A method for fabricating a stacked structure of flip chips

Abstract

This invention relates to the field of semiconductor manufacturing technology, specifically to a method for fabricating a stacked structure of flip chips. The method uses a lower-layer chip, an upper-layer chip, and a packaging substrate as starting materials, and pyrolyzed microspheres as the sole functional additive medium. Through the synergistic effect of pyrolyzed microspheres, programmed reflow soldering, and ultrasonic-assisted pre-fixation, it achieves the integration of chip bonding, microvia generation, and underfilling. This invention achieves the integration of chip bonding, microvia generation, and underfilling through a three-level synergy of pyrolyzed microspheres, programmed reflow soldering, and ultrasonic-assisted pre-fixation, solving the problems of premature flux failure, high solder joint void ratio, and the need for vacuum assistance for underfilling in traditional flip chip stacking. It provides a low-cost, high-yield one-step solution for high-reliability flip chip packaging.

Description

Technical Field

[0001] This invention relates to the field of semiconductor manufacturing technology, and more specifically to a method for fabricating a stacked structure of flip chips. Background Technology

[0002] In the semiconductor packaging field, flip-chip stacking suffers from inherent defects such as premature flux failure, high solder joint void ratio, and reliance on vacuum-assisted underfilling. These defects lead to decreased bonding interface reliability, increased process costs, and fluctuating packaging yields, becoming key bottlenecks restricting the industrial application of high-density 3D integrated packaging. Traditional flip-chip stacking relies on a serial process mode of independently applying flux, cleaning after reflow soldering, and vacuum-assisted underfilling. This multi-step, separate preparation mode has inherent defects: the flux oxidizes and fails prematurely during the reflow soldering preheating stage, resulting in poor solder wetting and decreased microbump bridging rate; vacuum-assisted underfilling equipment has high investment costs and a narrow process window, and the colloid filling time increases exponentially with chip size. At the same time, it is difficult to completely remove micro-air bubbles in the underfill adhesive, which can form voids and delamination defects, causing early failure of the package during temperature cycling tests. This severely restricts the practical application of flip-chip stacking in high-performance computing and mobile terminal fields.

[0003] In recent years, researchers have attempted to improve these issues by optimizing reflow soldering temperature profiles, developing low-volatility fluxes, and introducing vacuum dispensing equipment. However, temperature profile optimization cannot fundamentally solve the problem of flux oxidation failure during the preheating stage. While low-volatility fluxes can delay evaporation, their residues are difficult to clean, introducing additional cleaning steps and environmental pollution. Vacuum dispensing equipment is expensive and has limited filling effect on high-density, large-size chips, and micro-bubbles in the colloid are still difficult to completely eliminate. Therefore, how to construct an integrated stacking fabrication method that features delayed flux release, autonomous suppression of solder joint voids, and bottom filling without vacuum assistance has become a core technical challenge that urgently needs to be overcome in the field of flip-chip stacking packaging.

[0004] To address the above problems, the present invention provides a solution. Summary of the Invention

[0005] The purpose of this invention is to provide a method for fabricating a stacked structure of flip chips, in order to solve the problems of premature flux failure, high solder joint void rate, and reliance on vacuum assistance for bottom filling in the prior art.

[0006] To achieve the above objectives, the present invention adopts the following technical solution: A method for preparing a stacked structure of a flip chip uses the lower layer chip, the upper layer chip, and the packaging substrate as starting materials, and pyrolyzed bonding microspheres as the only functional additive medium. Through the synergistic effect of pyrolyzed bonding microspheres, programmed temperature rise reflow soldering, and ultrasonic-assisted pre-fixation, the chip bonding, micropore generation, and bottom filling are integrated.

[0007] Furthermore, a method for fabricating a stacked structure of flip chips includes the following steps:

[0008] S1: Take the packaged substrate and place it in a plasma cleaner. Set the cleaning power to 100W and mix argon and oxygen to obtain the cleaning gas. The volume ratio of argon to oxygen is 8:2. Then clean the packaged substrate for 60 seconds. After cleaning, place it on a heating table and preheat it to 120°C. Hold it at that temperature for 30 seconds. After holding it at that temperature, use screen printing to apply flux to the pads of the packaged substrate. The printing pressure is 0.3MPa, the printing speed is 30mm / s, and the flux thickness is 10-15um.

[0009] S2: Use a flip chip picker to pick up the lower-layer chip, align the bumps of the lower-layer chip with the pads of the packaging substrate, and after alignment, mount the chip onto the substrate with a mounting pressure of 0.5N. During the mounting process, keep the substrate temperature at 120℃, and then send it into a nitrogen-protected reflow oven. Nitrogen protection is maintained throughout the process. Set the reflow temperature profile, first heat up to 150℃ at 1.5℃ / s and hold for 30s, then heat up to 220℃ at 1℃ / s, then heat up to the peak temperature of 245℃ at 1℃ / s and hold for 20s, and finally cool down to room temperature at 2℃ / s. After the reflow is completed, use an X-ray inspection device to check the bump soldering quality to confirm that there are no bridging short circuits or open circuit defects, and obtain the soldered chip substrate assembly.

