Indium-based composite thermal interface material and preparation method and packaging application thereof

Abstract

The invention discloses a preparation method of an indium-based composite thermal interface material. According to the preparation method, a three-dimensional porous metal framework is filled with metal indium by adopting an electroplating process. The three-dimensional porous metal framework is used as a reinforcement body, the pores of the framework are filled with the metal indium by adopting an electroplating process to form a composite structure in which the metal indium and the three-dimensional porous metal framework are tightly combined, and the structure can effectively solve the pumping-out problem of a traditional indium prefabricated sheet TIM material in the repeated backflow process; the breaking strength of the indium TIM material body is improved, the effective service life of the indium TIM material is prolonged, and the interface stability of the indium TIM material is improved. The invention further discloses the indium-based composite thermal interface material and packaging application.

Description

Technical Field

[0001] This invention relates to the field of thermal interface materials technology, specifically to an indium-based composite thermal interface material, its preparation method, and its encapsulation application. Background Technology

[0002] With the rapid development of cutting-edge technologies such as 5G, artificial intelligence (AI), and high-performance computing (HPC), integrated circuit chips are evolving towards higher power density and higher operating frequencies. The ensuing challenge is the dramatic increase in chip power consumption. If the large amount of heat generated cannot be dissipated in a timely and efficient manner, it will cause the chip junction temperature to rise rapidly, leading to performance degradation, signal integrity deterioration, and even permanent damage, severely restricting system performance and reliability. Therefore, advanced thermal management technology has become one of the key bottlenecks in the development of the electronics industry.

[0003] In the heat dissipation path of chip packaging, thermal interface materials (TIMs) fill the tiny gaps between the chip and the heat sink (such as heat spreaders, vapor chambers, or fins) to replace residual air (air's thermal conductivity is only ~0.026 W / m·K), establishing an efficient heat conduction channel. The performance of the TIM directly determines the efficiency of the entire heat dissipation system, and its importance is self-evident. An ideal thermal interface material needs to simultaneously possess high thermal conductivity, low thermal resistance, good flexibility to conform to uneven surfaces, excellent long-term stability, and reliable mechanical properties.

[0004] Indium (In) is considered a candidate material for high-performance thermal interface materials (TIMs) due to its unique physicochemical properties. Specifically, indium has a thermal conductivity (approximately 86 W / m·K) that is much higher than that of conventional thermal greases and polymer-based TIMs; it is soft and highly malleable, capable of plastic deformation under low pressure, fully filling the microscopic unevenness between interfaces and achieving close contact; in addition, indium has a low melting point (156.6℃), which allows for reflow soldering under relatively mild process conditions (such as 210℃ used in this experiment) to form a metallurgical bond, thereby achieving extremely low interfacial contact thermal resistance.

[0005] Despite the above advantages, traditional indium preforms also exhibit many inherent defects in practical applications: (1) Secondary pumping effect, leading to uneven initial thickness of the thermal interface material during packaging, edge contamination, and even short circuits; irreversible thinning of the thermal interface material layer during device service, continuous increase in interface thermal resistance, and local overheating. (2) Insufficient mechanical strength and creep resistance, resulting in body fracture, interface cracking, etc., causing increased interface thermal resistance, deterioration of heat dissipation performance, and ultimately affecting product lifespan. (3) Fluctuations in interface bonding reliability, causing the indium layer to shrink and coalesce, forming a discontinuous island distribution; insufficient coverage will directly generate local hot spots, leading to increased thermal resistance. All of the above defects ultimately affect the lifespan and stability of the device.

[0006] To overcome the weaknesses of pure indium thermal interface materials (TIM), the industry has tried various improvement schemes. One is to develop indium-based alloys. Although this method can improve the strength to a certain extent, it often comes at the cost of sacrificing thermal conductivity and low-temperature welding performance. Another approach is to prepare composite materials, but these existing technologies also have inherent defects that are difficult to overcome, such as: (1) Composite materials using the "melt infiltration method" rely on high-temperature processes to melt solid indium and infiltrate it into a porous framework. This approach leads to three key defects: First, the interface bonding quality is inherently poor. Liquid indium and the framework mainly rely on physical wetting and mechanical interlocking, resulting in weak bonding force. Under thermomechanical stress, it is prone to become the starting point of failure. Second, the high-temperature environment causes indium to react with the framework metal to form brittle intermetallic compounds, which seriously damages the interface toughness. Third, intrinsic defects are difficult to avoid. The flow characteristics and solidification shrinkage behavior of molten metal make it easy to generate unfilled voids and shrinkage cavities in complex porous structures. These defects become bottlenecks in heat flow transmission and weak links in mechanical properties. Finally, the integrity of the material and interface is easily damaged. The high-temperature process exacerbates the oxidation and evaporation of the metal, which not only affects the intrinsic properties of the material but also significantly deteriorates the surface condition before welding. (2) The composite material using "electrodeposition microporous layer + melt infiltration" solves the metal overflow problem of melt infiltration by constructing a physical barrier. However, this "patchwork" improvement introduces new technical obstacles: First, the increased structural complexity leads to increased thermal resistance. Its "sandwich" multilayer structure artificially introduces multiple additional solid-solid interfaces, which become phonon scattering centers, significantly increasing the overall thermal resistance of the system and violating the basic requirement of pursuing the ultimate thermal conductivity of thermal interface materials. Second, it cannot fundamentally solve the inherent defects of melt infiltration, such as weak interface bonding and poor filling density. On the contrary, the dense characteristics of the microporous layer may hinder the full contact between the phase change metal and the external welding interface. In summary, these existing technologies either suffer from poor intrinsic material quality due to outdated process principles or limited thermal performance and reliability due to redundant structural design. They all fail to systematically meet the comprehensive requirements of high strength, high thermal conductivity, and high reliability for thermal interface materials in high-power chip packaging. Summary of the Invention

[0007] In view of this, the purpose of this application is to provide an indium-based composite thermal interface material, its preparation method and packaging application, in order to solve the problems of low mechanical strength, unstable interface bonding reliability and poor creep resistance of pure indium preforms as thermal interface materials in the prior art.

[0008] In a first aspect, this application provides a method for preparing an indium-based composite thermal interface material, which uses an electroplating process to fill metallic indium into a three-dimensional porous metal framework.

