Multi-chip stacked packaging structure and packaging method

Abstract

The invention discloses a multi-chip stacked packaging structure and a packaging method. The method comprises the following steps: providing a first chip with micro convex points at the bottom and a substrate with micro concave points at the top corresponding to the micro convex points; the surfaces of the micro convex points and the interiors of the micro concave points are evenly coated with welding flux and welding flux reinforcing agents respectively; respectively coating PCM on the top surface of the substrate and the bottom surface of the first chip, and laminating the first chip and the substrate after enabling the micro convex points to correspond to the micro concave points through a chip mounter; the joints of the pressed micro convex points and micro concave points are welded through microwaves, and temperature control curing is conducted on the welding flux and the welding flux reinforcing agent; after the solidified welding flux and the welding flux reinforcing agent are cooled to form a welding spot structure, a packaging structure is obtained; second chips are stacked on the packaging structure, the packaging structure is stacked and welded to the chips through a microwave welding method, and a multi-chip stacked packaging structure is obtained after a preset number of second chips are stacked and welded; the steps of the microwave welding method are the same as the previous steps. According to the invention, the problem of stress concentration caused by uneven temperature is solved.

Description

Technical Field

[0001] The present invention relates to the field of chip packaging, and in particular, to a multi-chip stacked packaging structure and a packaging method. Background Art

[0002] With the continuous development of integrated circuit technology, especially in the fields of high-performance computing and storage, 3DIC packaging technology has become the key to solving the limitations of traditional 2D packaging technology. 3DIC packaging greatly improves the chip integration and transmission speed by vertically stacking multiple chips and using micro-bump interconnections to connect different chips. However, micro-bumps still face a series of technical challenges during the soldering process, especially in terms of thermal management and soldering quality.

[0003] In the traditional soldering process, the soldering of micro-bumps requires high temperature to melt the solder. During the soldering process, due to uneven temperature control in the traditional soldering process and the difference in the coefficient of thermal expansion of the materials of the chip and the substrate, temperature changes will cause different expansion degrees in different parts of the chip and the substrate, resulting in stress concentration, structural deformation, and a decrease in the long-term reliability of soldering, which in turn affects the soldering quality and packaging performance. Correspondingly, uneven temperature control will also lead to solder joint failure, substrate warping, and uneven solder microstructure.

[0004] Therefore, a multi-chip stacked packaging structure and a packaging method are proposed to solve the problems of stress concentration, structural deformation, and a decrease in the long-term reliability of soldering caused by uneven temperature control in the traditional soldering process. Summary of the Invention

[0005] The purpose of the present invention is to provide a multi-chip stacked packaging structure and a packaging method to solve the problems of stress concentration, structural deformation, and a decrease in the long-term reliability of soldering caused by uneven temperature control in the traditional soldering process.

[0006] To achieve this purpose, the present invention adopts the following technical solutions: A multi-chip stacked packaging method, the packaging method comprising the following steps: Step S1, providing a first chip with micro-bumps at the bottom and a substrate with corresponding micro-indentations at the top; Step S2, uniformly coating solder and solder enhancer on the surface of the micro-bumps and inside the micro-indentations respectively; Step S3, coating PCM on the top surface of the substrate and the bottom surface of the first chip respectively, and after corresponding the micro-bumps and the micro-indentations through a mounter, pressing the first chip and the substrate; Step S4, welding the connection between the corresponding micro-bumps and micro-indentations by microwave, and controlling the temperature to cure the solder and the solder enhancer; Step S5: After the solder to be cured and the solder enhancer are cooled to form a solder joint structure, a packaged structure is obtained; Step S6: Stack a second chip on the packaged structure and weld the stacked packaged structure and the chip by microwave soldering. After stacking and welding the expected number of second chips, a multi-chip stacked packaged structure is obtained; the steps of the microwave soldering method are the same as those of Steps S1 to S5.

[0007] In Step S1, the diameter of the micro-dimples is 10 - 20 μm, the depth is 5 - 15 μm, and the pitch is 15 - 30 μm; the diameter of the micro-bumps is 12 - 30 μm, the height is 6 - 20 μm, and the pitch is 15 - 30 μm.

[0008] The solder on the surface of the micro-bumps in Step S2 is coated according to the following steps: Step S211: Fix the first chip on the workbench of the screen printer and keep it stable by vacuum adsorption; Step S212: Select a template with an opening diameter larger than the diameter of the micro-bumps according to the size of the arrangement of the micro-bumps; Step S213: Uniformly coat the solder on the coating surface of the template and coat the solder onto the surface of the micro-bumps through a squeegee; The solder enhancer on the surface of the micro-dimples in Step S2 is coated according to the following steps: Step S221: Low-pressure spray the solder enhancer using a sprayer; Step S222: Place the coated substrate in a thermal curing oven to cure the solder enhancer in the micro-dimples.