[0010] S3: Place the soldered chip substrate assembly in a plasma cleaner, set the cleaning power to 100W, and mix argon and oxygen to obtain the cleaning gas, with a volume ratio of argon to oxygen of 8:2. Clean the chip substrate assembly for 60 seconds. Then, add pyrolyzed bonded microspheres and isopropanol to the reaction vessel, set the magnetic stirrer speed to 300 rpm, and stir for 30 minutes to obtain a spray suspension. Then, use a precision spray valve with a nozzle diameter of 0.3 mm and an atomizing gas pressure of 0.2 MPa to spray the spray suspension onto the bonding pad area of ​​the upper chip, with a spraying amount of 0.5-1.0 mg / cm³. 2 The coating coverage is 50-70%, and the coating should be left to stand at room temperature for 5 minutes after spraying.

[0011] S4: After settling at room temperature, flip-chip the upper layer and align it using a flip-chip bonding machine. After alignment, apply low-frequency ultrasound using an ultrasonic flip-chip bonding machine, setting the ultrasonic frequency to 20kHz and the ultrasonic power density to 0.15W / cm². 2 The ultrasound time is 3-5 seconds, and a pre-fixed chip assembly is obtained after the ultrasound is completed;

[0012] S5: The pre-fixed chip assembly is sent into the reflow oven. The reflow oven is protected by nitrogen throughout the process. The temperature is increased from room temperature to 150°C at a rate of 1.5°C / s, held at 150°C for 30s, then increased from 150°C to 245°C at a rate of 1°C / s, held at 245°C for 20s, and finally cooled to room temperature at a rate of 2°C / s to obtain the primary chip assembly.

[0013] S6: Place the primary chip assembly on the dispensing machine's worktable and apply epoxy capillary underfill along the chip edge using a spray method. After application, place it in a hot plate for curing at 150℃ for 10 minutes. After curing, use X-ray inspection equipment to check the integrity of the underfill. The acceptable standard is no air bubbles with a diameter >20µm at the bottom of the chip and an edge glue overflow width ≤200µm. If X-ray inspection after curing reveals air bubbles with a diameter >20µm in the underfill, place the chip assembly in an ultrasonic cleaning tank, setting the ultrasonic frequency to 80kHz and the ultrasonic power density to 0.3W / cm². 2 Ultrasound for 2 minutes, and X-ray examination was performed again after ultrasonic treatment to confirm that the bubbles were eliminated;

[0014] S7: Use epoxy resin molding compound to encapsulate the chip assembly that has passed X-ray inspection. The molding temperature is 175℃, the molding pressure is 10MPa, the molding time is 120s, and after molding, cool to room temperature to obtain the finished flip-chip stacked structure.

[0015] Furthermore, the mass ratio of the thermally released bonded microspheres to isopropanol in step S3 is 1:10;

[0016] Furthermore, the preparation method of the thermally released bonded microspheres includes the following steps:

[0017] A1: Weigh polycarbonate and dichloromethane, add polycarbonate and dichloromethane to the reaction vessel, set the magnetic stirrer speed to 500 rpm, stir for 30 min to obtain an oil phase solution, wherein the mass ratio of polycarbonate to dichloromethane is 30:200;

[0018] A2: Weigh out rosin-based flux, nano Ag particles, and azodicarbonamide and add them to deionized water. Use an ultrasonic cell disruptor to assist dispersion. Set the ultrasonic cell disruptor power to 200W and the frequency to 20kHz. Disperse for 10 minutes to obtain an internal aqueous dispersion. The mass ratio of rosin-based flux, nano Ag particles, azodicarbonamide, and deionized water is 40:25:5:100.

[0019] A3: The aqueous phase dispersion was slowly added to the oil phase solution at a rate of 1 drop / s. After all the liquid was added, the high-shear homogenizer was set to a speed of 10,000 rpm and high-speed shear emulsification was performed for 5 minutes to obtain the primary emulsion. The mass ratio of the aqueous phase dispersion to the oil phase solution was 170:230.

[0020] A4: Weigh polyvinyl alcohol, add it to deionized water and heat to 80°C. Set the magnetic stirrer speed to 300 rpm and stir to dissolve for 30 min. Cool to room temperature to obtain a polyvinyl alcohol aqueous solution, wherein the mass ratio of polyvinyl alcohol to deionized water is 10:490. Then add the primary emulsion to the polyvinyl alcohol aqueous solution, set the magnetic stirrer speed to 400 rpm and stir for 3 h to obtain a thermally released bonded microsphere suspension, wherein the mass ratio of primary emulsion to polyvinyl alcohol aqueous solution is 400:500.

