Bga ball grid array heat conduction test sample, preparation method and testing device thereof

Abstract

The application discloses a BGA ball grid array heat conduction test sample and a preparation method and a test device thereof. The method comprises the following steps: the prepared BGA ball grid array heat conduction test sample is a sandwich structure, which is composed of two high-thermal-conductivity copper round substrates and a BGA ball grid array layer; BGA pads are formed on the surfaces of the copper round substrates through an ultraviolet exposure and development process; and the upper and lower substrates are aligned and attached after being subjected to a reflow soldering process through a handle and a ball mounting tool assembly. The test sample test device is composed of a heating source, a high-thermal-conductivity pure metal column, a test sample, a liquid cooling source and an outer heat preservation material. After a stable heat flow is applied to the test sample, the accurate thermal resistance parameters are calculated by a temperature measuring point in the metal column. The principle of the test device is based on a steady heat flow method, and the influences of the contact thermal resistance and the heat-conducting silicone grease on the calculation of the thermal characteristics of the test sample are eliminated through theoretical calculation. The application provides key measured data support for the heat transfer characteristics of the BGA ball grid array, and can effectively calibrate and verify a thermal simulation model constructed for the BGA ball grid array.

Description

Technical Field

[0001] This invention relates to the field of electronic component packaging and thermal control technology, and in particular to a BGA ball grid array thermal conductivity test sample, its preparation method, and testing device. Background Technology

[0002] As electronic components (mainly chips) within electronic device packages increasingly move towards higher power density and three-dimensional stacking, the heat generated during operation is increasing. If this heat cannot be dissipated through effective heat transfer paths, it will accumulate, potentially causing the components to malfunction or even fail. Therefore, thermal management and analysis of the BGA ball grid array micro-solder joint layer within electronic device packages are particularly important.

[0003] In engineering applications for heat dissipation design of micro solder joint layers in BGA ball grid arrays, the current method is the homogenized equivalent layer approximation analysis method. The calculation of the equivalent thermal conductivity parameters of the micro solder joint layer is only at the simulation level. There is an urgent need to propose a BGA ball grid array thermal conductivity test sample preparation process and corresponding testing device that can be used for thermal conductivity testing, so as to realize the actual testing of the equivalent thermal conductivity parameters and thermal resistance per unit area of ​​the micro solder joint layer.

[0004] Therefore, proposing a thermal conductivity test specimen for BGA ball grid arrays, its preparation method, and testing device to address the difficulties in the existing technology is a problem that urgently needs to be solved by those skilled in the art. Summary of the Invention

[0005] The purpose of this invention is to provide a BGA ball grid array thermal conductivity test sample, its preparation method and testing device, which provides key measured data support for the heat dissipation design of electronic packaging and can effectively calibrate and verify simulation models.

[0006] To achieve the above objectives, the present invention provides the following solution: A method for preparing a thermal conductivity test sample for a BGA ball grid array includes the following steps: S1. Coat the surface of the large copper substrate with solder resist ink, and heat the large copper substrate coated with solder resist ink to make it enter a semi-cured state; the large copper substrate includes multiple circular copper substrates; a film with multiple circular copper substrate solder resist window patterns is attached to the large copper substrate coated with solder resist ink, and multiple BGA pads with solder resist layer are formed by exposure and development ink process. S2. After printing the corresponding number on the surface of the circular copper substrate, cut out multiple circular copper substrates and perform chemical nickel-gold surface treatment on the BGA pads to form an anti-oxidation layer. S3. Fix a single copper substrate in a ball-planting fixture. First, print solder paste on the BGA pads using a solder paste stencil. Then, replace it with a ball-planting stencil and place the solder balls on the corresponding BGA pads through the openings in the ball-planting stencil to form a single-sided ball-planting substrate. S4. Reflow soldering is performed on the single-sided ball-mounted substrate to melt the solder paste and initially fix the solder balls. S5. Print solder paste on the BGA pads of another copper substrate, and align and mount the other copper substrate with the copper substrate that has completed the first reflow soldering through the positioning structure, so that the solder balls are located between the two substrates. S6. Reflow solder the mounted components again to melt all the solder and complete the final alignment and connection of the upper and lower pads under the action of surface tension. Repeat the above steps S1-S5 to form multiple test samples with symmetrical upper and lower copper substrates and a BGA ball grid array micro solder joint layer in the middle.

[0007] Preferably, in S1, multiple BGA pads with a solder resist layer are formed through an exposure and development ink process, specifically including: The solder resist ink under the transparent area of ​​the film is exposed to ultraviolet light and undergoes a photochemical reaction to cure. The solder resist ink under the black area of ​​the film, corresponding to the BGA pad pattern, is not exposed due to the film's obstruction and does not undergo a curing reaction. The uncured solder resist ink on the large copper substrate is cleaned with a weak alkaline solution. The unexposed and uncured solder resist ink is dissolved by the weak alkaline solution, while the exposed and cured solder resist ink is not dissolved and remains on the copper plate to form a surface solder resist layer. The large copper substrate is then placed in a high-temperature oven to complete the final curing of the solder resist ink and the opening of solder resist windows for all BGA pads.