[0009] Optionally, the metal in the three-dimensional porous metal framework includes copper, nickel, silver, aluminum and their alloys; the structural morphology of the three-dimensional porous metal framework includes foam metal, fiber metal, metal sponge, metal felt, mesh metal and sintered porous metal.

[0010] Preferably, the three-dimensional porous metal skeleton is foamed copper or 3D fiber copper.

[0011] Optionally, the electroplating process uses an electroplating solution comprising 95-115 g / L indium sulfamate, 40-50 g / L sodium chloride, 1.0-3.0 g / L glucose, and 0.3-0.7 g / L triethanolamine; the current density is 10-20 mA / cm², preferably 15 mA / cm².

[0012] Optionally, the electroplating process uses a control voltage of 0.7V-0.85V.

[0013] Optionally, the electroplating process includes stirring at a speed of 1500-4000 rpm, preferably 2000 rpm.

[0014] Optionally, the electroplating process further includes: pretreatment of the three-dimensional porous metal skeleton, the pretreatment including acid activation and vacuum pre-immersion;

[0015] The acid activation includes immersing the three-dimensional porous metal skeleton in an acidic solution with a volume concentration of 10-15% for 3-5 minutes, followed by rinsing with deionized water with a conductivity of less than or equal to 1 μS / cm.

[0016] The vacuum pre-impregnation process involves immersing the acid-activated three-dimensional porous metal skeleton in a 5% aminosulfonic acid solution under a vacuum pressure of less than or equal to -0.1 MPa for 10-20 minutes.

[0017] The preparation method provided in the application embodiments uses a three-dimensional porous metal framework as a carrier. An electroplating process under an applied electric field causes indium ions to gain electrons on the pore walls of the three-dimensional porous metal framework, reducing them to metallic indium atoms, which then deposit. This achieves a sealing and filling effect, where metallic indium is infiltrated and fixed within the three-dimensional porous metal framework. This preparation method possesses at least the following advantages: First, because the electroplating process allows for uniform deposition without dead zones, the metallic indium filler is dense and free of shrinkage cavities, forming a complete and continuous three-dimensional interpenetrating network structure with the framework. This maximizes the thermal conductivity and mechanical uniformity of the indium-based thermal interface material. Second, the electroplating process is typically carried out at room temperature, avoiding problems such as intermetallic compound formation, material oxidation, and grain coarsening caused by high temperatures. This facilitates the preparation of purer, more stable, and controllable indium-copper interface materials. Furthermore, by employing electroplating, indium atoms can directly grow epitaxially on the lattice of the three-dimensional porous metal framework or form strong metallic bonds. This metallic bonding force is far superior to physical adhesion. When the thermal interface material obtained using this preparation method is used, it can effectively resist thermal fatigue stress, effectively prevent interface cracking and debonding, and improve the effective service life and interface stability of the composite material. Finally, the three-dimensional porous metal framework itself possesses high porosity and interconnected channels, providing an ideal structural basis for indium filling and mechanical interlocking.

[0018] Secondly, this application provides an indium-based composite thermal interface material, which is obtained by the preparation method of the indium-based composite thermal interface material described in any one of the first aspects above.

[0019] Furthermore, the indium-based composite thermal interface material includes metallic indium and a three-dimensional porous metal framework, wherein the metallic indium and the three-dimensional porous metal framework are connected by chemical bonds; the volume ratio of the metallic indium to the three-dimensional porous metal framework is 1:1 to 4:1.

[0020] The indium-based composite thermal interface material provided in this application possesses at least the following advantages: First, the three-dimensional porous metal skeleton, acting as a rigid and incompressible "cage," mechanically confines the movement of metallic indium within the skeleton's pores, fundamentally preventing the lateral plastic flow and pumping out of molten or softened metallic indium from the outside, effectively solving the problems of "secondary pumping out" and creep. Second, the high-strength metal skeleton bears most of the mechanical load, making the overall strength and modulus of the composite material far higher than that of pure indium, thus improving the material's bulk strength. Furthermore, the metallic indium and the three-dimensional porous metal skeleton in this indium-based composite thermal interface material are bonded by atomic-scale metallic bonds. This metallic bonding force is far greater than physical adhesion, enabling the thermal interface material to effectively resist thermal fatigue stress, effectively prevent interface cracking and debonding, and improve the effective service life and interface stability of the composite material.

[0021] Optionally, this application also provides the application of the indium-based composite thermal interface material obtained by the above preparation method as a packaging solder.

[0022] This invention provides an encapsulation solder, comprising the indium-based composite thermal interface material described in any one of the above technical solutions.

[0023] Thirdly, this application also provides a packaging process for an indium-based composite thermal interface material, wherein the indium-based composite thermal interface material is prepared using any of the preparation methods in the first aspect above and used as a packaging solder, comprising the following steps:

[0024] Surface metallization of a semiconductor device includes the sequential deposition of an "adhesion-barrier-welding" metal stack on the surface of the semiconductor device; the material of the adhesion layer includes at least one of Ti, Cr, Ta, W, and Mo; the material of the barrier layer includes at least one of Ni, Pd, Pt, and NiV alloys; and the material of the weld layer includes at least one of Au and Ag.

[0025] The surface pretreatment of the welding interface includes cleaning and plasma activation of the surface of the semiconductor device, the surface of the cover plate, and the surface of the encapsulation solder after surface metallization;

[0026] The encapsulation assembly involves placing the encapsulation solder between the soldering surfaces of the semiconductor device and the cover plate, applying a contact pressure of 20-45000 Pa to the back of the semiconductor device and / or the cover plate, heating and reflowing in a vacuum environment, and then cooling to complete the encapsulation.

[0027] Optionally, the cleaning process includes sequentially cleaning with ethanol, a 5% (v / v) acidic solution, and deionized water, followed by drying.

[0028] The plasma activation uses argon gas as the medium, with a power of 50-300W, a flow rate of 20-400 sccm, and a time of 1-5min.

[0029] Optionally, the vacuum level of the vacuum environment is 20-200 Pa.

[0030] Optionally, the reflux temperature of the heating and reflux is 180-220℃, preferably 210℃, and the reflux time is 5-30 min, preferably 10-20 min.