[0021] A5: Place the pyrogenetic bonded microsphere suspension in a high-speed centrifuge, set the centrifuge speed to 8000 rpm, centrifuge for 15 min, discard the supernatant, wash the precipitate 3 times with deionized water, transfer it to a vacuum drying oven, set the temperature of the vacuum drying oven to 40℃ and the vacuum degree to -0.09 MPa, dry for 12 h, after drying, pass it through a 200-mesh stainless steel sieve, collect the sieve material to obtain pyrogenetic bonded microspheres, seal and dry at 4℃ for later use.

[0022] Furthermore, the thermal decomposition onset temperature of the azodicarbonamide is 200℃, the peak decomposition temperature is 210-220℃, and the gas emission is 200-220ml / g.

[0023] Furthermore, the average particle size of the nano-Ag particles is 50 nm, and the particle size distribution range is 30-80 nm.

[0024] Furthermore, the bottom filler is an epoxy capillary bottom filler with a viscosity of 300-500 mPa·s at 25°C, a glass transition temperature ≥120°C after curing, and a coefficient of thermal expansion ≤35ppm / °C.

[0025] In summary, due to the adoption of the above technical solution, the beneficial effects of the present invention are as follows:

[0026] 1: This invention creates a novel flip chip stacking method with three-stage synergy of pyrolyzed bonded microspheres, programmed temperature rise, and ultrasonic pre-fixation. By timing the thermal decomposition temperature of the polycarbonate shell in the pyrolyzed bonded microspheres with the foaming temperature of azodicarbonamide, the flux and nano-Ag are released on demand. The temperature gradient of programmed temperature rise reflow soldering induces the sequential rupture and foaming of the microspheres. Combined with ultrasonic-assisted pre-fixation for dry-state centering and positioning, this method systematically solves the key problems of premature flux failure, high solder joint void rate, and the need for vacuum assistance for bottom filling in traditional flip chip stacking.

[0027] 2: This invention uses self-synthesized pyrogenetic bonding microspheres as the core to construct an integrated stacking preparation system for flip chip bonding, microchannel generation, and underfilling. By encapsulating flux and nano-Ag within the microsphere shell, using programmed temperature rise to achieve delayed release of flux before the peak temperature and in-situ pore formation of foaming agent during cooling, and using ultrasonic pre-fixation to achieve glue-free temporary positioning of the chip, this invention achieves a three-in-one integrated manufacturing process that combines chip bonding, microchannel formation, and accelerated underfilling. It exhibits excellent adaptability, especially for flip chip stacking packaging systems that require high reliability and low void ratio.

[0028] 3: This invention integrates pyrolyzed bonded microspheres with standard reflow soldering equipment, compressing the traditional post-processing steps that require independent flux coating, vacuum-assisted underfilling, and multi-step cleaning into a one-step continuous packaging process with a single microsphere, single reflow soldering, and no vacuum assistance. During the entire reflow soldering process, it achieves programmable control over delayed flux release, in-situ generation of micropore channels, and spontaneous underfill penetration, providing a brand-new solution for low-cost, high-yield, and easy industrial production of high-performance flip chip stacking packaging. Detailed Implementation

[0029] To make the technical means, creative features, objectives and effects of this invention easier to understand, the invention will be further described below with reference to specific embodiments.

[0030] Unless otherwise specified, the experimental methods used in the following examples are conventional methods, performed according to the techniques or conditions described in the literature in this field or according to the product instructions. Unless otherwise specified, the materials and reagents used in the following examples are commercially available.

[0031] Example 1

[0032] 1: Weigh 300g of polycarbonate and 2kg of dichloromethane, add the polycarbonate and dichloromethane to the reaction vessel, set the magnetic stirrer speed to 500rpm, and stir for 30min to obtain 2.3kg of oil phase solution;

[0033] 2: Weigh 400g of NC-809 no-wash rosin flux, 250g of nano Ag particles and 50g of azodicarbonamide and add them to 1kg of deionized water. Use an ultrasonic cell disruptor to assist dispersion. Set the ultrasonic cell disruptor power to 200W and the frequency to 20kHz. Disperse for 10min to obtain 1.7kg of aqueous dispersion.

[0034] 3: Slowly add 1.7 kg of aqueous dispersion to 2.3 kg of oil solution at a rate of 1 drop / s. After all the solution has been added, set the high-shear homogenizer speed to 10,000 rpm and emulsify at high speed for 5 min to obtain 3.8 kg of primary emulsion.

[0035] 4: Weigh 100g of polyvinyl alcohol, add it to 4.9kg of deionized water and heat to 80℃. Set the magnetic stirrer speed to 300rpm and stir to dissolve for 30min. Cool to room temperature to obtain 5kg of polyvinyl alcohol aqueous solution. Then add 3.8kg of primary emulsion to 4.75kg of polyvinyl alcohol aqueous solution, set the magnetic stirrer speed to 400rpm and stir for 3h to obtain 8.5kg of thermally released bonded microsphere suspension.