[0008] Preferably, in S1, a square handle with a positioning hole is provided on the outer edge of the circular copper substrate. The electroless nickel-gold surface treatment is carried out by immersing the circular copper substrate in the molten metal through a fixing wire passing through the positioning hole.

[0009] Preferably, in S3, the ball-planting fixture is a square aluminum alloy fitting with a circular groove for embedding a circular copper substrate. The groove depth is less than the thickness of the circular copper substrate, so that after the circular copper substrate is embedded and fixed, its surface is higher than the surface of the ball-planting fixture, which facilitates the operation of the stencil.

[0010] A BGA ball grid array thermal conductivity test specimen is prepared based on any of the above-mentioned methods, comprising an upper circular copper substrate, an intermediate BGA ball grid array micro solder joint layer, and a lower circular copper substrate.

[0011] The present invention also provides a testing apparatus for a BGA ball grid array thermal conductivity test sample, used to test the BGA ball grid array thermal conductivity test sample prepared according to any of the above-described methods, comprising, from bottom to top: Liquid cooling source, used to provide a constant temperature; The cold-end metal column has its lower surface in thermal contact with the liquid cooling source for heat removal and temperature gradient measurement. The thermal conductivity test sample of the BGA ball grid array to be tested is placed on the upper surface of the cold end metal pillar; A hot-end metal pillar, the lower surface of which contacts the upper surface of the BGA ball grid array thermal conductivity test sample to be tested, is used for heat conduction, temperature gradient measurement and heat flow shaping. The heating source is in thermal contact with the upper surface of the hot-end metal pillar to simulate the heating of electronic components. The pressure application mechanism is used to apply axial preload to the stack consisting of a heating source, a hot-end metal column, a sample, a cold-end metal column, and a liquid cooling source.

[0012] Preferably, the sides of the hot-end metal column and the cold-end metal column are covered with thermal insulation material, and multiple temperature measuring holes are uniformly arranged axially inside the hot-end metal column and the cold-end metal column to arrange temperature sensors to measure the axial temperature gradient inside the column.

[0013] Preferably, the heating source is a circular polyimide (PI) heating film powered by a DC power supply, and its area is equal to the area of ​​the end face of the hot-end metal pillar and the thermal conductivity test sample of the BGA ball grid array.

[0014] Preferably, the hot-end metal column and the cold-end metal column are copper columns or aluminum columns; the liquid cooling source is a copper water cooling head, which is connected to a constant temperature chiller.

[0015] According to specific embodiments provided by the present invention, the present invention discloses the following technical effects: (1) This invention creatively provides a complete solution from sample preparation to parameter measurement, achieving for the first time a leap from "pure simulation estimation" to "precise physical measurement" of the equivalent thermal conductivity of BGA micro solder joint layers. By designing a dedicated circular substrate, adapting tooling, and a two-step reflow soldering process, sandwich samples with consistent structures that can be used for testing are prepared, solving the problem of high-precision manufacturing of non-standard shaped test pieces; at the same time, the matching steady-state heat flow method test device, through copper pillar temperature measurement, axial pressure, and lateral heat preservation, constructs ideal one-dimensional heat flow conditions in the experiment, which can accurately separate and measure the thermal resistance and thermal conductivity of the micro solder joint layer itself. Ultimately, this invention provides key measured data support for the heat dissipation design of electronic packaging, effectively calibrating and verifying simulation models, thereby improving the reliability and design accuracy of thermal management of high power density chips.

[0016] (2) The sample preparation method and corresponding testing device of the present invention can realize the thermal conductivity test of the micro solder joint layer of BGA ball grid array, accurately calculate the actual thermal conductivity parameters and thermal resistance per unit area of ​​the micro solder joint layer, and provide physical test reference verification for the finite element simulation method of heat dissipation analysis. Attached Figure Description

[0017] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the embodiments will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0018] Figure 1 This is an external view of the tooling assembly for embedding a copper circular substrate provided by the present invention. Figure 2 This is a surface morphology diagram of the BGA thermal conductivity test sample before mounting after ball implantation according to the present invention. Figure 3 This is a schematic diagram of the sandwich sample mounting and reflow soldering process of the present invention; Figure 4 This is a schematic diagram of the testing device used in this invention; Figure 5 The image shows the test results of the thermal conductivity test sample of the BGA ball grid array of the present invention. Figure 6 This is a schematic diagram of the window and handle of the solder resist layer of the circular copper substrate pad in this invention. Figure 7 This is a schematic diagram of the upper and lower substrates of the BGA ball grid array thermal conductivity test sample after mounting. Detailed Implementation

[0019] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0020] To make the above-mentioned objects, features and advantages of the present invention more apparent and understandable, the present invention will be further described in detail below with reference to the accompanying drawings and specific embodiments.