[0031] The encapsulation application of the indium-based composite thermal interface material provided in this application, because it uses the indium-based composite thermal interface material prepared in the first aspect above as the encapsulation solder, possesses all the excellent properties of the indium-based composite thermal interface material described above. Furthermore, the encapsulation application provided in this application employs plasma activation and vacuum welding processes, ensuring a perfect bond between the composite material and the interface, achieving a coverage rate of up to 94%, thereby minimizing interface thermal resistance. Attached Figure Description

[0032] To more clearly illustrate the technical solutions in the embodiments of this application or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are only embodiments of this application. For those skilled in the art, other drawings can be obtained based on the provided drawings without creative effort.

[0033] Figure 1 The diagram shows the structure of the copper-indium foam composite material provided in Example 1, where (a) is a photograph of the copper-indium foam composite material, (b) shows the surface microstructure under an optical microscope, and (c) shows the cross-sectional SEM microstructure.

[0034] Figure 2 The diagram shows the structure of the 3D fiber copper-indium composite material provided in Example 2; where (a) is a photograph of the 3D fiber copper-indium composite material, and (b) and (c) show the surface microstructure under optical microscopes at different magnifications.

[0035] Figure 3 This is a comparison chart of the mechanical properties of the foamed copper-indium and 3D fiber copper-indium composite thermal interface materials obtained in Examples 1 and 2 with those of conventional indium preform thermal interface materials;

[0036] Figure 4 This application provides a schematic diagram of a packaging structure.

[0037] Figure 5 This is a comparison chart of the interface coverage in the packaging of Embodiment 3 and Comparative Example 1 of this application;

[0038] Figure 6 A comparison chart showing the thermal resistance values ​​of foamed copper-indium, 3D fiber copper-indium, and pure indium preforms in packaging applications. Detailed Implementation

[0039] The technical solutions of the embodiments of this application will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of this application, and not all embodiments. Based on the embodiments of this application, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of this application.

[0040] Research has revealed that "secondary pumping out" and creep in pure indium preforms refer to the phenomenon where molten indium is continuously squeezed out from the interface gap between the chip and the heat sink during packaging manufacturing and service. This is a dynamic, irreversible, and continuously escalating reliability failure process. Its possible failure mechanisms and resulting defects are roughly as follows:

[0041] 1. Failure Mechanism and Process

[0042] Thermomechanical fatigue drives electronic devices to undergo power cycles during power-on / off or load changes, resulting in repeated temperature fluctuations in the chip. Due to the severe mismatch in the coefficients of thermal expansion of the chip (e.g., silicon, CTE ~2.6 ppm / ℃), indium (CTE ~33 ppm / ℃), and heat sink (e.g., copper, CTE ~17 ppm / ℃), asynchronous expansion and contraction occur in each thermal cycle. This periodic relative motion creates a "pumping" effect on the soft indium layer sandwiched in between, acting like a miniature pump that pushes and extrudes molten or softened indium from the high-pressure central region to the lower-pressure edge region.

[0043] Intrinsic properties of indium: Pure indium has a melting point of only 156.6℃. At chip operating temperatures (e.g., 80-150℃), its strength decreases sharply, exhibiting superplasticity or even localized melting. This near-fluid behavior makes it unable to resist shear stress caused by CTE mismatch, and it is extremely prone to plastic flow.

[0044] 2. Direct consequences

[0045] As the TIM layer thickness continues to decrease, thermal resistance rises, leading to chip breakage: With the continuous pumping out of indium, the effective thickness of the TIM layer irreversibly decreases. According to the basic thermal resistance formula R = L / (kA), where L is the thickness and k is the thermal conductivity, the thermal resistance R is directly proportional to the thickness L. The thinning of the TIM layer directly results in a continuous increase in interface thermal resistance, a gradual decline in heat dissipation efficiency, and a continuous rise in the chip junction temperature throughout its lifespan, ultimately causing performance degradation or overheating damage.

[0046] Localized drying and hot spots can occur because the pumping process is not uniform, and is typically more severe in areas where the structure is weakest or where pressure is uneven. This can lead to excessive thinning of the TIM layer in localized areas, or even detachment from the interface, creating "dry zones." The heat flow channels in these areas are blocked, instantly generating extremely high localized hot spots, posing a fatal threat to chip reliability.

[0047] Risk of short circuits: The pumped-out indium can form indium beads or whiskers around the chip or substrate. If these conductive materials migrate to nearby precision circuits or pins, they can easily cause electrical short circuits, leading to the functional failure of the entire component.

[0048] Loss of interfacial contact pressure creates a vicious cycle: Initial interfacial contact pressure is crucial for ensuring good heat conduction. As indium is lost, the interfacial filler decreases, and the contact pressure gradually relaxes. This pressure drop further weakens the thermal contact at the interface and reduces the constraint on the remaining indium, thereby accelerating the subsequent pumping process and forming a continuously worsening positive feedback loop.

[0049] Long-term reliability is not guaranteed: "Secondary pumping out" is a damage process that accumulates over time. This means that even if a device using a pure indium preform performs well in initial testing, it cannot be guaranteed to continue to dissipate heat effectively after months or years of actual use. This unpredictable long-term reliability is the fundamental reason why its application in high-end, long-lifecycle products (such as servers, communication equipment, and automotive electronics) is strictly limited.

[0050] "Secondary pumping out" is a systemic failure inevitably caused by the combined effect of the low melting point and high ductility of pure indium preforms and the thermal mismatch of the packaging structure. It is not an accidental phenomenon, but an inevitable result of its material properties under specific operating conditions, fundamentally restricting the application of pure indium preforms in high-reliability heat dissipation scenarios.

[0051] Furthermore, pure indium has extremely low yield strength (typically <10 MPa) and a very low Young's modulus. This makes it prone to creep under sustained pressure and easily deformed under mechanical vibration or impact, making it unable to maintain a stable interface structure and pressure. Moreover, when creep and other factors cause imperfect bonding between pure indium and the chip / cover plate interface, resulting in microscopic voids, a huge interfacial contact thermal resistance is generated, becoming a heat dissipation bottleneck.

[0052] This application innovatively introduces a three-dimensional porous metal framework as a support and fixing carrier for indium metal, and uses an electroplating process to fill the pores of the framework with indium metal. This alters and enhances the interaction properties and filling density between indium metal and the three-dimensional porous metal framework, forming a composite structure in which indium metal and the three-dimensional porous metal framework are tightly bonded. This structure effectively solves the pumping problem during multiple reflow processes in the application of traditional indium preform TIM materials, improves the fracture strength of the indium TIM material itself, and extends the effective service life and interface stability of the indium TIM material.