[0036] 5. Place 8.5 kg of the pyrogenetic bonded microsphere suspension in a high-speed centrifuge, set the centrifuge speed to 8000 rpm, centrifuge for 15 min, discard the supernatant, wash the precipitate three times with deionized water, transfer it to a vacuum drying oven, set the temperature of the vacuum drying oven to 40℃ and the vacuum degree to -0.09 MPa, dry for 12 h, after drying, pass it through a 200-mesh stainless steel sieve, collect the sieve material, and obtain 650 g of the pyrogenetic bonded microspheres prepared in Example 1, seal and dry at 4℃ for later use.

[0037] Example 2

[0038] The packaging substrate used in this embodiment is a BT resin substrate with a substrate size of 10mm×10mm, a pad array pitch of 100um, and a pad material of Cu / OSP; the lower layer chip used is a silicon-based logic chip with a chip size of 8mm×8mm, a bump material of SAC305, a bump height of 50um, and a bump pitch of 100um; the upper layer chip used is a silicon-based memory chip with a chip size of 6mm×6mm and a pad material of Cu, used for flip-chip stacking packaging.

[0039] 1. Take the packaged substrate and place it in a plasma cleaner. Set the cleaning power to 100W and mix 8L of argon and 2L of oxygen to obtain the cleaning gas. Then clean the packaged substrate for 60 seconds. After cleaning, place it on a heating table and preheat it to 120°C. Hold it at that temperature for 30 seconds. After holding it at that temperature, use screen printing to apply flux to the pads of the packaged substrate. The printing pressure is 0.3MPa, the printing speed is 30mm / s, and the flux thickness is 10um.

[0040] 2. Use a flip chip mounter to pick up the lower-layer chip, align the bumps of the lower-layer chip with the pads of the packaging substrate, and mount the chip onto the substrate with a mounting pressure of 0.5N. During the mounting process, maintain the substrate temperature at 120℃, and then send it into a nitrogen-protected reflow oven. Nitrogen protection is maintained throughout the process. Set the reflow temperature profile, first heat up to 150℃ at 1.5℃ / s and hold for 30s, then heat up to 220℃ at 1℃ / s, then heat up to the peak temperature of 245℃ at 1℃ / s and hold for 20s, and finally cool down to room temperature at 2℃ / s. After the reflow is completed, use an X-ray inspection device to check the bump soldering quality. The bump bridging rate is 100%, the void rate is 3.2%, and there are no bridging short circuits or open circuit defects, thus obtaining the soldered chip substrate assembly.

[0041] 3. Place the soldered chip substrate assembly in a plasma cleaner, set the cleaning power to 100W, and mix 8L of argon and 2L of oxygen to obtain the cleaning gas. Clean the chip substrate assembly for 60 seconds. Then, add 100g of the pyrolyzed bonded microspheres prepared in Example 1 and 1000g of isopropanol to the reaction vessel. Set the magnetic stirrer speed to 300rpm and stir for 30 minutes to obtain a spray suspension. Then, use a precision spray valve with a nozzle diameter of 0.3mm and an atomizing gas pressure of 0.2MPa to spray the spray suspension onto the bonding pad area of ​​the upper chip, with a spraying amount of 0.5mg / cm³. 2 The coating coverage is 50%, and the coating is left to stand at room temperature for 5 minutes after spraying.

[0042] 4. After allowing the chip to stand at room temperature, flip-chip it and align it using a flip-chip bonding machine. After alignment, apply low-frequency ultrasound using an ultrasonic flip-chip bonding machine, setting the ultrasonic frequency to 20kHz and the ultrasonic power density to 0.15W / cm³. 2 The ultrasound time is 3 seconds, and a pre-fixed chip assembly is obtained after the ultrasound is completed;

[0043] 5: The pre-fixed chip assembly is sent into the reflow oven. The reflow oven is protected by nitrogen throughout the process. The temperature is increased from room temperature to 150°C at a rate of 1.5°C / s, held at 150°C for 30s, then increased from 150°C to 245°C at a rate of 1°C / s, held at 245°C for 20s, and finally cooled to room temperature at a rate of 2°C / s to obtain the primary chip assembly.

[0044] 6: Place the primary chip assembly on the dispensing machine table and apply epoxy capillary underfill material along the chip edge using a spray method. After application, place it in a hot plate for curing at 150℃ for 10 minutes. After curing, use an X-ray inspection device to check the integrity of the underfill material. There should be no bubbles with a diameter >20µm at the bottom of the chip, and the width of the edge overflow of the material should be 150µm. The pass standard is no bubbles with a diameter >20µm at the bottom of the chip and the width of the edge overflow of the material ≤200µm.

[0045] 7: Panasonic CV8713 epoxy resin molding compound was used to encapsulate the chip assembly that passed X-ray inspection. The molding temperature was 175°C, the molding pressure was 10MPa, and the molding time was 120s. After molding, the chip was cooled to room temperature to obtain the flip-chip stacked structure product prepared in Example 2.

[0046] After the packaging process is completed, the finished flip-chip stacked structure is subjected to reliability testing. The interface bonding is inspected using a scanning acoustic microscope, and no delamination defects are found.