[0009] In Step S213, the solder is Sn - Ag - Cu solder with a thickness of 5 - 15 μm, and the scraping pressure of the squeegee is 10 - 30 N; In Step S221, the solder enhancer is a rosin-based flux with a thickness of 1 - 3 μm, the spraying pressure of the sprayer is 0.05 - 0.2 MPa, and the spraying time is 1 - 2 seconds; In Step S222, the curing temperature of the thermal curing oven is 100 - 150 °C, and the curing time of the thermal curing oven is 1 - 3 minutes.

[0010] The PCM on the top surface of the substrate in Step S3 is coated according to the following steps: Step S311: Fix the substrate on the workbench of the precision sprayer to ensure that the substrate is horizontal and stably fixed; Step S312: The precision sprayer avoids the micro-dimples and uniformly covers the PCM on the top surface of the substrate; Step S313: After the coating is completed, place the substrate in a thermal curing oven for curing to stabilize the coated PCM; In step S3, the bottom surface PCM of the first chip is coated according to the following steps: Step S321: Fix the first chip on the working platform of the screen printing equipment, and make the openings of the screen template completely correspond to the arrangement of the microbumps; Step S322: Uniformly introduce PCM into the screen template, and coat the PCM on the bottom surface of the first chip through a squeegee; Step S323: After coating, place the first chip in a thermal curing device for curing to stabilize the coated PCM; In step S3, the first chip and the substrate are pressed together according to the following steps: Step S331: Align the microdents with the microbumps through the optical alignment system of the mounter, and align the substrate with the first chip; Step S332: Press the substrate and the first chip through the mounter, and position the substrate and the first chip after pressing.

[0011] In step S312, the spraying pressure of the precision spraying machine is 0.05 - 0.2 MPa, the rate is 10 - 20 mm / s, the distance is 5 - 20 mm, and the coating thickness of the precision spraying machine is 5 - 15 μm; In step S313, the curing temperature of the thermal curing device is 50 - 100 °C, and the time is 1 - 3 minutes; In step S322, the scraping pressure of the squeegee is 10 - 30 N, the scraping angle of the squeegee is 45° - 60°, the scraping speed of the squeegee is 10 - 50 mm / s, and the coating thickness is 5 - 15 μm; In step S332, the pressing force of the mounter is 50 - 200 gf, and the pressing time is 2 - 5 s.

[0012] Step S4 specifically includes: Step S411: Perform preheating treatment on the connection between the microbumps and the microdents through microwave to form preheating microwave in a nitrogen environment; Step S412: Increase the microwave power to form melting microwave to melt the solder and the solder enhancer at the connection between the microbumps and the microdents; Step S413: Decrease the microwave power to form temperature control microwave to control the temperature and cure the melted solder and the solder enhancer, so as to form a solder joint structure prototype at the connection between the microbumps and the microdents.

[0013] In step S411, the flow rate of the nitrogen is 1 - 2 L / min, the vacuum degree is 10 - 50 Pa, the power of the preheating microwave is 10 - 30 W, the welding temperature is 150 - 180 °C, and the preheating time is 5 - 10 seconds; In the step S412, the melting microwave power is 30 - 60 W, the welding temperature is 220 - 260 °C, and the melting time is 8 - 15 s; In the step S413, the temperature control microwave power is 10 - 20 W, the temperature control curing time is 10 - 20 s, and the corresponding cooling rate is ≤ 2 °C / s.

[0014] The cooling time of the solidified solder and the solder enhancer is 20 - 60 s, and the corresponding cooling rate is 0.5 - 2 °C / s.

[0015] A multi - chip stacked packaging structure is obtained by using the multi - chip stacked packaging method as described above.

[0016] Compared with the prior art, the present invention has the following beneficial effects: 1. For the multi - chip stacked packaging structure and packaging method of the present invention, through the combined use of microwave selective heating and PCM, precise temperature control of the welding area is achieved. Microwave welding heats only the welding joints of the micro - bumps and micro - depressions in a local heating manner. The set PCM absorbs the excess heat during the welding process and reduces the temperature rise amplitude through the phase - change buffering mechanism, resulting in a reduced temperature gradient in the welding area, further flattening the temperature change, avoiding the temperature unevenness caused by overall heating in the traditional process, and thus preventing problems such as stress concentration, structural deformation, and a decline in the long - term reliability of welding.