[0053] Using a three-dimensional porous metal framework as a carrier, an electroplating process under an applied electric field causes indium ions to gain electrons on the pore walls of the framework, reducing them to metallic indium atoms, which then deposit. This achieves a sealing and filling effect, with metallic indium permeating and fixing into the three-dimensional porous metal framework. This preparation method possesses at least the following advantages: First, because the electroplating process allows for uniform deposition without dead zones, the metallic indium filler is dense and free of shrinkage cavities, forming a complete and continuous three-dimensional interpenetrating network structure with the framework. This maximizes the thermal conductivity and mechanical uniformity of the indium-based thermal interface material. Second, the electroplating process is usually carried out at room temperature, avoiding problems such as intermetallic compound formation, material oxidation, and grain coarsening caused by high temperatures. This facilitates the preparation of purer, more stable, and controllable indium-copper interface materials. Furthermore, by employing electroplating, indium atoms can directly grow epitaxially on the lattice of the three-dimensional porous metal framework or form strong metallic bonds. This metallic bonding force is far superior to physical adhesion. When the thermal interface material obtained using this preparation method is used, it can effectively resist thermal fatigue stress, effectively prevent interface cracking and debonding, and improve the effective service life and interface stability of the composite material. Finally, the three-dimensional porous metal framework itself possesses high porosity and interconnected channels, providing an ideal structural basis for indium filling and mechanical interlocking.

[0054] Specifically, the three-dimensional porous metal framework is composed of metals or alloys with high thermal conductivity and high strength. Its structural forms include, but are not limited to, foamed metals, fiber metals, metal sponges, metal felts, mesh metals, and sintered porous metals. The preferred metal materials are copper, silver, nickel, aluminum, and their alloys; among them, foamed copper or fiber copper with a three-dimensional interconnected network structure is the most preferred in terms of overall performance. The synergistic combination of these specific structures and high thermal conductivity metals provides an ideal foundation for the uniform filling and robust composite of indium, jointly ensuring the high thermal conductivity and excellent mechanical properties of the composite thermal interface material. By mechanically locking the metallic indium within the rigid framework, the path for its macroscopic plastic flow and pumping out is fundamentally cut off.

[0055] Optionally, one of the key aspects of the electroplating process is the selection of the plating solution formulation. The electroplating solution used in this application includes indium sulfamate 95-115 g / L, sodium chloride 40-50 g / L, glucose 1.0-3.0 g / L, and triethanolamine 0.3-0.7 g / L.

[0056] Specifically, indium sulfamate, as the source of indium ions, is reduced to metallic indium during the electroplating process and deposited on the pore wall surface of the three-dimensional porous metal framework. Because electroplating is an ion-to-atom level deposition, indium atoms directly epitaxially grow on the crystal lattice of the three-dimensional porous metal framework or form strong metallic bonds. The strength of the metallic bonds formed by this in-situ synthesis is much higher than that of physical adhesion. Furthermore, the concentration of indium sulfamate can be selected from 100 g / L, 105 g / L, 110 g / L, etc. Sodium chloride, as a conductive salt, can increase the conductivity of the solution and improve current efficiency. It can also improve the dispersion ability of the plating solution, making the electroplated deposition distribution more uniform. Furthermore, the concentration of sodium chloride can be selected from 41 g / L, 42 g / L, 43 g / L, 44 g / L, 45 g / L, 46 g / L, 47 g / L, 48 g / L, 49 g / L, etc. Glucose can act as a reducing protectant, preventing indium ions from oxidizing and causing plating instability and a rough coating. Because it can adsorb onto the surface of a three-dimensional porous metal framework, it can improve cathodic polarization and refine the coating crystals. Furthermore, the concentration of glucose can be selected at 1.5 g / L, 2.0 g / L, 2.5 g / L, etc. Triethanolamine can form stable complexes with indium ions (such as [In(TEA)]⁺), slowing down the deposition rate, improving cathodic polarization, and making the coating denser and smoother. Furthermore, the concentration of triethanolamine can be selected at 0.4 g / L, 0.5 g / L, 0.6 g / L, etc.

[0057] Another key factor affecting the electroplating process is the precise control of process parameters. In this application, a constant current mode is adopted, and the current density is maintained in the range of 10-20 mA / cm². Specifically, 11 mA / cm², 12 mA / cm², 13 mA / cm², 14 mA / cm², 15 mA / cm², 16 mA / cm², 17 mA / cm², 18 mA / cm², 19 mA / cm², etc. can be selected; preferably, 15 mA / cm² is preferred.

[0058] Optionally, the control voltage used in the electroplating process is 0.7V-0.85V.

[0059] Optionally, the electroplating process includes stirring at a speed of 1500-4000 rpm, preferably 2000 rpm. Specifically, the stirring speed can be 1500 rpm, 1700 rpm, 1900 rpm, 2100 rpm, 2300 rpm, 2500 rpm, 2700 rpm, 2900 rpm, 3100 rpm, 3300 rpm, 3500 rpm, 3700 rpm, 3900 rpm, or 4000 rpm.

[0060] Stirring during the electroplating process ensures that the plating solution flows sufficiently within the complex porous structure, thereby achieving dense and uniform deposition of indium.

[0061] Optionally, the electroplating process further includes: pretreatment of the three-dimensional porous metal skeleton, the pretreatment including acid activation and vacuum pre-immersion;

[0062] The acid activation includes immersing the three-dimensional porous metal skeleton in an acidic solution with a volume concentration of 10-15% for 3-5 minutes, followed by rinsing with deionized water with a conductivity of less than or equal to 1 μS / cm.

[0063] The vacuum pre-impregnation process involves immersing the acid-activated three-dimensional porous metal skeleton in a 5% aminosulfonic acid solution under a vacuum pressure of less than or equal to -0.1 MPa for 10-20 minutes.

[0064] Specifically, the acidic solution can be at least one of hydrochloric acid, sulfuric acid, or sulfonic acid. Acid activation ensures that the pH of the base metallized surface remains acidic, preventing oxide recombination and protecting the indium sulfamate plating bath from the infiltration of activating chemicals. Vacuum pre-dip treatment effectively removes bubbles and impurities from within the three-dimensional porous metal framework, preventing bubbles from remaining inside the three-dimensional porous metal framework and the plating layer during electroplating, thus avoiding plating defects.