[0021] The present invention provides a method for preparing a thermal conductivity test sample for a BGA ball grid array, comprising the following steps: S1. Coat the surface of the large copper substrate with solder resist ink, and heat the large copper substrate coated with solder resist ink to make it enter a semi-cured state; the large copper substrate includes multiple circular copper substrates; a film with multiple circular copper substrate solder resist window patterns is attached to the large copper substrate coated with solder resist ink, and multiple BGA pads with solder resist layer are formed by exposure and development ink process. S2. After printing the corresponding number on the surface of the circular copper substrate, cut out multiple circular copper substrates and perform chemical nickel-gold surface treatment on the BGA pads to form an anti-oxidation layer. S3. Fix a single circular copper substrate in the ball-mounting fixture. First, print solder paste on the BGA pads using a solder paste stencil. Then, replace the stencil with a ball-mounting stencil and place the solder balls onto the corresponding BGA pads through the openings in the stencil, forming a single-sided ball-mounting substrate. Figures 1-2 As shown; S4. Reflow soldering is performed on the single-sided ball-mounted substrate to melt the solder paste and initially fix the solder balls. S5. Print solder paste on the BGA pads of another circular copper substrate, and align and mount the other circular copper substrate with the circular copper substrate that has completed the first reflow soldering through the positioning structure, so that the solder balls are located between the two substrates. S6. Reflow soldering is performed on the mounted components again to melt all the solder. Under the action of surface tension, the final alignment and connection of the upper and lower pads are completed. Steps S1-S5 are repeated to form multiple test samples with symmetrical upper and lower copper substrates and a BGA ball grid array micro-solder joint layer in the middle, such as... Figure 3 or Figure 7 As shown, the dashed line indicates the location of the handle opening.

[0022] 1. Specifically, the above preparation method includes: (1) coating a 2mm thick copper substrate with solder resist ink (liquid photosensitive solder resist ink), and heating it moderately in an oven at a temperature of about 70-90℃ to evaporate the solvent in the solder resist ink, so that the whole substrate enters a semi-cured state. A film with 24 copper wafer substrate solder resist window patterns is attached to the copper substrate coated with solder resist ink, and exposed to ultraviolet light (UV) on an exposure machine. The solder resist ink under the transparent area of ​​the film is exposed to ultraviolet light and undergoes a photochemical reaction to cure, while the solder resist ink under the black area of ​​the film, which corresponds to the BGA pad pattern, is not exposed due to the film's shielding and does not undergo a curing reaction.

[0023] (2) Clean the uncured solder resist ink on the copper substrate with a weak alkaline solution. The uncured solder resist ink is dissolved by the weak alkaline solution, while the cured solder resist ink is not dissolved and remains on the copper plate to form a surface solder resist layer. Then, place the large substrate in a high-temperature oven at 140-150℃ to complete the final curing of the solder resist ink and the solder resist opening of all BGA pads. By panelizing 24 copper wafer substrates on the large substrate, one-time curing and development of the solder resist ink in the solder resist area can be achieved, effectively saving processing steps and costs. After printing the corresponding number on the surface of the copper wafer substrate, cut out 24 copper wafer substrates and drill a 3mm round hole in the handle, such as Figure 6 As shown, a fixing wire is passed through the positioning hole, and the substrate is immersed in the molten metal for chemical nickel-gold plating. The resulting surface nickel-gold layer mainly prevents the pads from being oxidized during subsequent reflow soldering. After chemical nickel-gold plating, the thickness of the nickel plating layer on the pad surface is about 2-5 μm, and the gold plating layer is about 0.05 μm.

[0024] (3) The copper wafer substrate is embedded with the designed tooling fittings, and the BGA solder ball placement stage is rotated counterclockwise (e.g., Figure 6 (As shown) The lower turntable clamps the tooling components. Lock the turntable, then cover the ball-mounting platform with the magnetic stencil lower cover frame. The SMT solder paste stencil is magnetically attached to the stencil frame of the ball-mounting platform. Adjust the stencil height by rotating the copper pillar screws at the four corners of the ball-mounting platform, ensuring the solder paste stencil is flush against the substrate. Align each pad of the copper substrate with the corresponding laser-cut opening on the SMT solder paste stencil. Then, cover the upper cover frame of the stencil to prevent movement during the ball-mounting process. Spray solder paste onto the openings to cover the solder mask pads. At the window opening, remove the solder paste stencil from the mold and replace it with a ball-planting stencil. Rotate the copper pillar screws to make the ball-planting stencil 0.1-0.15mm above the substrate to prevent solder paste from sticking to the stencil and ensure the stencil surface is dry and free of foreign objects. This will ensure that the solder balls can pass smoothly through the stencil holes. Then, pour out the solder ball bottle to allow the solder balls to fall through the opening of the ball-planting stencil onto the corresponding pads. After each solder ball has completely filled and fallen into the corresponding stencil hole, remove the ball-planting stencil. The solder balls will remain in the corresponding pad position due to the adhesion effect of the previously printed solder paste.

[0025] (4) The copper substrate with the solder ball clamped and fixed is placed into the reflow soldering equipment for high-temperature reflow soldering, and the solder ball melts on one side of the copper substrate.