[0017] 2. For the multi - chip stacked packaging structure and packaging method of the present invention, through the high - precision design of micro - bumps and micro - depressions and the use of solder enhancers, the mechanical strength and electrical performance of the solder joints are enhanced. The precise geometric size matching of micro - bumps and micro - depressions provides sufficient welding area. Combined with the wetting effect of the solder enhancer, the welding interface is more uniform and reliable. The whole process of microwave welding is in a controlled environment, and local heating avoids high - temperature thermal shock to the substrate and the whole chip, effectively reducing the stress failure problem of the solder joints caused by thermal expansion.

[0018] 3. For the multi - chip stacked packaging structure and packaging method of the present invention, by coating PCM on the top surface of the substrate and the bottom surface of the first chip, the excess heat is absorbed during the welding process, effectively reducing the temperature rise amplitude in the welding area. At the same time, microwave selective heating only locally heats the connection between micro - bumps and micro - depressions, and other areas of the substrate are limitedly heated, thus avoiding the problem of substrate warping due to overall heating in the traditional heating method. In the subsequent cooling stage, the PCM gradually releases heat, reducing the cooling speed and the change amplitude of the temperature gradient, making the thermal expansion of the substrate more uniform, further avoiding the warping risk caused by rapid cooling, and ensuring the welding quality.

[0019] 4. In a multi-chip stacked packaging structure and a packaging method of the present invention, by providing a PCM, the heat dissipation capacity of the overall packaging is improved during the operation of the packaging structure, helping the solder joints to remain stable in a high heat load environment, delaying the aging process of the solder joints, enhancing the long-term reliability of the solder joints and the packaging structure, and ensuring its continuous and stable operation in a complex working environment. BRIEF DESCRIPTION OF THE DRAWINGS

[0020] In order to more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the following will briefly introduce the drawings required for use in the description of the embodiments or the prior art. Obviously, the following drawings are only some embodiments of the present invention. For those of ordinary skill in the art, without creative efforts, other drawings can be obtained based on these drawings.

[0021] The structures, proportions, sizes, etc. shown in the drawings of this specification are only used to cooperate with the content disclosed in the specification for those familiar with this technology to understand and read, and are not used to limit the limiting conditions for the implementation of the present invention. Therefore, they do not have a substantial technical meaning. Any modification of the structure, change in the proportional relationship, or adjustment of the size, without affecting the effects that the present invention can produce and the purposes that can be achieved, should still fall within the scope that can be covered by the technical content disclosed in the present invention.

[0022] Figure 1 It is a flowchart of a multi-chip stacked packaging method of the present invention. DETAILED DESCRIPTION OF THE EMBODIMENTS

[0023] In order to make the object, features, and advantages of the present invention more obvious and understandable, the following will clearly and completely describe the technical solutions in the embodiments of the present invention with reference to the drawings in the embodiments of the present invention. Obviously, the following described embodiments are only some embodiments of the present invention, rather than all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative efforts belong to the scope of protection of the present invention.

[0024] In the description of the present invention, it should be understood that the orientation or positional relationship indicated by terms such as "upper", "lower", "top", "bottom", "inner", "outer", etc. is based on the orientation or positional relationship shown in the drawings, and is only for the convenience of describing the present invention and simplifying the description, rather than indicating or implying that the device or element referred to must have a specific orientation, be constructed and operated in a specific orientation, and therefore should not be construed as a limitation of the present invention. It should be noted that when a component is considered to be "connected" to another component, it can be directly connected to the other component or there may be an intermediate component present at the same time.

[0025] Embodiment 1: Please refer toFigure 1 , a multi-chip stacked packaging method in this embodiment, the packaging method includes the following steps: Step S1, provide a first chip with micro-bumps at the bottom and a substrate with micro-dimples corresponding to the micro-bumps at the top; Further, in step S1, the diameter of the micro-dimples is 10-20 μm, the depth is 5-15 μm, the pitch is 15-30 μm, the diameter of the micro-bumps is 12-30 μm, the height is 6-20 μm, and the pitch is 15-30 μm.

[0026] It should be noted that through the diameter and pitch of the micro-bumps and micro-dimples and the corresponding height and depth, the high-precision docking of the first chip and the substrate is ensured, and the problem of uneven solder joints during the welding process is avoided.

[0027] Preferably, the diameter of the micro-dimples is 15 μm, the depth is 10 μm, the pitch is 20 μm, the diameter of the micro-bumps is 20 μm, the height is 15 μm, and the pitch is 20 μm.

[0028] It can be understood that the diameter and pitch of the micro-bumps and micro-dimples and the corresponding height and depth provide sufficient welding area, which is conducive to forming a reliable solder joint structure in the subsequent welding process.