[0065] An indium-based composite thermal interface material can be obtained using any of the above selective preparation methods. The material comprises metallic indium and a three-dimensional porous metal framework, with the indium and framework connected by chemical bonds. The volume ratio of indium to the three-dimensional porous metal framework in this composite thermal interface material is 1:1 to 4:1. Specifically, it can be 1:1, 2:1, 3:1, or 4:1.

[0066] This invention provides the application of indium-based composite thermal interface materials obtained by any of the above preparation methods as encapsulation solders.

[0067] The present invention also provides an encapsulation solder, comprising the indium-based composite thermal interface material described in any one of the above technical solutions.

[0068] This invention provides a packaging process for an indium-based composite thermal interface material, comprising the following steps:

[0069] A) Surface metallization of semiconductor devices: This includes the sequential deposition of an "adhesion-block-weld" metal stack on the surface of semiconductor devices;

[0070] The material of the adhesion layer includes at least one of Ti, Cr, Ta, W, and Mo; the material of the barrier layer includes at least one of Ni, Pd, Pt, and NiV alloys; and the material of the welding layer includes at least one of Au and Ag.

[0071] B) Surface pretreatment of the welding interface: including cleaning and plasma activation of the surface of the semiconductor device, the cover plate, and the encapsulation solder after surface metallization;

[0072] C) Welding assembly: The encapsulation solder described in the above technical solution is placed between the welding surfaces of the semiconductor device and the cover plate. A contact pressure of 20-45000Pa is applied to the back of the semiconductor device and / or the cover plate. The device is heated and reflowed in a vacuum environment and then cooled to complete the encapsulation.

[0073] Specifically, semiconductor devices include, but are not limited to, high-performance computing chips, power electronics, lasers, and radio frequency power amplifiers.

[0074] The encapsulation application of the indium-based composite thermal interface material provided in this application, because it uses the indium-based composite thermal interface material obtained by the above-described preparation method as the encapsulation solder, possesses all the excellent properties of the indium-based composite thermal interface material described above. Furthermore, the encapsulation application provided in this application employs plasma activation and vacuum welding processes, ensuring a perfect bond between the composite material and the interface, achieving a coverage rate of up to 94%, thereby minimizing interface thermal resistance.

[0075] It should be noted that the contact pressure can also be provided by any pressure supply device with self-adjusting function in the prior art. As long as it can provide a stable and controllable pressure effect on the welding interface, it falls under the concept protected by this application.

[0076] The present invention first deposits an "adhesion-block-weld" metal stack on the surface of a semiconductor device in sequence; specifically, the sequence can be an adhesion layer, a block layer, and a weld layer.

[0077] Specifically, the material of the adhesion layer includes at least one of Ti, Cr, Ta, W, and Mo; the material of the barrier layer includes at least one of Ni, Pd, Pt, and NiV alloys; and the material of the welding layer includes at least one of Au and Ag.

[0078] Surface pretreatment of the welding interface includes cleaning and plasma activation of the surface of the semiconductor device, the cover plate, and the encapsulation solder after surface metallization. The cleaning process of this invention includes sequential cleaning with ethanol, a 5% (v / v) acidic solution, and deionized water, followed by drying. The acidic solution can be at least one of hydrochloric acid, sulfuric acid, or sulfonic acid. Specifically, ethanol removes oil and other organic impurities from the surface to be welded, the 5% (v / v) acidic solution removes oxides from the surface, and further cleaning with deionized water followed by drying with N2 achieves thorough cleaning of the surfaces of the semiconductor device, cover plate, and encapsulation solder to be welded.

[0079] The plasma activation uses argon gas as the medium, with a power of 50-300W, a flow rate of 20-400 sccm, and a time of 1-5 min. Specifically, the power can be 50W, 60W, 70W, 80W, 90W, 100W, 110W, 120W, 130W, 140W, 150W, 160W, 170W, 180W, 190W, 200W, 210W, 220W, 230W, 240W, 250W, 260W, 270W, 280W, 290W, or 300W. The flow rate can be specifically 20 sccm, 30 sccm, 40 sccm, 50 sccm, 60 sccm, 70 sccm, 80 sccm, 90 sccm, 100 sccm, 110 sccm, 120 sccm, 130 sccm, 140 sccm, 150 sccm, 160 sccm, 170 sccm, 180 sccm, 190 sccm, 200 sccm, 210 sccm, 220 sccm, 230 sccm, 240 sccm, 250 sccm, 260 sccm, 270 sccm, 280 sccm, 290 sccm, 300 sccm, 310 sccm, 320 sccm, 330 sccm, 340 sccm, 350 sccm, 360 sccm, 370 sccm, 380 sccm, 390 sccm, 400 sccm. The time can be 1 min, 2 min, 3 min, 4 min or 5 min.

[0080] This activation treatment can significantly improve the interfacial energy of the welding interface and promote wetting and bonding during subsequent reflow welding.

[0081] The welding assembly involves placing the encapsulation solder described in the above technical solution between the welding surfaces of the semiconductor device and the cover plate, applying a contact pressure of 20-45000Pa to the back of the semiconductor device and / or the cover plate, heating and reflowing in a vacuum environment, and then cooling to complete the encapsulation. Specifically, the contact pressure is 20 Pa, 100 Pa, 1100 Pa, 2100 Pa, 3100 Pa, 4100 Pa, 5100 Pa, 6100 Pa, 7100 Pa, 8100 Pa, 9100 Pa, 10100 Pa, 11100 Pa, 12100 Pa, 13100 Pa, 14100 Pa, 15100 Pa. Pa, 16100 Pa, 17100 Pa, 18100 Pa, 19100 Pa, 20100 Pa, 21100 Pa, 22100 Pa, 23100 Pa, 24100 Pa, 25100 Pa, 26100 Pa, 27100 Pa, 28100 Pa, 29100 Pa, 30100 Pa, 31100 Pa、32100 Pa、33100 Pa、34100 Pa, 35100 Pa, 36100 Pa, 37100 Pa, 38100 Pa, 39100 Pa, 40100 Pa, 41100 Pa, 42100 Pa, 43100 Pa, 44100 Pa, 45100 Pa.