[0047] The void rate of the weld joints was checked using an X-ray inspection instrument. The void rate was 2.1%, which is less than 5% and is acceptable.

[0048] The shear strength of the chip was measured using a shear force tester, and the average value was 28.5 MPa, which is greater than 20 MPa and is considered acceptable.

[0049] Temperature cycling tests were conducted using a thermal cycling tester, with conditions ranging from -55℃ to 125℃ and held for 15 minutes at each temperature. After 1000 cycles, the X-ray detection showed that the void ratio increased to 3.8% and the shear strength decreased to 24.2 MPa, meeting industry standards.

[0050] Table 1, Reflow Soldering Process Parameters for Example 2

[0051]

[0052] Table 2, Bottom Filling Process Parameters for Example 2

[0053]

[0054] Table 3, Reliability Test Results of Finished Products in Example 2

[0055]

[0056] Example 3

[0057] The packaging substrate used in this embodiment is a ceramic substrate with a size of 15mm × 15mm, a pad array pitch of 120um, and a pad material of Ni / Au; the lower-layer chip used is a silicon-based power management chip with a size of 10mm × 10mm, a bump material of SAC305, a bump height of 60um, and a bump pitch of 120um; the upper-layer chip used is a silicon-based radio frequency chip with a size of 8mm × 8mm and a pad material of Cu, used for flip-chip stacking packaging.

[0058] 1. Take the packaged substrate and place it in a plasma cleaner. Set the cleaning power to 100W and mix 8L of argon and 2L of oxygen to obtain the cleaning gas. Then clean the packaged substrate for 60 seconds. After cleaning, place it on a heating table and preheat it to 120°C. Hold it at that temperature for 30 seconds. After holding it at that temperature, use screen printing to apply flux to the pads of the packaged substrate. The printing pressure is 0.3MPa, the printing speed is 30mm / s, and the flux thickness is 12um.

[0059] 2. Use a flip chip mounter to pick up the lower-layer chip, align the bumps of the lower-layer chip with the pads of the packaging substrate, and mount the chip onto the substrate with a mounting pressure of 0.5N. During the mounting process, maintain the substrate temperature at 120℃, and then send it into a nitrogen-protected reflow oven. Nitrogen protection is maintained throughout the process. Set the reflow temperature profile, first heat up to 150℃ at 1.5℃ / s and hold for 30s, then heat up to 220℃ at 1℃ / s, then heat up to the peak temperature of 245℃ at 1℃ / s and hold for 20s, and finally cool down to room temperature at 2℃ / s. After the reflow is completed, use an X-ray inspection device to check the bump soldering quality. The bump bridging rate is 100%, the void rate is 2.8%, and there are no bridging short circuits or open circuit defects, thus obtaining the soldered chip substrate assembly.

[0060] 3. Place the soldered chip substrate assembly in a plasma cleaner, set the cleaning power to 100W, and mix 8L of argon and 2L of oxygen to obtain the cleaning gas. Clean the chip substrate assembly for 60 seconds. Then, add 100g of the pyrolyzed bonded microspheres prepared in Example 1 and 1000g of isopropanol to the reaction vessel. Set the magnetic stirrer speed to 300rpm and stir for 30 minutes to obtain a spray suspension. Then, use a precision spray valve with a nozzle diameter of 0.3mm and an atomizing gas pressure of 0.2MPa to spray the spray suspension onto the pad area of ​​the upper chip, with a spraying amount of 0.8mg / cm³. 2 The coating coverage is 60%, and the coating is left to stand at room temperature for 5 minutes after spraying.

[0061] 4. After allowing the chip to stand at room temperature, flip-chip it and align it using a flip-chip bonding machine. After alignment, apply low-frequency ultrasound using an ultrasonic flip-chip bonding machine, setting the ultrasonic frequency to 20kHz and the ultrasonic power density to 0.15W / cm³. 2 The ultrasound time is 4 seconds, and a pre-fixed chip assembly is obtained after the ultrasound is completed;

[0062] 5: The pre-fixed chip assembly is sent into the reflow oven. The reflow oven is protected by nitrogen throughout the process. The temperature is increased from room temperature to 150°C at a rate of 1.5°C / s, held at 150°C for 30s, then increased from 150°C to 245°C at a rate of 1°C / s, held at 245°C for 20s, and finally cooled to room temperature at a rate of 2°C / s to obtain the primary chip assembly.

[0063] 6: Place the primary chip assembly on the dispensing machine table and apply epoxy capillary underfill material along the chip edge using a spray method. After application, place it in a hot plate for curing at 150℃ for 10 minutes. After curing, use an X-ray inspection device to check the integrity of the underfill material. There should be no bubbles with a diameter >20µm at the bottom of the chip, and the edge glue overflow width should be 120µm. The pass standard is no bubbles with a diameter >20µm at the bottom of the chip and the edge glue overflow width ≤200µm.