[0029] Specifically, the welding area of the micro-bumps and micro-dimples is: ; When the diameter of the micro-dimples is 15 μm, a larger contact area can be formed, improving the welding strength and reducing the possibility of mechanical failure of the solder joints. In addition, the optimized design of the pitch and height reduces the stress transfer during multi-chip stacking. At the same time, setting the diameter of the micro-dimples to 15 μm achieves the best balance among welding strength, packaging density, processing accuracy, and thermal management compared to setting it to 10 μm or 20 μm, which is suitable for the actual application requirements of multi-chip stacked packaging. Setting the diameter of the micro-dimples to 10 μm results in higher packaging density but insufficient welding strength, insufficient solder volume, difficult filling, and excessive processing difficulty. While setting the diameter of the micro-dimples to 20 μm results in higher welding strength but decreased packaging density, increased risk of thermal stress concentration, increased processing cost, and decreased solder filling efficiency.

[0030] Step S2, uniformly coat solder and solder enhancer on the surface of the micro-bumps and inside the micro-dimples respectively; The solder on the surface of the micro-bumps in step S2 is coated according to the following steps: Further, step S211, fix the first chip on the workbench of the screen printing machine and keep it stable by vacuum adsorption; Step S212: Select a template with an opening diameter larger than the diameter of the micro bumps according to the size of the arrangement of the micro bumps; Step S213: Uniformly apply solder on the coating surface of the template, and use a squeegee to apply the solder onto the surface of the micro bumps; In step S2, the solder enhancer on the surface of the micro dents is applied according to the following steps: Step S221: Use a spraying machine to spray the solder enhancer at a low pressure; Step S222: Place the coated substrate in a thermal curing machine to cure the solder enhancer inside the micro dents.

[0031] It should be noted that the uniform distribution of the solder provides sufficient wetting and filling capabilities, avoiding pores and voids after welding. At the same time, under the action of the solder enhancer, the wettability of the solder is significantly improved, enhancing the mechanical strength and electrical performance of the solder joints.

[0032] Furthermore, in step S213, the solder is Sn-Ag-Cu solder, with a thickness of 5 - 15 μm, and the scraping pressure of the squeegee is 10 - 30 N; Preferably, the solder thickness is 10 μm, and the scraping pressure of the squeegee is 20 N.

[0033] In step S221, the solder enhancer is a rosin-based flux, with a thickness of 1 - 3 μm, the spraying pressure of the spraying machine is 0.05 - 0.2 MPa, and the spraying time is 1 - 2 seconds; Preferably, the thickness of the solder enhancer is 2 μm, the spraying pressure of the spraying machine is 0.1 MPa, and the spraying time is 1.5 seconds.

[0034] In step S222, the curing temperature of the thermal curing machine is 100 - 150 °C, and the curing time of the thermal curing machine is 1 - 3 minutes.

[0035] Preferably, the curing temperature of the thermal curing machine is 120 °C, and the curing time of the thermal curing machine is 2 minutes.

[0036] It can be understood that under the specific printing process of the solder and the specific spraying process of the solder enhancer, the efficient and precise coating of the solder and the solder enhancer is achieved, ensuring the uniformity of the solder and the solder enhancer after coating.

[0037] Specifically, the screen printing machine uses a scraping pressure of 20 N through a squeegee to coat the solder thickness to 10 μm, ensuring that the solder is appropriately and uniformly covered. The screen template with an opening diameter larger than the diameter of the micro bumps also avoids coating deviation. At the same time, the spraying machine sprays the solder enhancer thickness to 2 μm through a spraying pressure of 0.1 MPa, enabling the solder enhancer to be evenly distributed inside the micro dents, thereby reducing the surface tension during the subsequent welding process and enhancing the bonding strength between the micro bumps and the micro dents.

[0038] Step S3: Coat PCM (Phase Change Material) on the top surface of the substrate and the bottom surface of the first chip respectively. After corresponding the microbumps with the microdents through a mounter, press the first chip and the substrate together. Further, in step S3, the PCM on the top surface of the substrate is coated according to the following steps: Step S311: Fix the substrate on the workbench of a precision spraying machine to ensure that the substrate is horizontal and stably fixed. Step S312: Avoid the microdents with the precision spraying machine and evenly cover the top surface of the substrate with PCM. Step S313: After the coating is completed, place the substrate in a thermal curing machine for curing to stabilize the coated PCM. In step S3, the PCM on the bottom surface of the first chip is coated according to the following steps: Step S321: Fix the first chip on the working platform of a screen printing device and make the openings of the screen template completely correspond to the arrangement of the microbumps. Step S322: Evenly introduce PCM into the screen template and coat the PCM on the bottom surface of the first chip through a squeegee. Step S323: After the coating is completed, place the first chip in a thermal curing machine for curing to stabilize the coated PCM. In step S3, the first chip and the substrate are pressed together according to the following steps: Step S331: Align the microdents with the microbumps through the optical alignment system of the mounter to align the substrate with the first chip. Step S332: Press the substrate and the first chip together through the mounter and position the substrate and the first chip after pressing.