[0082] The vacuum level of the vacuum environment is 20-200 Pa; specifically, it can be 20 Pa, 30 Pa, 40 Pa, 50 Pa, 60 Pa, 70 Pa, 80 Pa, 90 Pa, 100 Pa, 110 Pa, 120 Pa, 130 Pa, 140 Pa, 150 Pa, 160 Pa, 170 Pa, 180 Pa, 190 Pa, or 200 Pa. Vacuum welding prevents air from entering and remaining in the welding interface and inside the cooled encapsulation interface, thus avoiding defects in the welding layer.

[0083] The reflux temperature for the heating and reflux process is 180-220℃; specifically, it can be 180℃, 190℃, 200℃, 210℃, or 220℃.

[0084] Reflow time is 5-30 min. Specifically, it can be 5 min, 7 min, 9 min, 11 min, 13 min, 15 min, 17 min, 19 min, 21 min, 23 min, 25 min, 27 min, or 29 min. Specifically, the welding heat is slowly increased from room temperature (25°C) to a reflow temperature of 210°C over a heating time of approximately 50-80 min, and held at this temperature for 5-20 minutes. This heating operation allows the indium on the surface of the composite material and some of the indium in the pores to fully melt and flow, forming a strong metallurgical bond with the interface metal.

[0085] The indium-based composite thermal interface material provided in this application possesses at least the following advantages: First, the three-dimensional porous metal skeleton, acting as a rigid, incompressible "cage," has a large specific surface area and strong capillary attraction, mechanically constraining the movement of metallic indium within the skeleton's pores. This fundamentally cuts off the lateral plastic flow and pumping out of molten or softened metallic indium from external sources, effectively solving the problems of "secondary pumping out" and creep. Second, the high-strength metal skeleton bears most of the mechanical load, resulting in an overall strength and modulus of the composite material that are far higher than those of pure indium, thus enhancing the material's bulk strength. Furthermore, the metallic indium and the three-dimensional porous metal skeleton in this indium-based composite thermal interface material are bonded by atomic-scale metallic bonds. This metallic bonding force is far greater than physical adhesion, enabling the thermal interface material to effectively resist thermal fatigue stress, effectively prevent interface cracking and debonding, optimize interface thermal resistance, and improve interface stability.

[0086] The preparation method and excellent effects of this application are illustrated in detail below through two specific embodiments.

[0087] Example 1

[0088] In this embodiment, a three-dimensional porous metal skeleton copper foam with a size of 87mm*45mm and a thickness of 80µm is first subjected to electroplating pretreatment, which includes acid activation and vacuum pre-immersion.

[0089] The acid activation process involves immersing the copper foam in a 15% sulfuric acid solution for 3 minutes to obtain the cathode workpiece, followed by rapid rinsing with deionized water with a conductivity of 0.07 μS / cm to complete the acid activation of the cathode workpiece.

[0090] After pickling and activation, the cathode workpiece is immersed in a 5% aminosulfonic acid solution for vacuum pre-immersion. The vacuum pressure is set to -0.1 MPa and the time is controlled at 20 minutes to ensure that the pH value of the base metallized surface remains acidic, prevent the recombination of oxides, and protect the indium aminosulfonic acid plating solution from the dragging in of activator chemicals, while removing bubbles and impurities inside the foamed copper.

[0091] Subsequently, a double-sided electroplating process is performed in a dedicated indium plating solution, comprising 106 g / L indium sulfamate, 45.8 g / L sodium chloride, 2 g / L glucose, and 0.57 g / L triethanolamine. The current density is maintained in the range of 10-20 mA / cm², preferably 15 mA / cm², while mechanical stirring at 1500-4000 rpm, preferably 2000 rpm, is applied to ensure sufficient flow of the plating solution within the complex porous structure, thereby achieving dense and uniform indium deposition. The control voltage for the electroplating process is typically between 0.7-0.85 V. After the indium deposition in the foamed copper pores is complete, a further deposition of a certain thickness of indium is performed on the surface of the foamed copper framework. This deposition thickness is precisely calculated using the deposition rate (approximately 0.5 μm / min) and the target thickness, and the electroplating time is controlled to be 170 min. After electroplating, the final foamed copper-indium composite thermal interface material is obtained through cleaning and drying.

[0092] like Figure 1 As shown, Figure 1 This is a schematic diagram of the structure of the copper-indium foam composite material obtained in this embodiment. (a) is a photograph of the copper-indium foam composite material, (b) shows its surface microstructure under an optical microscope, and (c) shows its cross-sectional SEM microstructure, clearly showing that metallic indium is fully filled into the three-dimensional network pores of the copper-indium foam skeleton. The metallic indium is uniformly deposited and completely filled into the pores of the skeleton through an electroplating process, while forming a continuous indium layer on the two main outer surfaces of the skeleton, ultimately resulting in an overall double-sided total thickness of 250 micrometers for the copper-indium foam composite material.

[0093] Example 2

[0094] In this embodiment, the basic operation is the same as in Embodiment 1 above; specifically:

[0095] First, a three-dimensional porous metal skeleton (3D fiber copper) measuring 87mm*45mm and 20µm thick was subjected to pretreatment before electroplating. The pretreatment included acid activation and vacuum pre-immersion. Acid activation involved immersing the 3D fiber copper in a 10% hydrochloric acid solution for 5 minutes to obtain the cathode workpiece, followed by rapid rinsing with deionized water (0.07μS / cm) to complete the acid activation. The activated cathode workpiece was then immersed in a 5% aminosulfonic acid solution for vacuum pre-immersion at a vacuum pressure of -0.1MPa for 10 minutes. This ensured the pH of the base metallized surface remained acidic, preventing oxide recombination and protecting the indium aminosulfonic acid plating bath from the infiltration of activating chemicals. Vacuum pre-immersion effectively removed air bubbles and impurities from the 3D fiber copper, preventing residual air bubbles within the 3D fiber copper and the plating layer during electroplating, which could lead to plating defects.

[0096] Subsequently, a double-sided electroplating process is performed in a dedicated indium plating solution, comprising 115 g / L indium sulfamate, 50 g / L sodium chloride, 3 g / L glucose, and 0.7 g / L triethanolamine. The current density is maintained within the range of 15-20 mA / cm², while mechanical stirring at 2000-3000 rpm is employed to ensure sufficient flow of the plating solution within the complex porous structure, thereby achieving dense and uniform indium deposition. The control voltage for the electroplating process is typically between 0.75-0.80 V. After the deposition of metallic indium in the 3D fiber copper voids is complete, a further deposition of metallic indium of a certain thickness is carried out on the surface of the foam copper skeleton. This deposition thickness is precisely calculated using the deposition rate (approximately 0.5 μm / min) and the target thickness, and the electroplating time is controlled to be 230 min. After electroplating, cleaning and drying yield the final 3D fiber copper-indium composite thermal interface material.