[0026] (5) The other copper substrate in the same group is printed with solder paste using a ball-mounting platform and solder paste stencil. Then, with the help of the handle, the copper substrate is aligned and mounted onto the single copper substrate that has been ball-mounted. After high-temperature reflow, the BGA sample forms a symmetrical sandwich structure. During the reflow soldering process, even if there is a slight deviation in the alignment of the upper and lower pads after the solder balls melt, the upper and lower pads can still be automatically aligned under the pull of the surface tension and wetting force of the solder before it is balanced. The overall force of the BGA ball grid array causes the upper and lower copper substrates to align naturally. The consistent solder ball height ensures the coplanarity of the upper and lower copper substrates, ensuring that all solder joints melt simultaneously and form a good connection during reflow soldering, avoiding internal stress caused by uneven force. After the reflow soldering temperature cools down, the sandwich structure sample is taken out, and the preparation of the BGA thermal conductivity test sample is completed.

[0027] 2. In the embodiments, the sample preparation method includes: (1) Each test sample is a circle with a diameter of 30 mm.

[0028] (2) A square handle is designed on the outer edge of the round copper substrate of each BGA ball grid array thermal conductivity test sample. A 3mm round hole is drilled on the handle for the subsequent electroless nickel-gold process for wire threading and fixing. Since the commonly used BGA solder ball placement stage can only clamp square substrates, an additional tooling fitting part that can be embedded in round substrates is designed to assist in completing the ball placement operation.

[0029] (3) The tooling assembly is made of aluminum alloy and is machined by a machine tool. The designed external dimensions are 45×45mm, which is slightly smaller than the maximum square substrate that the BGA solder ball placement station can hold. The groove thickness for embedding the circular substrate is 1.5mm, which is less than the 2mm thickness of the circular copper substrate. When clamped, the circular copper substrate used for ball placement is higher than the tooling assembly, which facilitates the installation and removal of multiple sets of copper substrates. The circular substrate and the groove adopt an interference fit tolerance. The outer handle of the copper substrate is tightly embedded with the square part of the groove to ensure that the circular copper substrate remains fixed during the ball placement process. This ensures that the openings of the solder paste stencil and the ball placement stencil strictly correspond to the positions of the solder pads for ball placement, and that the solder paste spraying and the placement of each solder ball are accurate.

[0030] (4) Solder paste stencils and ball-planting stencils were designed to meet the required experimental parameters. The openings in the stencils were produced using a high-precision laser cutting process on the surface of 304 stainless steel. All openings corresponded to the designed BGA pad positions, quantities, and diameters. During the ball-planting process, the stencil holes guided the solder balls to achieve ultra-high positioning accuracy, avoiding soldering defects such as misalignment and solder bridging, and improving BGA operation efficiency. The solder balls used for ball-planting had a nominal diameter tolerance of approximately 8μm and were uniform in size, ensuring that the final drum-shaped solder joints formed on each pad had a nearly uniform volume shape.

[0031] (5) The reflow soldering process typically uses a typical reflow oven to achieve solder joints. This equipment heats the mounted substrate using forced convection of hot air. The high-temperature hot air blown out gradually heats up the solder paste and solder balls on the substrate surface and melts them in the peak area. Subsequently, they solidify during the cooling process to form solder joints, thereby achieving a reliable connection between the upper and lower substrates. The reflow temperature profile consists of four continuous zones: heating, holding, reflow, and cooling. The residence time of the sample in each temperature zone of the reflow oven is controlled by adjusting the conveyor chain speed in the reflow oven.

[0032] (6) When conducting thermal conductivity tests, the high thermal conductivity of the copper material in the copper substrate enables the heat to spread rapidly in the lateral direction, reducing the bending of the heat flow in the heat transfer process of the substrate. At the same time, it ensures that the temperature difference between the two ends of the BGA solder ball with low thermal conductivity under the same heat flux density is greater than the temperature difference inside the copper substrate, reducing the influence of the relative error of temperature measurement on theoretical calculation.

[0033] (7) The process variables that can be controlled during the design are solder ball diameter, pad diameter, solder joint X-axis spacing, solder joint Y-axis spacing, and stencil window diameter. The process parameter settings for the BGA ball grid array thermal conductivity test sample are shown in Table 1.