[0039] It should be noted that coating PCM on the top surface of the substrate and the bottom surface of the first chip can absorb excess heat during the welding process, reduce the temperature gradient in the welding area, relieve thermal stress, and achieve precise alignment of the microdents and microbumps through the optical alignment system of the mounter to ensure mechanical contact between the microbumps and microdents.

[0040] Furthermore, in step S312, the spraying pressure of the precision spraying machine is 0.05 - 0.2 MPa, the rate is 10 - 20 mm / s, the distance is 5 - 20 mm, and the coating thickness of the precision spraying machine is 5 - 15 μm. Preferably, the spraying pressure of the precision spraying machine is 0.1 MPa, the rate is 15 mm / s, the distance is 10 mm, and the coating thickness of the precision spraying machine is 10 μm.

[0041] In step S313, the curing temperature of the thermal curing machine is 50 - 100 °C and the time is 1 - 3 minutes. Preferably, the curing temperature of the thermal curing device is 75 °C and the time is 2 minutes.

[0042] In step S322, the scraping pressure of the squeegee is 10 - 30 N, the scraping angle of the squeegee is 45° - 60°, the scraping speed of the squeegee is 10 - 50 mm / s, and the coating thickness is 5 - 15 μm. Preferably, the scraping pressure of the squeegee is 20 N, the scraping angle of the squeegee is 50°, the scraping speed of the squeegee is 30 mm / s, and the coating thickness is 10 μm.

[0043] In step S332, the pressing force of the mounter is 50 - 200 gf and the pressing time is 2 - 5 s.

[0044] Preferably, the pressing force of the mounter is 100 gf and the pressing time is 3 s.

[0045] It should be noted that the coated PCM absorbs the excess heat during the welding process, alleviates the thermal stress caused by the inconsistent thermal expansion between the first chip and the substrate, reduces the risk of solder joint cracks and substrate warping. The PCM also provides a thermal buffering function, making the temperature gradient in the cooling stage after welding smoother, further improving the packaging reliability.

[0046] It should also be noted that after the mounter accurately aligns the microbumps and microdents through the optical alignment system, it then uses a pressing force of 100 gf and a pressing time of 3 s to ensure sufficient and stable mechanical contact between the first chip and the substrate, solving the problem of uneven solder joints caused by alignment errors between the microbumps and microdents. The precise pressing provides a reliable physical basis for subsequent microwave welding.

[0047] In addition, during subsequent heat absorption, the coated PCM can absorb and store the heat around the solder when the solder melts, alleviating the heat concentration situation. And during the operation process after packaging, the PCM can also help the packaging structure achieve better heat dissipation performance, extend the service life of the solder joint structure and the first chip. At the same time, through the thermal buffering and heat dissipation effects of the PCM, the thermal management ability of the solder joints is greatly improved, and the packaging structure remains stable under high heat loads. The combination of microwave welding and PCM heat dissipation can delay the aging problem of solder joints caused by high-temperature environments, improving long-term reliability and performance.

[0048] Step S4: Weld the connection between the pressed microbumps and microdents by microwave, and control the temperature and cure the solder and solder enhancer. Furthermore, step S4 specifically includes: Step S411: Perform a preheating treatment on the connection between the microbumps and microdents by forming a preheating microwave in a nitrogen environment. Step S412: Increase the microwave power to form melting microwaves, causing the solder and the solder enhancer at the connection between the microbumps and the microdents to melt; Step S413: Decrease the microwave power to form temperature-controlled microwaves, controlling the temperature of the melted solder and the solder enhancer for curing, so that a solder joint structure prototype is formed at the connection between the microbumps and the microdents.

[0049] It should be noted that by heating the solder and the solder enhancer with the preheating microwaves, the solder enhancer can be promoted to remove oxides and improve wettability. Subsequently, under the action of the melting microwaves, the solder is heated to the melting temperature, enabling the solder to flow and fill the microdents, forming a reliable bond with the surface of the microdents. Finally, under the action of the temperature-controlled curing microwaves, the melted solder forms a welding structure prototype and firmly connects the microbumps and the microdents.

[0050] It can be understood that by welding the connection between the microbumps and the microdents with microwaves, a reliable connection between the microbumps and the microdents is achieved, avoiding the difference in the coefficient of thermal expansion between the first chip and the substrate that is easily caused by high temperatures during the welding process, resulting in warping of the package structure or solder joint cracks.