[0097] like Figure 2 As shown, Figure 2 This is a schematic diagram of the structure of the 3D fiber copper-indium composite material obtained in this embodiment. (a) is a photograph of the 3D fiber copper-indium composite material, and (b) and (c) show its surface microstructure under different magnification optical microscopes. It can be clearly seen that metallic indium is fully filled into the three-dimensional network pores of the foamed copper skeleton. The metallic indium is uniformly deposited and completely filled into the pores of the skeleton through an electroplating process, while a continuous indium layer is formed on the two main outer surfaces of the skeleton, ultimately resulting in a total double-sided thickness of 250 micrometers for the 3D fiber copper-indium composite thermal interface material.

[0098] Mechanical performance analysis and comparison: Figure 3 The graph shows a comparison of the mechanical properties of the foamed copper-indium and 3D fiber copper-indium composite thermal interface materials obtained in Examples 1 and 2 with those of conventional indium preforms (commercially available from Indium Corporation, USA). Figure 3As shown, the mechanical properties of foamed copper-indium and 3D fiber copper-indium composite thermal interface materials are significantly improved compared to pure indium preforms. The maximum tensile strengths of the foamed copper-indium and 3D fiber copper-indium composite thermal interface materials are 13.91 MPa and 12.60 MPa, respectively, which are much higher than the 2.42 MPa of pure indium preforms. More importantly, the elastic modulus (Young's modulus) of the foamed copper-indium and 3D fiber copper-indium composite thermal interface materials are as high as 2203.68 MPa and 1333.39 MPa, respectively, which are much higher than the 98.26 MPa of pure indium preforms. This means that the indium-based composite thermal interface materials prepared by the method of this application have better resistance to deformation and can maintain the stability of the interface morphology under external force or long-term thermal stress, effectively overcoming the fatal flaw of pure indium's easy creep, thereby ensuring the high efficiency and reliability of the heat dissipation interface throughout the product life cycle.

[0099] The following describes the encapsulation application and superior performance of the indium-based composite thermal interface material of this application, using a specific embodiment and a comparative example, to illustrate a reflow soldering method for achieving efficient thermal connection between a chip and a heat sink (such as a cover plate).

[0100] Example 3

[0101] like Figure 4 As shown, it illustrates a schematic diagram of a packaging structure where a composite thermal interface material is soldered between a chip and a heat sink. In this embodiment, the foamed copper-indium composite material obtained in Example 1 above is used as the packaging solder.

[0102] First, a 18mm*26mm chip is subjected to surface metallization treatment. Specifically, this involves sequentially depositing a Ti (50nm) / Ni (100nm) / Au (300nm) metal stack via magnetron sputtering. The titanium (Ti) layer serves as an adhesion layer, enhancing the bonding force between the subsequent metal layers and the diamond substrate. The nickel (Ni) layer acts as a barrier layer, preventing gold atoms from diffusing into the diamond interface while providing good surface flatness. The gold (Au) layer serves as a crucial wetting layer, and its excellent surface activity and good affinity with indium create ideal conditions for subsequent Au-In eutectic welding. Similarly, a 300nm Au layer is sputtered in the middle region of the copper-nickel cover plate (consistent with the chip size of 18mm*26mm) as a crucial wetting layer.

[0103] Secondly, this method requires rigorous surface pretreatment of the welding interface, namely, thorough cleaning and plasma activation of the chip (typically the back gold surface), the copper-nickel cover plate, and the foamed copper-indium composite thermal interface material of this invention. The cleaning steps include (sequentially using ethanol, deionized water, and finally drying with N2) and activation. The activation steps include hydrochloric acid pickling of the chip and argon plasma treatment of all welding components (composite material, cover plate, chip). Typical plasma activation parameters are 200W power, 300 sccm argon flow rate, and 2 minutes. This activation treatment significantly improves the interface energy, promoting wetting and bonding during subsequent reflow soldering.

[0104] Finally, after soldering assembly, the assembly is performed in a vacuum environment (vacuum degree 80-200 Pa), applying a stable adaptive contact pressure of 20-45000 Pa to the back of the chip. The soldering thermal profile involves slowly heating from room temperature (25°C) to a reflow temperature of 210°C over a heating time of approximately 70 minutes, and holding at this temperature for 10 minutes. This allows the indium on the surface of the composite material and some of the indium in the pores to melt and flow, forming a strong metallurgical bond with the interface metal. Subsequently, the material is cooled in a vacuum environment to complete the encapsulation.

[0105] Comparative Example 1

[0106] In this comparative example, a pure indium preform from the prior art is used as the encapsulation solder. Apart from this, the materials and encapsulation steps for other parts are the same as in Example 3. They will not be repeated here.

[0107] Analysis of the welding interface coverage revealed that the interface coverage of the copper-indium composite thermal interface material was more stable than that of the pure indium preform. Figure 5 A comparison chart of interface coverage in the package is shown for Example 3 and Comparative Example 1; as shown Figure 5 As shown, the copper-indium composite thermal interface material prepared using Example 1 of this invention exhibits consistently high interface coverage rates with both the cover plate (Lidto TIM) and the die (TIM to Die) after welding (reaching a maximum of 89.19% and 92.54%, respectively), significantly better than the coverage rate data of the comparative example—pure indium preform—and with smaller fluctuations. This demonstrates that the three-dimensional skeleton structure can effectively constrain the flow of molten indium, preventing its uneven shrinkage, thereby achieving more perfect and reliable interface filling and bonding.

