antinodes of the transient pressure oscillation field naturally fall in the transition region between the center and edge of the three-dimensional slit gap, coinciding with or adjacent to the transition region of the rupture gradient levels of the adjacent capsule walls. This coincidence is determined by both the spatial gradient rupture characteristics and the physical symmetry of the transient pressure oscillation field. On the one hand, the rupture front of the spatial gradient rupture characteristics advances from the center to the edge, generating abrupt changes in the density and rupture timing of microcapsule units in the gradient level transition region; on the other hand, the antinodes of the transient pressure oscillation field are determined by the boundary reflection conditions, and for a center-edge symmetrical structure, the antinodes naturally fall in the transition region.

[0110] The overlap of the two produces a synergistic effect, with the pressure oscillation amplitude being the largest at the antinode. This, combined with the stress concentration caused by the abrupt change in thickness of the capsule wall in the gradient transition region microcapsule unit, results in the actual rupture threshold of the transition region microcapsule unit being lower than the nominal rupture threshold temperature.

[0111] The complete process of the wavefront-pressure oscillation positive feedback loop is as follows: The first stage is the wavefront formation stage. The microcapsule units in the central region preferentially rupture, releasing the phase change working fluid, which vaporizes and expands, forming a vaporized exhaust wavefront that propagates from the center to the edge.

[0112] The second stage is the wavefront propagation and reflection stage. The vaporized exhaust wavefront propagates towards the edge, and is reflected upon reaching the gap boundary. The reflected wave propagates towards the center and superimposes with the subsequent incident wave to form a transient pressure oscillation field.

[0113] The third stage is the pre-damage stage of the antinodes. The antinodes of the transient pressure oscillation field fall in the gradient level transition region. The pressure oscillation amplitude and thermal stress at the antinodes are superimposed, causing pre-damage to the capsule wall of the microcapsule unit in the transition region, that is, the initiation and propagation of microcracks, which reduces its actual rupture threshold temperature.

[0114] The fourth stage is the pre-rupture and wavefront acceleration stage. The microcapsule unit in the transition region undergoes pre-rupture before reaching the nominal rupture threshold temperature. The released phase change working fluid is added to the vaporized exhaust wavefront, providing additional energy to the wavefront and accelerating its propagation speed.

[0115] The fifth stage is the positive feedback loop stage. After wavefront acceleration, the microcapsule units reach the next gradient level transition region more quickly. In this region, the microcapsule units are also pre-ruptured due to the pre-damage effect of the antinodes of the oscillation field, releasing more phase change working fluid to further replenish the wavefront energy, forming a positive feedback loop of rupture, degassing, pressure oscillation, pre-damage, pre-rupture, more degassing, and stronger oscillation.

[0116] The exhaust process of the positive feedback loop is not a passive exhaust with uniform propulsion, but a self-reinforcing active exhaust. The wavefront continuously gains energy during propulsion, and the exhaust efficiency increases with the propulsion distance, ensuring that even if the microcapsule units in the gap edge region have not yet ruptured, the wavefront's powerful exhaust capability has effectively discharged the residual gas in the edge region.

[0117] After the vaporized exhaust wavefront reaches the boundary of the three-dimensional slit gap, it is reflected. The reflected wave superimposed with the subsequent incident wave to form a transient pressure oscillation field. The antinode of the transient pressure oscillation field coincides with or is adjacent to the transition region of the adjacent gradient level. The enhanced pressure oscillation at the antinode and the superposition of thermal stress cause pre-damage to the capsule wall of the microcapsule unit in the transition region, reducing its actual rupture threshold. Pre-rupture occurs before reaching the nominal rupture threshold temperature. The phase change working fluid released by the pre-rupture is added to the vaporized exhaust wavefront, forming a positive feedback loop of rupture-exhaust-pressure oscillation. This realizes the use of the wave effect of the transient pressure oscillation field to accelerate the rupture of the microcapsule unit in the gradient level transition region, so that the propulsion of the exhaust wavefront obtains self-reinforcing power and significantly improves the exhaust efficiency.

[0118] The above description is only a preferred embodiment of the present invention. For those skilled in the art, there will be changes in the specific implementation and application scope based on the ideas of the present invention. The content of this specification should not be construed as a limitation of the present invention.

Claims

1. A method for assembling 3D stacked printed circuit boards with self-collimation and thermal buffer venting, characterized in that, Includes the following steps: S1. Apply a first melting point solder (12) to the substrate pad (11) of the bottom printed circuit board assembly (1) and apply a second melting point solder (22) to the connection part (21) of the top printed circuit board assembly (2), forming a three-dimensional slit gap between the two; fill the three-dimensional slit gap with a phase change heat buffer medium (3) to form the assembly to be soldered. The melting point of the first melting point solder (12) is higher than that of the second melting point solder (22); the phase change heat buffer medium (3) includes multiple heat storage units, each heat storage unit includes a shell and a phase change working medium wrapped by the shell; the shell has temperature-sensitive cracking characteristics, and the cracking threshold temperature is not lower than the upper limit of the phase change temperature of the phase change working medium; S2. Heat the component to be welded. When the temperature reaches the liquidus temperature range of the second melting point solder (22), use the isothermal effect of the phase change heat buffer medium (3) to keep the second melting point solder (22) in a liquid state and the temperature of the bottom printed circuit board component (1) is lower than the remelting threshold of the first melting point solder (12). S3. During the window period when the second melting point solder (22) is in a liquid state, the physical inflection point of the phase change endothermic stage is identified by monitoring the temperature rise rate change trend in the three-dimensional slit gap in situ. Accordingly, a horizontal ultrasonic excitation force parallel to the substrate plane of the bottom printed circuit board assembly (1) is applied to the component to be soldered, driving the top printed circuit board assembly (2) to slide laterally relative to the bottom printed circuit board assembly (1) to complete self-alignment. S4. After self-alignment, the temperature continues to rise. When the temperature exceeds the rupture threshold temperature of the shell, the shell ruptures and releases the phase change working medium. The phase change working medium vaporizes and expands in the three-dimensional slit gap, and discharges the residual air and volatile substances generated during the welding process. The temperature was then lowered to allow the second melting point solder (22) to solidify.