[0051] Furthermore, in step S411, the flow rate of nitrogen is 1 - 2 L / min, the vacuum degree is 10 - 50 Pa, the preheating microwave power is 10 - 30 W, the welding temperature is 150 - 180 °C, and the preheating time is 5 - 10 seconds; Preferably, the flow rate of nitrogen is 1.5 L / min, the vacuum degree is 30 Pa, the preheating microwave power is 20 W, the welding temperature is 160 °C, and the preheating time is 8 seconds.

[0052] In step S412, the melting microwave power is 30 - 60 W, the welding temperature is 220 - 260 °C, and the melting time is 8 - 15 s; Preferably, the melting microwave power is 45 W, the welding temperature is 240 °C, and the melting time is 10 s; In step S413, the temperature-controlled curing microwave power is 10 - 20 W, the curing time is 10 - 20 s, and the corresponding cooling rate is ≤2 °C / s.

[0053] Preferably, the temperature-controlled curing microwave power is 15 W, and the curing time is 15 s.

[0054] It should also be noted that microwave soldering precisely controls the temperature of the solder by locally heating the connection between the microbumps and microdents, avoiding damage to irrelevant areas due to high temperatures. While using microwaves to melt the solder, it ensures that other sensitive areas of the substrate and the first chip are at a lower temperature during the soldering process, reducing the concentrated effect of thermal stress. During the soldering process, the PCM alleviates the problem of a sharp temperature rise by absorbing heat. When the PCM undergoes a phase change, it can smoothly regulate the temperature change, gradually releasing the thermal stress in the soldering area and avoiding thermal expansion mismatch and the fracture of the solder joint structure prototype caused by sudden temperature changes. Therefore, the combination of microwave selective heating and the thermal buffering effect of the PCM significantly reduces the thermal stress caused by changes in the soldering temperature, avoids the fracture and delamination failure of the microbumps, and alleviates the thermal expansion difference during the soldering process due to heating limitations and the adjustment effect of the PCM, ensuring more uniform thermal expansion of the solder joint structure prototype and the substrate.

[0055] Specifically, after obtaining the predicted heat absorption of the PCM through a finite element model, the actual heat absorption of the PCM is measured using a test tool, and then the actual mass of the PCM is obtained based on the predicted heat absorption and the actual heat absorption of the PCM to provide feedback for adjusting subsequent PCM coating. The predicted heat absorption of the PCM is obtained according to the following calculation formula: ; In the formula: is the predicted mass of the PCM; is the latent heat of phase change of the PCM; The actual mass of the PCM is obtained according to the following calculation formula: ; In the formula: is the actual mass of the PCM; is the actual heat absorption of the PCM; It can be understood that the predicted heat absorption of the PCM can be obtained from a finite element model, and the actual heat absorption of the PCM can be obtained from a test tool. The specific finite element model and test tool are well-known to those skilled in the art. The ratio calculation formulas in which the latent heat of phase change, the heat absorption, and the mass of the PCM are proportional are also well-known to those skilled in the art, and will not be described in this embodiment. In addition, after obtaining the actual heat absorption of the PCM, compare the actual heat absorption of the PCM with the predicted heat absorption of the PCM. When the actual heat absorption of the PCM is greater than the predicted heat absorption of the PCM, the actual mass of the PCM can be accurately reduced according to the ratio of the heat absorption of the PCM to the mass of the PCM, and the coating thickness of the PCM can be accurately reduced to reduce the production cost. On the contrary, when the actual heat absorption of the PCM is less than the predicted heat absorption of the PCM, the actual mass of the PCM can be increased according to the ratio of the heat absorption of the PCM to the mass of the PCM, and the coating thickness of the PCM can be increased to ensure that the coated PCM can achieve the predicted heat absorption effect, improve the welding quality and the service life of the packaging structure.

[0056] It should be noted that when the PCM absorbs the excess heat during the welding process, the temperature change range in the area near the solder joint can be reduced, thereby reducing the initial temperature difference in the welding area. When the initial temperature difference in the welding area decreases, the thermal stress between the substrate and the first chip during welding will also decrease. Specifically, the calculation formula for thermal stress is: ; In the formula: is the thermal stress; is the Young's modulus of the material; is the coefficient of thermal expansion of the material; is the initial temperature difference in the welding area; It can be understood that the Young's modulus of the material and the coefficient of thermal expansion of the material are well-known to those skilled in the art. The initial temperature difference in the welding area can be obtained from the heat absorption of the PCMD, which will not be described in this embodiment.