[0034] Table 1

[0035] A BGA ball grid array thermal conductivity test specimen is prepared based on any of the above-mentioned methods. The thermal conductivity test specimen has a sandwich structure, including an upper circular copper substrate, a middle BGA ball grid array micro-solder joint layer, and a lower circular copper substrate. The middle BGA ball grid array micro-solder joint layer with low thermal conductivity is located between the two layers of high thermal conductivity copper material, so that the temperature difference between the two ends of the high thermal conductivity substrate is much smaller than the temperature difference between the two ends of the BGA ball grid array micro-solder joint layer during the thermal conductivity test, thereby improving the accuracy of the heat transfer characteristic calculation. Figure 4 As shown, the present invention also provides a testing device for a BGA ball grid array thermal conductivity test sample, used to test the BGA ball grid array thermal conductivity test sample prepared according to any of the above-described methods, comprising, from bottom to top: Liquid cooling source, used as a constant temperature boundary for the device; The cold-end metal column has its lower surface in thermal contact with the liquid cooling source for heat removal and temperature gradient measurement. The thermal conductivity test sample of the BGA ball grid array to be tested is placed on the upper surface of the cold end metal pillar; A hot-end metal pillar, the lower surface of which contacts the upper surface of the BGA ball grid array thermal conductivity test sample to be tested, is used for heat conduction, temperature gradient measurement and heat flow shaping. The heating source is in thermal contact with the upper surface of the hot-end metal pillar to simulate the heating of electronic components. The pressure application mechanism is used to apply axial preload to the stack consisting of a heating source, a hot-end metal column, a sample, a cold-end metal column, and a liquid cooling source.

[0036] 1. Specifically, the testing apparatus includes: (1) The core test section consists of a heating source, a hot-end copper pillar, a test sample, a cold-end copper pillar and a liquid cooling source (purple copper water cooling head) from top to bottom. The 30mm diameter circular thermal conductivity test sample and the two copper pillars and the circular heat source have the same contact heat dissipation area. The core principle of the experiment is the steady-state heat flow method.

[0037] (2) Use 400 grit, 600 grit, 1000 grit, 1500 grit and 2000 grit sandpaper to polish the upper and lower surfaces of the copper column, remove the copper oxide layer on the surface and ensure that the contact surface is smooth, so as to avoid the surface oxide layer from affecting the transfer of steady-state heat flow. Four holes are evenly arranged inside the copper column along the tooling axis as temperature measuring points. Each hole is 1 mm in diameter and 15 mm deep, and is drilled to the center position. The spacing of the holes on the outer column surface is designed to be 15 mm, which is the arrangement position of the K-type thermocouple.

[0038] (3) The heating source uses a customizable polyimide (PI) heating film. The circular polyimide heating film has the same end face area as the hot-end copper pillar and the cold-end copper pillar. The heating element has a maximum temperature resistance of 180℃ and a thickness of about 0.2mm, which can realize heat input in ultra-thin space. After the heating element is powered on, the pressure plate connected to the digital push-pull force gauge and the pre-tightening force of the hot-end copper pillar can make the heating surface of the heating film and the contact surface of the hot-end copper pillar fit tightly. The pressing action can ensure good heat conduction between the heating film and the hot-end copper pillar, avoiding gaps between the heating surface and the heat-conducting surface of the hot-end copper pillar, which would hinder heat transfer and cause heat loss. In particular, when there are gaps or air bubbles under the heating film, it will cause local heat loss, which will increase the local surface heat of the heating film, and cause the heating wire surface temperature to be too high, resulting in melting and circuit breakage. The end of the heating wire is soldered with solder wire. The connection between the power cord and the DC power supply uses a pure copper gold-plated 4mm lantern-shaped banana plug, and a male to female extension cable is also connected. The circular polyimide heating film contains uniformly arranged heating wires with similar spacing, ensuring that the heat generated by the PI heating film flows downwards in a uniform circular pattern. The high thermal conductivity of the copper material within the copper column allows for rapid lateral heat diffusion, reducing heat flow deflection during heat transfer. This makes the contact surface between the copper column end and the sample closer to a one-dimensional isothermal boundary, minimizing lateral temperature gradients caused by localized temperature measurement points. Simultaneously, it ensures that, under equal heat flux density, the temperature difference between the two ends of the test sample with lower thermal conductivity is greater than the temperature difference between the drilled holes in the copper column, making it easier to determine steady-state conditions and reducing the impact of relative temperature measurement errors on theoretical calculations. The above theoretical calculation method is unaffected by the DC power input, chiller flow rate, and supply liquid temperature.

[0039] 2. In this embodiment, the corresponding testing device includes: (1) The core test section consists of a heating source, a hot-end copper pillar, a test sample, a cold-end copper pillar, and a liquid cooling source (purple copper water cooling head) from top to bottom. The contact heat dissipation area of ​​the 30mm diameter circular thermal conductivity test sample, the two copper pillars, and the circular heat source is the same.

[0040] (2) Remove rust from the heat-conducting surfaces at both ends of the copper column. Use 400 grit, 600 grit, 1000 grit, 1500 grit and 2000 grit sandpaper to polish the upper and lower surfaces of the copper column in sequence. Remove the copper oxide layer and rust on the surface while ensuring the contact surface is smooth. Avoid the surface oxide layer from affecting the transfer of steady-state heat flow. Arrange four drill holes evenly along the tooling axis inside the copper column as temperature measuring points. Each drill hole is 1 mm in diameter and 15 mm deep, just to the center position. The spacing of the drill holes on the outer column surface is designed to be 15 mm, which is the arrangement position of the K-type thermocouple.