[0108] Furthermore, to objectively quantify the heat dissipation performance of the composite thermal interface material of this invention under real working conditions, three fully encapsulated simulation test structures were constructed: foamed copper-indium, 3D fiber copper-indium, and pure indium prefabricated "chip-TIM-heat dissipation cover" (the construction method described in this invention can be fabricated according to any existing technology, such as the method in the literature Patel AH, Yogi K, Sahu G, et al. A Novel Multi-Chip Cooling System Using Direct-on-Chip Jet Impingement for High-Performance Interposer Package[C] / / 2024International 3D Systems Integration Conference (3DIC). IEEE, 2024: 1-6.), and the schematic diagram of the encapsulation structure is shown below. Figure 4 As shown, the overall thermal resistance of the structure was measured under steady-state conditions with a constant heating power of 10W. Figure 6 A comparative graph showing the thermal resistance values ​​of foamed copper-indium (TIM) in Example 1, 3D fiber copper-indium (TIM) in Example 1, and existing pure indium TIM preforms in packaging applications is presented. Test data shows that the overall thermal resistance of the traditional pure indium preform is 3.6982 K·cm² / W; in contrast, the overall thermal resistance of the foamed copper-indium and 3D fiber copper-indium composite materials prepared in this invention is significantly reduced to 2.5690 K·cm² / W and 2.5745 K·cm² / W, respectively, with reductions exceeding 30%. This comparison directly demonstrates that, under simulated actual power and packaging conditions, the composite material constructed by introducing a three-dimensional porous metal framework and a precision electroplating process can systematically reduce the overall thermal conductivity impedance from the chip to the heat sink. The significant reduction in thermal resistance is attributed to the additional efficient thermal pathways provided by the framework, the defect-free metallurgical bonding interface between indium and the framework, and the resulting ultra-high welding coverage with the chip and the heat sink. It not only fundamentally solves the mechanical reliability problem of pure indium materials, but also achieves a leap in performance in its core heat transfer function, providing high-power chips with an integrated thermal management solution that combines high reliability and excellent heat dissipation performance.

[0109] In summary, this application achieves metallurgical-grade indium filling on a three-dimensional porous metal framework using a low-temperature electroplating process, combined with a systematic packaging application method, fundamentally overcoming the limitations of existing technologies. This solution not only eliminates the problematic high-temperature melting process, avoiding the resulting interface reactions and filling defects, but also eliminates the thermal resistance problem caused by excess interfaces through an optimized homogeneous structure design. More importantly, the structural design of filling indium on a three-dimensional porous metal framework can leverage the large specific surface area and strong capillary suction of the framework to solve the pumping problem of traditional indium TIM. Filling in indium on the three-dimensional porous metal framework improves the bulk structural strength of the indium TIM, and the alloy-grade interface between the three-dimensional porous metal framework and electroplated indium achieves low thermal resistance. This invention organically combines material preparation and packaging application, forming a complete process closed loop, providing a systematic technical guarantee for achieving ultra-high interface coverage and high-reliability thermal management, and providing an effective solution to the heat dissipation problem of high-power-density chips.

Claims

1. A method for preparing an indium-based composite thermal interface material, characterized in that, Indium metal was filled into a three-dimensional porous metal framework using an electroplating process.

2. The preparation method according to claim 1, characterized in that, The metals in the three-dimensional porous metal framework include copper, nickel, silver, aluminum and their alloys; the structural forms of the three-dimensional porous metal framework include foam metal, fiber metal, metal sponge, metal felt, mesh metal and sintered porous metal.

3. The preparation method according to claim 2, characterized in that, The three-dimensional porous metal skeleton is made of foamed copper or 3D fiber copper.

4. The preparation method according to claim 1, characterized in that, The electroplating process uses an electroplating solution comprising 95-115 g / L indium sulfamate, 40-50 g / L sodium chloride, 1.0-3.0 g / L glucose, and 0.3-0.7 g / L triethanolamine; the current density is 10-20 mA / cm².

5. The preparation method according to claim 4, characterized in that, The electroplating process includes stirring, and the stirring speed is 1500-4000 rpm.

6. The preparation method according to claim 1, characterized in that, The electroplating process further includes: pretreatment of the three-dimensional porous metal skeleton, the pretreatment including acid activation and vacuum pre-immersion; The acid activation includes immersing the three-dimensional porous metal skeleton in an acidic solution with a volume concentration of 10-15% for 3-5 minutes, and then rinsing it with deionized water with a conductivity of less than or equal to 1 µS / cm. The vacuum pre-impregnation process involves immersing the acid-activated three-dimensional porous metal skeleton in a 5% aminosulfonic acid solution under a vacuum pressure of less than or equal to -0.1 MPa for 10-20 minutes.

7. An indium-based composite thermal interface material, characterized in that, It is obtained by the preparation method according to any one of claims 1-6.

8. The indium-based composite thermal interface material according to claim 7, characterized in that, It includes metallic indium and a three-dimensional porous metal framework, wherein the metallic indium and the three-dimensional porous metal framework are connected by chemical bonds; the volume ratio of the metallic indium to the three-dimensional porous metal framework is 1:1 to 4:

1.

9. The application of the indium-based composite thermal interface material obtained by the preparation method according to any one of claims 1-6 as an encapsulation solder.

10. A type of encapsulation solder, characterized in that, Including the indium-based composite thermal interface material as described in any one of claims 7 to 8.

11. A packaging process for an indium-based composite thermal interface material, characterized in that, Includes the following steps: A) Surface metallization of semiconductor devices: including the sequential deposition of an "adhesion-barrier-welding" metal stack on the surface of a semiconductor device; the material of the adhesion layer includes at least one of Ti, Cr, Ta, W, and Mo; the material of the barrier layer includes at least one of Ni, Pd, Pt, and NiV alloys; and the material of the weld layer includes at least one of Au and Ag. B) Surface pretreatment of the welding interface: including cleaning and plasma activation of the surface of the semiconductor device, the cover plate, and the encapsulation solder surface as described in claim 10 after surface metallization; C) Welding assembly: The encapsulation solder of claim 10 is placed between the welding surfaces of the semiconductor device and the cover plate. A contact pressure of 20-45000 Pa is applied to the back of the semiconductor device and / or the cover plate. The device is heated and reflowed in a vacuum environment and then cooled to complete the encapsulation.

12. The packaging process for the indium-based composite thermal interface material according to claim 11, characterized in that, The cleaning process includes sequentially cleaning with ethanol, a 5% (v / v) acidic solution, and deionized water, followed by drying. The plasma activation uses argon gas as the medium, with a power of 50-300W, a flow rate of 20-400 sccm, and a time of 1-5min.

13. The packaging process for the indium-based composite thermal interface material according to claim 11, characterized in that, The vacuum level of the vacuum environment is 20-200 Pa; the reflux temperature of the heating and reflux is 180-220℃; and the reflux time is 5-30 min.

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