[0057] Step S5: After the solder to be cured and the solder enhancer are cooled to form a solder joint structure, a packaging structure is obtained; The cooling time of the cured solder and the solder enhancer is 40 s, and the corresponding cooling rate is 1.5 °C / s.

[0058] It should be noted that by controlling the temperature during cooling to form the solder joint structure from the solder joint structure prototype, it is ensured that no cracks or internal defects are generated in the solder joint structure prototype during the cooling process, and the thermal stress concentration caused by rapid cooling is avoided.

[0059] It should also be noted that during the welding process, the microwave power gradually increases, causing the solder to gradually heat up, reducing the thermal shock to the substrate and the first chip. Moreover, during welding, the PCM adjusts the temperature by absorbing heat, which can alleviate the difference in the coefficients of thermal expansion between the substrate and the first chip. During this process, the PCM releases heat again to ensure that the temperature in the welding area gradually decreases, further avoiding the problem of substrate warping caused by rapid cooling.

[0060] In addition, by reducing the substrate warping, the accuracy of the microbumps and microdents during welding is effectively improved, enhancing the reliability.

[0061] Step S6: Stack the second chip on the encapsulation structure and weld the encapsulation structure stack and the chip by microwave soldering. After stacking and welding the expected number of second chips, a multi-chip stacked encapsulation structure is obtained; the steps of the microwave soldering method are the same as those of steps S1 to S5.

[0062] Specifically, the microwave soldering method includes the following steps: Step 1: Provide a second chip with micro-bumps at the bottom and an encapsulation structure with corresponding micro-dents at the top. Step 2: Uniformly coat solder and solder enhancer on the surface of the micro-bumps and inside the micro-dents respectively. Step 3: Coat PCM on the top surface of the encapsulation structure and the bottom surface of the second chip respectively. After aligning the micro-bumps and micro-dents through a mounter, press the second chip and the encapsulation structure together. Step 4: Weld the connection between the pressed micro-bumps and micro-dents by microwave, and control the temperature of the solder and solder enhancer to cure. Step 5: After the cured solder and solder enhancer cool to form a solder joint structure, a multi-chip stacked encapsulation structure is obtained. It should be noted that in multi-chip stacked encapsulation, multiple chips are stacked and placed on a substrate for welding. Therefore, since the second chip is provided with micro-bumps and micro-dents at corresponding positions similar to the first chip, and when the second chip is welded to the encapsulation structure in the above manner, the heat generated during selective heating by microwave will not affect the encapsulation structure. At the same time, the encapsulation structure also undergoes the pressure of pressing by the mounter during welding, and the pressure generated by the mounter during pressing of the encapsulation structure and the second chip will not affect the encapsulation structure.

[0063] Example 2: A multi-chip stacked encapsulation structure in this example is obtained by using the multi-chip stacked encapsulation method as in Example 1.

[0064] The above, the above embodiments are only used to illustrate the technical solutions of the present invention and are not intended to limit them; although the present invention has been described in detail with reference to the foregoing embodiments, those of ordinary skill in the art should understand that they can still modify the technical solutions described in the foregoing embodiments, or perform equivalent replacements for some of the technical features; and these modifications or replacements do not make the essence of the corresponding technical solutions deviate from the spirit and scope of the technical solutions of the embodiments of the present invention.

Claims

1. A multi-chip stacking packaging method, characterized in that: The multi-chip stacking packaging method comprises the following steps: Step S1, providing a first chip with micro-bumps on the bottom and a substrate with micro-dimples on the top corresponding to the micro-bumps; Step S2, uniformly coating solder and solder enhancer on the surface of micro-convex points and inside micro-concave points respectively; Step S3, coating PCM on the top surface of the substrate and the bottom surface of the first chip respectively, and after aligning the micro-convex points with the micro-concave points by a chip mounter, pressing the first chip and the substrate together; Step S4, welding the connection between the pressed micro-convex points and the micro-concave points by microwave, and curing the solder and the solder reinforcing agent by temperature control; Step S5, after the solder to be solidified and the solder reinforcing agent are cooled to form a solder joint structure, a packaging structure is obtained; Step S6, stacking the second chip on the packaging structure, and welding the packaging structure and the chip by microwave welding, and obtaining a multi-chip stacked packaging structure after stacking and welding the expected number of second chips; the steps of the microwave welding method are the same as steps S1 to step S5.

2. The multi-chip stacking packaging method according to claim 1, characterized in that: In the step S1, the diameter of the micro-concave points is 10-20 μm, the depth is 5-15 μm, and the spacing is 15-30 μm; the diameter of the micro-convex points is 12-30 μm, the height is 6-20 μm, and the spacing is 15-30 μm.