[0041] (3) The heating source uses a customizable polyimide (PI) heating film. The 30mm diameter circular polyimide heating film has the same end face area as the hot and cold end copper pillars. The heating element has a maximum temperature resistance of 180℃ and a thickness of about 0.2mm, enabling heat input within an ultra-thin space. After the heating element is powered on, the pressure plate connected to the digital push-pull force gauge and the pre-tightening force of the hot end copper pillar can make the heating surface of the heating film and the contact surface of the hot end copper pillar fit tightly together. The pressing action can ensure good heat conduction between the heating film and the hot end copper pillar, avoiding gaps between the heating surface and the heat-conducting surface of the hot end copper pillar, which would hinder heat transfer and cause heat loss. In particular, when there are gaps or air bubbles under the heating film, local heat loss will occur, causing the local surface heat of the heating film to increase, and the surface temperature of the heating wire will be too high, resulting in melting and circuit breakage. The end of the heating wire is soldered with solder wire. The connection between the power cord and the DC power supply uses a pure copper gold-plated 4mm lantern-shaped banana plug, and a male-to-female extension cable is also connected. The high thermal conductivity of the copper material inside the copper column allows heat to diffuse rapidly in the lateral direction, reducing heat flow bends during heat transfer. This makes the contact surface of the copper column, i.e. the sample, closer to a one-dimensional isothermal boundary, reducing the lateral temperature gradient caused by local temperature measurement points. At the same time, it ensures that the temperature difference between the two ends of the test sample with low thermal conductivity under the same heat flux density is greater than the temperature difference between the holes drilled in the copper column. This makes it easier to determine steady-state conditions and reduces the impact of relative temperature measurement errors on theoretical calculations.

[0042] (4) A 40mm×40mm copper water-cooling head is placed below the cold-end copper pillar. The water-cooling head is an integrated welded heat exchanger with polished surfaces on both sides. A 9.5mm outer diameter pagoda-shaped water nozzle connects to a PVC soft water pipe. The soft water pipe is connected to a chiller with a built-in 13mm outer diameter soft water pipe via a pagoda-shaped adapter. Heat from the circular heat source is conducted downwards through the cold-end copper pillar and the water-cooling head to achieve heat dissipation. A chiller without a compressor is used to avoid fluctuations in the supply temperature of the coolant caused by the compressor's effect during operation, ensuring a constant surface temperature of the copper water-cooling head. The DC power supply is an IT6720 model, which has a maximum output voltage of 60V and a maximum output power of 100W, meeting the requirements of this experiment. The insulation cotton is a porous vacuum silicone ultra-thin aerogel insulation cotton.

[0043] (5) In order to minimize the contact thermal resistance caused by the contact between the hot and cold end copper pillars and the surface of the test sample during heat transfer, thermal grease must be used to fill the gaps between each contact surface during actual testing, including the contact surface between the copper pillar and the sample and the contact surface between the copper pillar and the water cooling head. This enhances the heat conduction effect of the contact part, eliminates the error caused by the contact thermal resistance in the theoretical calculation, and speeds up the time for the system to reach steady-state heat conduction.

[0044] 3. In this embodiment, the operation steps of the testing device include: (1) Place the vertical frame with digital push-pull force gauge on a flat table. Use the side of the scraper to slowly and evenly apply thermal grease to the lower surface of the cold end copper column and the upper surface of the copper water cooling head and press it in the initial stage. Then apply thermal grease to the lower surface of the hot end copper column and the upper surface of the cold end copper column. Place the thermal conductivity test sample between the copper columns, align the upper and lower copper columns and the thermal conductivity test sample so that their circular parts are completely overlapped and press them in the initial stage.

[0045] (2) Attach the PI heating film to the upper surface of the hot end copper column. Place a layer of heat insulation cotton that can completely cover the sample between the pressure plate below the digital push-pull force gauge and the PI heating film. Adjust the screw device of the vertical frame to slowly apply pressure to the sample through the upper pressure plate with a diameter of 60mm. The loading pressure of the sample is 50N. After the pre-tightening force reading of the digital push-pull force gauge tends to stabilize, wipe the thermal conductive silicone grease overflowing from the outer column surface of the pressed copper column with a non-woven cloth soaked in 95% medical alcohol and check again to ensure that the contact surface between the sample and the copper column is completely overlapped.

[0046] (3) At the pre-drilled hole in the copper column, the temperature measuring point of the K-type nickel-chromium-nickel-silicon thermocouple with thermal grease applied is probed to the center position along the radius of the copper column. The temperature compensation wire at the other end of the K-type thermocouple is connected to the channel of the multi-channel temperature acquisition instrument through the Y-type terminal.