2. The method for assembling a 3D stacked printed circuit board with self-collimation and thermal buffer venting according to claim 1, characterized in that, The phase change heat buffer medium (3) is a microcapsule phase change heat buffer medium (3), which contains multiple microcapsule units as the heat storage unit; the shell is the capsule wall of the microcapsule unit; The phase change temperature range of the phase change working medium is between the liquidus temperature of the second melting point solder (22) and the solidus temperature of the first melting point solder (12). During the phase change heat absorption stage, the microcapsule phase change heat buffer medium (3) experiences a significant decrease in the temperature rise rate due to phase change heat absorption, forming a plateau characteristic segment on the temperature rise curve. The rupture threshold temperature of the microcapsule unit is not lower than the upper limit of the phase change temperature of the phase change working fluid, and is not higher than 1.2 times the upper limit of the phase change temperature.

3. The method for assembling a 3D stacked printed circuit board with self-collimation and thermal buffer venting according to claim 2, characterized in that, The microcapsule phase change heat buffer medium (3) also includes a dispersion matrix. The multiple microcapsule units are core-shell structured microcapsules with organic phase change materials as the core and inorganic materials doped with nano-metal oxides as the capsule walls, and are dispersed in the dispersion matrix. The nano-metal oxides are arranged in a transverse orientation in the capsule wall to form an anisotropic thermal conductive network. The transverse thermal conductivity of the anisotropic thermal conductive network increases stepwise when the volume fraction of the microcapsule unit in the microcapsule phase change heat buffer medium (3) reaches the percolation threshold. The permeation threshold is the critical value at which the volume fraction of microcapsule units in the microcapsule phase change heat buffer medium (3) reaches the point where a continuous heat conduction path is formed, and the critical value is between 15% and 30%.

4. The method for assembling a 3D stacked printed circuit board with self-collimation and thermal buffer venting according to claim 3, characterized in that, The edge of the three-dimensional slit gap is also provided with at least one thermally conductive connection terminal, one end of which is embedded in the gap and in thermal contact with the microcapsule phase change heat buffer medium (3), and the other end extends to the outer surface of the top printed circuit board assembly (2) or the bottom printed circuit board assembly (1). The thermally conductive connection terminal is made of an insulating thermally conductive material; In S2, the heat conducted to the gap edge by the anisotropic heat-conducting network is discharged through the heat-conducting connection terminal.

5. The method for assembling a 3D stacked printed circuit board with self-collimation and thermal buffer venting according to claim 2, characterized in that, In S3, the physical inflection point is detected in the following way: Monitor the temperature rise curve within the three-dimensional slit gap. When the temperature rise rate drops to less than 10% of the initial temperature rise rate during the phase change endothermic stage and remains so for at least 2 seconds, confirm the physical inflection point. The physical inflection point triggers horizontal ultrasonic excitation.

6. The method for assembling a 3D stacked printed circuit board with self-collimation and thermal buffer venting according to claim 5, characterized in that, The activation of horizontal ultrasonic excitation in step S3 and the triggering of cyst wall rupture in step S4 are achieved through the following adaptive temperature control method: The heating temperature rise curve is obtained, the change in surface temperature rise rate is detected, and the inflection point of the phase change plateau period is confirmed after a preset lag time as the physical inflection point. The preset lag time is the heat penetration time required for heat to be conducted from the gap surface to the phase change working medium inside the microcapsule unit. The heat penetration time is not less than the ratio of the square of the capsule wall thickness to the thermal diffusivity of the capsule wall material. Upon confirmation, horizontal ultrasonic excitation is triggered. After ultrasonic excitation, the temperature is controlled to rise to a preset threshold exceeding the upper limit of the phase change temperature, so that the temperature exceeds the rupture threshold temperature of the capsule wall, causing the capsule wall to rupture and release the phase change working fluid, and then the temperature is cooled and solidified.

7. The method for assembling a 3D stacked printed circuit board with self-collimation and thermal buffer venting according to claim 2, characterized in that, The multiple microcapsule units possess spatial gradient rupture characteristics, specifically: The thickness of the capsule wall of the microcapsule unit is distributed in a gradient increasing from the center to the edge within the three-dimensional slit gap, and correspondingly, the capsule wall rupture threshold temperature is distributed in a gradient increasing from the center to the edge. As the temperature continues to rise in S4, when the temperature exceeds the rupture threshold temperature of the microcapsule unit in the central region, the microcapsule unit in the central region ruptures preferentially over the edge region, forming a directional and ordered rupture sequence and vaporization exhaust wavefront that propagates from the center to the edge.

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