3. The multi-chip stacking packaging method according to claim 1, characterized in that: In step S2, the solder on the surface of the micro-bump is coated according to the following steps: Step S211, fixing the first chip on the working table of the screen printer and keeping it stable by vacuum adsorption; Step S212, selecting a template whose opening diameter is larger than the diameter of the micro-bumps according to the size of the micro-bumps arrangement; Step S213, uniformly coating the solder on the coating surface of the template, and coating the solder on the surface of the micro-bumps by a scraper; In step S2, the solder enhancer on the surface of the micro-pits is applied according to the following steps: Step S221, using a sprayer to spray the solder enhancer at low pressure; Step S222: placing the coated substrate in a thermal curing device to cure the solder reinforcing agent in the micro-pits.

4. The multi-chip stacking packaging method according to claim 3, characterized in that: In the step S213, the solder is Sn-Ag-Cu solder, the thickness of which is 5-15 μm, and the scraping pressure of the scraper is 10-30 N; In the step S221, the solder reinforcing agent is a rosin-based flux with a thickness of 1-3 μm, the spraying pressure of the sprayer is 0.05-0.2 MPa, and the spraying time is 1-2 seconds; In the step S222, the curing temperature of the thermal curing device is 100-150°C, and the curing time of the thermal curing device is 1-3 minutes.

5. The multi-chip stacking packaging method according to claim 1, characterized in that: In step S3, the PCM on the top surface of the substrate is coated according to the following steps: Step S311, fix the substrate on the workbench of the precision spraying machine to ensure that the substrate is level and stably fixed; Step S312, the precision spraying machine avoids the micro-pits and evenly covers the PCM on the top surface of the substrate; Step S313: After coating, the substrate is placed in a heat curing device for curing to stabilize the coated PCM; In step S3, the PCM on the bottom surface of the first chip is coated according to the following steps: Step S321, fixing the first chip on the working platform of the screen printing equipment, and making the openings of the screen template completely correspond to the arrangement of the micro bumps; Step S322, PCM is uniformly introduced into the screen template, and the PCM is coated on the bottom surface of the first chip by a scraper; Step S323: After coating, the first chip is placed in a thermal curing device for curing to stabilize the coated PCM; In step S3, the first chip and the substrate are pressed together according to the following steps: Step S331, aligning the micro-concave points with the micro-convex points through the optical alignment system of the chip mounter, so as to align the substrate with the first chip; Step S332: Press the substrate and the first chip together using a chip mounter, and position the substrate and the first chip after pressing together.

6. The multi-chip stacking packaging method according to claim 5, characterized in that: In the step S312, the spraying pressure of the precision spraying machine is 0.05-0.2MPa, the rate is 10-20mm / s, the distance is 5-20mm, and the coating thickness of the precision spraying machine is 5-15μm; In step S313, the curing temperature of the thermal curing device is 50-100° C. and the curing time is 1-3 minutes; In step S322, the scraping pressure of the scraper is 10-30N, the scraping angle of the scraper is 45°-60°, the scraping speed of the scraper is 10-50mm / s, and the coating thickness is 5-15μm; In the step S332, the pressing force of the placement machine is 50-200 gf, and the pressing time is 2-5 s.

7. The multi-chip stacking packaging method according to claim 1, characterized in that: The step S4 specifically includes: Step S411, preheating the connection between the micro-convex points and the micro-concave points by forming preheating microwaves in a nitrogen environment; Step S412, increasing the microwave power to form a melting microwave to melt the solder and solder reinforcing agent at the connection between the micro-convex points and the micro-concave points; Step S413, reducing the microwave power to form a temperature-controlled microwave, and temperature-controlling and solidifying the melted solder and solder enhancer to form a solder joint structure prototype at the connection between the micro-bumps and micro-pits.

8. The multi-chip stacking packaging method according to claim 7, characterized in that: In the step S411, the flow rate of the nitrogen is 1-2 L / min, the vacuum degree is 10-50 Pa, the preheating microwave power is 10-30 W, the welding temperature is 150-180° C., and the preheating time is 5-10 seconds; In the step S412, the melting microwave power is 30-60W, the welding temperature is 220-260°C, and the melting time is 8-15s; In the step S413, the temperature-controlled microwave power is 10-20 W, the temperature-controlled curing time is 10-20 s, and the corresponding cooling rate is ≤ 2°C / s.

9. The multi-chip stacking packaging method according to claim 1, characterized in that: The solidified solder and solder enhancer are cooled for 20-60 seconds, and the corresponding cooling rate is 0.5-2°C / s.

10. A multi-chip stacked packaging structure, characterized in that: The multi-chip stacking packaging method is used as described in any one of claims 1 to 9.

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