[0047] (4) Turn on the chiller and set the cooling water to a constant temperature of 22.5℃. After the water temperature reaches the set temperature, open the valve and let the constant temperature chilled water flow continuously into the copper cold head through the Teflon hose. Cut a piece of insulation cotton slightly larger than the outer surface area of ​​the two copper columns. Use a strap to fully wrap the insulation cotton around the core test part, i.e., the outer surface of the copper column, to prevent heat from dissipating into the surrounding environment along the side of the copper column during the test. Ensure that the heat flow is uniformly transferred downward along the Z-axis. The thermocouple wire is led out from the bottom of the insulation cotton and the contact part with the water cold head. Then wrap another layer of insulation cotton with aluminum foil on the outer surface to prevent the system from exchanging heat with the outside through thermal radiation during the test. When the chiller is working, turn on the maximum water speed. Due to the high-speed flow of the coolant, the temperature of the lower surface of the cold end copper column can be regarded as an isothermal surface equal to the temperature of the inlet liquid when it reaches a steady state. The heat generated by the heat source is quickly carried away by the cooling water when it flows through the contact surface between the cold end copper column and the water cold head.

[0048] (5) Turn on the DC power supply to power the circular heat source. The PI heating film heats up evenly, and the temperatures collected by the four thermocouples rise first and then tend to stabilize. Wait half an hour until the surface temperature of the copper cold head is constant and all thermocouple readings are stable. Ensure that the temperature curve of the temperature acquisition instrument is stable within 5 minutes and the temperature drift of the four thermocouples is less than 0.05℃. Record the values ​​of the four temperature measurement points and the current and voltage input of the DC power supply. Then remove the two layers of insulation cotton, rotate the screw device clockwise to unload the pressure on the hot and cold copper pillars, take out the sample, and wipe the sample and the surface of the copper pillars with a non-woven cloth soaked in alcohol to remove the residual thermal grease. Then repeat the above steps to complete the thermal conductivity test of other samples. The test results are as follows: Figure 5 As shown.

[0049] Theoretical calculation Assuming that during the experiment, the heat flow generated by the heat source, wrapped in insulating cotton, is not dissipated into the surrounding environment through the surfaces of the two copper pillars, and the heat flow is uniformly transferred downwards along the Z-axis of the platform, let the heat flowing through the hot and cold copper pillars be Q. This heat can be calculated from the voltage reading U and current reading I of the DC power supply, i.e. I Before formally calculating the thermal resistance of the BGA ball grid array thermal conductivity test sample, the temperature difference caused by applying thermal grease was measured. Under steady-state heat flux, the temperature gradients inside the hot-end and cold-end copper pillars are assumed to be uniformly distributed. Let the distance between the first and second temperature measuring points on the hot-end copper pillar be... The distance between the second temperature measuring point and the heat-conducting surface of the lower surface of the hot-end copper pillar is... The distance between the heat-conducting surface on the upper surface of the cold-end copper pillar and the third temperature measuring point located on the cold-end copper pillar is... The distance between the third and fourth temperature measurement points is The temperatures at the four measuring points were respectively , , and ,but

[0050] After calculating the temperature difference of the thermal grease in the steady-state thermal flux and thermal resistance measurement platform system, a BGA ball grid array thermal conductivity test was performed. When the measurement platform system reached steady state, the average temperature Th of the upper surface and the average temperature Th of the lower surface of the sample were measured. It can be calculated from the distance between each temperature measuring point and the distance from the temperature measuring point to the heat-conducting surface.

[0051]

[0052]

[0053] Given that the total thickness of the BGA test sample is H, and the height of the drum-shaped solder joints in the BGA ball grid array layer is... The thickness H2 of the upper and lower copper wafer substrates is then...

[0054] Actual temperature difference of BGA ball grid array layer Equal to the average temperature Th of the upper surface and the average temperature of the lower surface of the test sample The difference is then subtracted from the temperature difference caused by the thickness of the upper and lower copper wafer substrates.

[0055]

[0056] The thermally conductive area A of a 30mm diameter copper pillar is 706.84mm², and the measured thermal conductivity is... and measured thermal resistance per unit area The actual temperature difference of the BGA ball grid array layer The height H1 of the drum-shaped solder joint is calculated.

[0057]

[0058]

[0059] Through the above description of the embodiments, those skilled in the art can clearly understand that each embodiment can be implemented by means of software plus necessary general-purpose hardware platforms, and of course, it can also be implemented by hardware. Based on this understanding, the above technical solutions, in essence or the part that contributes to the prior art, can be embodied in the form of a software product. This computer software product can be stored in a computer-readable storage medium, such as ROM / RAM, magnetic disk, optical disk, etc., and includes several instructions to cause a computer device (which may be a personal computer, server, or network device, etc.) to execute the methods described in the various embodiments or some parts of the embodiments.

[0060] This document uses specific examples to illustrate the principles and implementation methods of the present invention. The descriptions of the above embodiments are only for the purpose of helping to understand the method and core ideas of the present invention. Furthermore, those skilled in the art will recognize that, based on the ideas of the present invention, there will be changes in the specific implementation methods and application scope. Therefore, the content of this specification should not be construed as a limitation of the present invention.