[0064] 7: The X-ray-tested chip assembly was encapsulated using Resonac CEL-9240 HF10 epoxy resin molding compound. The molding temperature was 175℃, the molding pressure was 10MPa, and the molding time was 120s. After molding, the assembly was cooled to room temperature to obtain the flip-chip stacked structure product prepared in Example 3.

[0065] After the packaging process is completed, the finished flip-chip stacked structure is subjected to reliability testing. The interface bonding is inspected using a scanning acoustic microscope, and no delamination defects are found.

[0066] The void rate of the weld joints was checked using an X-ray inspection instrument. The void rate was 1.9%, which is less than 5% and is acceptable.

[0067] The shear strength of the chip was measured using a shear force tester, and the average value was 30.2 MPa, which is greater than 20 MPa and is considered acceptable.

[0068] Temperature cycling tests were conducted using a thermal cycling tester, with conditions ranging from -55℃ to 125℃ and held for 15 minutes at each temperature. After 1000 cycles, the X-ray detection showed that the void ratio increased to 3.2% and the shear strength decreased to 26.5 MPa, meeting industry standards.

[0069] Table 4, Reflow Soldering Process Parameters for Example 3

[0070]

[0071] Table 5, Bottom Filling Process Parameters for Example 3

[0072]

[0073] Table 6, Reliability Test Results of Finished Products in Example 3

[0074]

[0075] Example 4

[0076] The packaging substrate used in this embodiment is a silicon-based adapter board with a substrate size of 20mm×20mm, a pad array pitch of 150um, and a pad material of Cu; the lower-layer chip used is a silicon-based AI accelerator chip with a chip size of 15mm×15mm, a bump material of SAC305, a bump height of 70um, and a bump pitch of 150um; the upper-layer chip used is a silicon-based high-bandwidth memory chip with a chip size of 12mm×12mm and a pad material of Cu, used for flip-chip stacking packaging.

[0077] 1. Take the packaged substrate and place it in a plasma cleaner. Set the cleaning power to 100W and mix 8L of argon and 2L of oxygen to obtain the cleaning gas. Then clean the packaged substrate for 60 seconds. After cleaning, place it on a heating table and preheat it to 120°C. Hold it at that temperature for 30 seconds. After holding it at that temperature, use screen printing to apply flux to the pads of the packaged substrate. The printing pressure is 0.3MPa, the printing speed is 30mm / s, and the flux thickness is 15um.

[0078] 2. Use a flip chip mounter to pick up the lower-layer chip, align the bumps of the lower-layer chip with the pads of the packaging substrate, and mount the chip onto the substrate with a mounting pressure of 0.5N. During the mounting process, maintain the substrate temperature at 120℃, and then send it into a nitrogen-protected reflow oven. Nitrogen protection is maintained throughout the process. Set the reflow temperature profile, first heat up to 150℃ at 1.5℃ / s and hold for 30s, then heat up to 220℃ at 1℃ / s, then heat up to the peak temperature of 245℃ at 1℃ / s and hold for 20s, and finally cool down to room temperature at 2℃ / s. After the reflow is completed, use an X-ray inspection device to check the bump soldering quality. The bump bridging rate is 100%, the void rate is 2.5%, and there are no bridging short circuits or open circuit defects, thus obtaining the soldered chip substrate assembly.

[0079] 3. Place the soldered chip substrate assembly in a plasma cleaner, set the cleaning power to 100W, and mix 8L of argon and 2L of oxygen to obtain the cleaning gas. Clean the chip substrate assembly for 60 seconds. Then, add 100g of the pyrolyzed bonded microspheres prepared in Example 1 and 1000g of isopropanol to the reaction vessel. Set the magnetic stirrer speed to 300rpm and stir for 30 minutes to obtain a spray suspension. Then, use a precision spray valve with a nozzle diameter of 0.3mm and an atomizing gas pressure of 0.2MPa to spray the spray suspension onto the pad area of ​​the upper chip. The spraying amount is 1.0mg / cm³. 2 The coating coverage is 70%, and the coating is left to stand at room temperature for 5 minutes after spraying.

[0080] 4. After allowing the chip to stand at room temperature, flip-chip it and align it using a flip-chip bonding machine. After alignment, apply low-frequency ultrasound using an ultrasonic flip-chip bonding machine, setting the ultrasonic frequency to 20kHz and the ultrasonic power density to 0.15W / cm³. 2The ultrasound time is 5 seconds, and a pre-fixed chip assembly is obtained after the ultrasound is completed;

[0081] 5: The pre-fixed chip assembly is sent into the reflow oven. The reflow oven is protected by nitrogen throughout the process. The temperature is increased from room temperature to 150°C at a rate of 1.5°C / s, held at 150°C for 30s, then increased from 150°C to 245°C at a rate of 1°C / s, held at 245°C for 20s, and finally cooled to room temperature at a rate of 2°C / s to obtain the primary chip assembly.