Claims

1. A method for preparing a BGA ball grid array thermally conductive test coupon, comprising: providing a BGA ball grid array test coupon having a plurality of BGA balls; and applying a thermally conductive material to the BGA balls. Includes the following steps: S1. Coat the surface of a large copper substrate with solder resist ink, and heat the large copper substrate coated with solder resist ink to bring it into a semi-cured state; the large copper substrate includes multiple circular copper substrates. A film with multiple circular copper substrate solder mask window patterns is attached to a large copper substrate coated with solder mask ink. Multiple BGA pads with solder mask layer are formed by exposure and development ink processes. S2. After printing the corresponding number on the surface of the circular copper substrate, cut out multiple circular copper substrates and perform chemical nickel-gold surface treatment on the BGA pads to form an anti-oxidation layer. S3. Fix a single circular copper substrate in the ball-planting fixture. First, print solder paste on the BGA pads using a solder paste stencil. Then, replace it with a ball-planting stencil and place the solder balls on the corresponding BGA pads through the openings in the ball-planting stencil to form a single-sided ball-planting substrate. S4. Reflow soldering is performed on the single-sided ball-mounted substrate to melt the solder paste and initially fix the solder balls. S5. Print solder paste on the BGA pads of another circular copper substrate, and align and mount the other circular copper substrate with the circular copper substrate that has completed the first reflow soldering through the positioning structure, so that the solder balls are located between the two substrates. S6. Reflow solder the mounted components again to melt all the solder and complete the final alignment and connection of the upper and lower pads under the action of surface tension. Repeat the above steps S1-S5 to form multiple test samples with symmetrical upper and lower copper substrates and a BGA ball grid array micro solder joint layer in the middle.

2. The method for preparing a BGA ball grid array thermal conductivity test sample according to claim 1, characterized in that, In step S1, multiple BGA pads with a solder resist layer are formed through an exposure and development ink process, specifically including: The solder resist ink under the transparent area of ​​the film is exposed to ultraviolet light and undergoes a photochemical reaction to cure. The solder resist ink under the black area of ​​the film, corresponding to the BGA pad pattern, is not exposed due to the film's obstruction and does not undergo a curing reaction. The uncured solder resist ink on the large copper substrate is cleaned with a weak alkaline solution. The unexposed and uncured solder resist ink is dissolved by the weak alkaline solution, while the exposed and cured solder resist ink is not dissolved and remains on the copper plate to form a surface solder resist layer. The large copper substrate is then placed in a high-temperature oven to complete the final curing of the solder resist ink and the opening of solder resist windows for all BGA pads.

3. The method for preparing a BGA ball grid array thermal conductivity test sample according to claim 1, characterized in that, In S1, a square handle with a positioning hole is provided on the outer edge of the circular copper substrate. The chemical nickel-gold surface treatment is carried out by immersing the circular copper substrate in the molten metal through a fixing wire passing through the positioning hole.

4. The method for preparing a BGA ball grid array thermal conductivity test sample according to claim 1, characterized in that, In S3, the ball-planting fixture is a square aluminum alloy fitting with a circular groove for embedding the circular copper substrate. The depth of the groove is less than the thickness of the circular copper substrate, so that after the circular copper substrate is embedded and fixed, its surface is higher than the surface of the ball-planting fixture, which facilitates the operation of the steel mesh.

5. A BGA ball grid array thermal conductivity test specimen, prepared according to the preparation method of any one of claims 1-4, characterized in that, It includes an upper circular copper substrate, a middle BGA ball grid array micro solder joint layer, and a lower circular copper substrate.

6. A testing apparatus for a BGA ball grid array thermal conductivity test sample, used to test any of the BGA ball grid array thermal conductivity test samples prepared according to claims 1-4, characterized in that, From bottom to top, including: Liquid cooling source, used to provide a constant temperature; A cold-end metal column, the lower surface of which is in thermal contact with the liquid cooling source, is used for heat removal and temperature gradient measurement. The BGA ball grid array thermal conductivity test sample to be tested is placed on the upper surface of the cold end metal pillar; A hot-end metal pillar, the lower surface of which contacts the upper surface of the BGA ball grid array thermal conductivity test sample to be tested, is used for heat conduction, temperature gradient measurement and heat flow shaping; A heating source is in thermal contact with the upper surface of the hot-end metal pillar to simulate the heating of electronic components; A pressure application mechanism is used to apply axial preload to the stack consisting of the heating source, the hot-end metal column, the sample, the cold-end metal column, and the liquid cooling source.

7. The testing apparatus for a BGA ball grid array thermal conductivity test sample according to claim 6, characterized in that, The hot-end and cold-end metal columns are covered with thermal insulation material on their sides. Multiple temperature measuring holes are uniformly arranged axially inside the hot-end and cold-end metal columns to accommodate temperature sensors for measuring the axial temperature gradient inside the column.

8. The testing apparatus for a BGA ball grid array thermal conductivity test sample according to claim 6, characterized in that, The heating source is a circular polyimide (PI) heating film powered by a DC power supply, and its area is equal to the area of ​​the end face of the hot-end metal pillar and the area of ​​the BGA ball grid array thermal conductivity test sample.

9. The testing apparatus for a BGA ball grid array thermal conductivity test sample according to claim 6, characterized in that, The hot-end metal column and the cold-end metal column are copper columns or aluminum columns; the liquid cooling source is a copper water cooling head, which is connected to a constant temperature chiller.

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