[0082] 6: Place the primary chip assembly on the dispensing machine worktable and apply epoxy capillary underfill material along the chip edge using a spray method. After application, place it in a hot plate for curing at 150℃ for 10 minutes. After curing, use an X-ray inspection device to check the integrity of the underfill material. There should be no bubbles with a diameter >20µm at the bottom of the chip, and the edge glue overflow width should be 180µm. The pass standard is no bubbles with a diameter >20µm at the bottom of the chip and edge glue overflow width ≤200µm.

[0083] 7: Panasonic CV8713 epoxy resin molding compound was used to encapsulate the chip assembly that passed X-ray inspection. The molding temperature was 175°C, the molding pressure was 10MPa, and the molding time was 120s. After molding, the chip was cooled to room temperature to obtain the flip-chip stacked structure product prepared in Example 4.

[0084] After the packaging process is completed, the finished flip-chip stacked structure is subjected to reliability testing. The interface bonding is inspected using a scanning acoustic microscope, and no delamination defects are found.

[0085] The void rate of the weld joints was checked using an X-ray inspection instrument. The void rate was 1.7%, which is less than 5% and is acceptable.

[0086] The shear strength of the chip was measured using a shear force tester, and the average value was 31.5 MPa, which is greater than 20 MPa and is considered acceptable.

[0087] Temperature cycling tests were conducted using a thermal cycling tester, with conditions ranging from -55℃ to 125℃ and held for 15 minutes at each temperature. After 1000 cycles, the X-ray detection showed that the void ratio increased to 2.9% and the shear strength decreased to 27.8 MPa, meeting industry standards.

[0088] Table 7, Reflow Soldering Process Parameters for Example 4

[0089]

[0090] Table 8, Bottom Filling Process Parameters for Example 4

[0091]

[0092] Table 9, Reliability Test Results of Finished Products in Example 4

[0093]

[0094] The foregoing has shown and described the basic principles, main features, and advantages of the present invention. Those skilled in the art should understand that the present invention is not limited to the above embodiments. The embodiments and descriptions in the specification are merely illustrative of the principles of the invention. Various changes and modifications can be made to the invention without departing from its spirit and scope, and all such changes and modifications fall within the scope of the present invention as claimed. The scope of protection of this invention is defined by the appended claims and their equivalents.

Claims

1. A method for fabricating a stacked structure of flip chips, characterized in that, Using the lower-layer chip, upper-layer chip, and packaging substrate as starting materials, and pyrolyzed microspheres as the sole functional additive medium, the synergistic effect of pyrolyzed microspheres, programmed temperature reflow soldering, and ultrasonic-assisted pre-fixation achieves the integration of chip bonding, micropore generation, and bottom filling.

2. The method for fabricating a stacked structure of a flip chip according to claim 1, characterized in that, Includes the following steps: S1: Take the packaged substrate and place it in a plasma cleaner. Set the cleaning power to 100W and mix argon and oxygen to obtain the cleaning gas. The volume ratio of argon to oxygen is 8:

2. Then clean the packaged substrate for 60 seconds. After cleaning, place it on a heating table and preheat it to 120°C. Hold it at that temperature for 30 seconds. After holding it at that temperature, use screen printing to apply flux to the pads of the packaged substrate. The printing pressure is 0.3MPa, the printing speed is 30mm / s, and the flux thickness is 10-15um. S2: Use a flip chip picker to pick up the lower-layer chip, align the bumps of the lower-layer chip with the pads of the packaging substrate, and after alignment, mount the chip onto the substrate with a mounting pressure of 0.5N. During the mounting process, keep the substrate temperature at 120℃, and then send it into a nitrogen-protected reflow oven. Nitrogen protection is maintained throughout the process. Set the reflow temperature profile, first heat up to 150℃ at 1.5℃ / s and hold for 30s, then heat up to 220℃ at 1℃ / s, then heat up to the peak temperature of 245℃ at 1℃ / s and hold for 20s, and finally cool down to room temperature at 2℃ / s. After the reflow is completed, use an X-ray inspection device to check the bump soldering quality to confirm that there are no bridging short circuits or open circuit defects, and obtain the soldered chip substrate assembly. S3: Place the soldered chip substrate assembly in a plasma cleaner, set the cleaning power to 100W, and mix argon and oxygen to obtain the cleaning gas, with a volume ratio of argon to oxygen of 8:

2. Clean the chip substrate assembly for 60 seconds. Then, add pyrolyzed microspheres and isopropanol to the reaction vessel, set the magnetic stirrer speed to 300 rpm, and stir for 30 minutes to obtain a spray suspension, with a mass ratio of pyrolyzed microspheres to isopropanol of 1:

10. Then, use a precision spray valve with a nozzle diameter of 0.3 mm and an atomizing gas pressure of 0.2 MPa to spray the spray suspension onto the bonding pad area of ​​the upper chip, with a spraying amount of 0.5-1.0 mg / cm³. 2 The coating coverage is 50-70%, and the coating should be left to stand at room temperature for 5 minutes after spraying. S4: After settling at room temperature, flip-chip the upper layer and align it using a flip-chip bonding machine. After alignment, apply low-frequency ultrasound using an ultrasonic flip-chip bonding machine, setting the ultrasonic frequency to 20kHz and the ultrasonic power density to 0.15W / cm². 2 The ultrasound time is 3-5 seconds, and a pre-fixed chip assembly is obtained after the ultrasound is completed; S5: The pre-fixed chip assembly is sent into the reflow oven. The reflow oven is protected by nitrogen throughout the process. The temperature is increased from room temperature to 150°C at a rate of 1.5°C / s, held at 150°C for 30s, then increased from 150°C to 245°C at a rate of 1°C / s, held at 245°C for 20s, and finally cooled to room temperature at a rate of 2°C / s to obtain the primary chip assembly. S6: Place the primary chip assembly on the dispensing machine's worktable and apply epoxy capillary underfill along the chip edge using a spray method. After application, place it in a hot plate for curing at 150℃ for 10 minutes. After curing, use X-ray inspection equipment to check the integrity of the underfill. The acceptable standard is no air bubbles with a diameter >20µm at the bottom of the chip and an edge glue overflow width ≤200µm. If X-ray inspection after curing reveals air bubbles with a diameter >20µm in the underfill, place the chip assembly in an ultrasonic cleaning tank, setting the ultrasonic frequency to 80kHz and the ultrasonic power density to 0.3W / cm². 2 Ultrasound for 2 minutes, and X-ray examination was performed again after ultrasonic treatment to confirm that the bubbles were eliminated; S7: Use epoxy resin molding compound to encapsulate the chip assembly that has passed X-ray inspection. The molding temperature is 175℃, the molding pressure is 10MPa, the molding time is 120s, and after molding, cool to room temperature to obtain the finished flip-chip stacked structure.

3. The method for fabricating a stacked structure of a flip chip according to claim 2, characterized in that, The mass ratio of the thermally released bonded microspheres to isopropanol in step S3 is 1:

10.

4. The method for fabricating a stacked structure of a flip chip according to claim 2, characterized in that, The method for preparing the thermally bonded microspheres includes the following steps: A1: Weigh polycarbonate and dichloromethane, add polycarbonate and dichloromethane to the reaction vessel, set the magnetic stirrer speed to 500 rpm, stir for 30 min to obtain an oil phase solution, wherein the mass ratio of polycarbonate to dichloromethane is 30:200; A2: Weigh out rosin-based flux, nano Ag particles, and azodicarbonamide and add them to deionized water. Use an ultrasonic cell disruptor to assist dispersion. Set the ultrasonic cell disruptor power to 200W and the frequency to 20kHz. Disperse for 10 minutes to obtain an internal aqueous dispersion. The mass ratio of rosin-based flux, nano Ag particles, azodicarbonamide, and deionized water is 40:25:5:

100. A3: The aqueous phase dispersion was slowly added to the oil phase solution at a rate of 1 drop / s. After all the liquid was added, the high-shear homogenizer was set to a speed of 10,000 rpm and high-speed shear emulsification was performed for 5 minutes to obtain the primary emulsion. The mass ratio of the aqueous phase dispersion to the oil phase solution was 170:

230. A4: Weigh polyvinyl alcohol, add it to deionized water and heat to 80°C. Set the magnetic stirrer speed to 300 rpm and stir to dissolve for 30 min. Cool to room temperature to obtain a polyvinyl alcohol aqueous solution, wherein the mass ratio of polyvinyl alcohol to deionized water is 10:

490. Then add the primary emulsion to the polyvinyl alcohol aqueous solution, set the magnetic stirrer speed to 400 rpm and stir for 3 h to obtain a thermally released bonded microsphere suspension, wherein the mass ratio of primary emulsion to polyvinyl alcohol aqueous solution is 400:

500. A5: Place the pyrogenetic bonded microsphere suspension in a high-speed centrifuge, set the centrifuge speed to 8000 rpm, centrifuge for 15 min, discard the supernatant, wash the precipitate 3 times with deionized water, transfer it to a vacuum drying oven, set the temperature of the vacuum drying oven to 40℃ and the vacuum degree to -0.09 MPa, dry for 12 h, after drying, pass it through a 200-mesh stainless steel sieve, collect the sieve material to obtain pyrogenetic bonded microspheres, seal and dry at 4℃ for later use.

5. The method for fabricating a stacked structure of a flip chip according to claim 4, characterized in that, The thermal decomposition initiation temperature of the azodicarbonamide is 200℃, the peak decomposition temperature is 210-220℃, and the gas emission is 200-220ml / g.

6. The method for fabricating a stacked structure of a flip chip according to claim 4, characterized in that, The nano-Ag particles have an average particle size of 50 nm and a particle size distribution range of 30-80 nm.

7. The method for fabricating a stacked structure of a flip chip according to claim 4, characterized in that, The underfill adhesive is an epoxy capillary underfill material with a viscosity of 300-500 mPa·s at 25°C, a glass transition temperature ≥120°C after curing, and a coefficient of thermal expansion ≤35ppm / °C.

Images