Bi-sn-in low melting point alloy thermal interface material and preparation method thereof

Abstract

This invention belongs to the field of electronic packaging heat dissipation materials technology, and relates to a Bi-Sn-In system low-melting-point alloy thermal interface material and its preparation method. The material is composed of 53% Bi, 42% Sn, and 5% In by mass percentage. The preparation method includes: melting the raw materials at 300-350℃ under high-purity argon protection, followed by cooling at a rate of 5-10℃ / min to obtain a composite structure with a primary Bi-rich dendritic framework and an intergranular low-melting-point eutectic phase; then performing a low-temperature stabilization treatment at 100-110℃. This process enables the material to form a stable composite structure of "solid dendritic network + liquid eutectic phase" within the semi-solid working range, where the solid dendritic network provides mechanical support and inhibits pumping out, and the liquid eutectic phase ensures excellent thermal interface wetting and heat transfer performance. The material prepared by this method has a low melting point, high thermal conductivity, and good dimensional stability. The process is simple and suitable for the industrial production of heat dissipation interfaces for electronic devices.

Description

Technical Field

[0001] This invention relates to the field of thermal interface materials technology, specifically to a low-melting-point thermal interface material based on Bi-Sn-In alloys, and more particularly to a composite thermal interface material with a dendritic skeleton structure and its preparation method. Background Technology

[0002] With the rapid development of electronic devices towards miniaturization and high performance, the integration of electronic components is constantly increasing, leading to a significant increase in heat generation per unit area and making heat dissipation a more prominent issue. Thermal interface materials, as key materials connecting heat-generating components and heat dissipation devices, directly affect the heat dissipation efficiency and long-term reliability of electronic devices.

[0003] Low-melting-point alloy thermal interface materials undergo a solid-liquid phase transition at their phase transformation temperature, effectively filling interfacial gaps and reducing contact thermal resistance, while also possessing good thermal conductivity. Bi-Sn-In alloys are an important class of low-melting-point alloy materials. By adding an appropriate amount of In, the eutectic melting point can be further reduced and wettability improved, making them an ideal choice for thermal interface materials.

[0004] However, traditional low-melting-point alloy thermal interface materials have significant drawbacks: when the material reaches its phase transformation temperature, the low-melting-point phase melts into a liquid state. During long-term use, the liquid metal is prone to pumping out, meaning it is squeezed out of the interface. This leads to increased contact thermal resistance and decreased heat dissipation performance, severely limiting the service life and reliability of the thermal interface material. Especially in the semi-solid range of 140-160℃, the alloy exists in a solid-liquid coexistence state, resulting in a significant decrease in yield strength and an increased risk of pumping out. Summary of the Invention

[0005] To address the pumping problem in existing low-melting-point alloy thermal interface materials, this invention provides a Bi-Sn-In system low-melting-point alloy thermal interface material and its preparation method. By controlling the solidification conditions to obtain a continuous dendritic framework structure, and then improving the interface bonding through low-temperature stabilization treatment, the pumping phenomenon is effectively alleviated and the stability of the material in the semi-solid range is improved.

[0006] The Bi-Sn-In based low-melting-point alloy thermal interface material provided by this invention has the following composition by mass percentage: Bi 53%, Sn 42%, In 5%, possessing a composite structure of primary Bi-rich dendritic framework and intergranular low-melting-point eutectic phase. Referring to the Bi-Sn-In ternary phase diagram, the 5% In content maintains the Bi-rich primary phase as the main precipitated phase while lowering the eutectic phase melting point to approximately 116°C, achieving a reasonable separation between the framework melting temperature and the eutectic melting temperature, while simultaneously maintaining the Bi-rich primary phase as a high-temperature supporting framework.

[0007] DSC testing shows that the solidus temperature of this alloy is approximately 116℃, with the eutectic phase mainly melting in the 116-152℃ range. The primary Bi-rich dendrites begin to melt at around 151℃, and the complete liquidus temperature is approximately 223-232℃. Within the operating temperature range of 140-160℃, the intergranular low-melting-point eutectic phase has essentially melted into a liquid state, while the primary Bi-rich dendrite network still maintains a certain degree of connectivity and skeletal structure, forming a composite structure of "dendritic network + liquid eutectic phase." This provides a certain degree of mechanical support and resistance to deformation in the semi-solid state, effectively mitigating the pumping out phenomenon of the liquid phase.

[0008] The preparation method of the present invention includes the following steps:

[0009] (1) Weigh the raw materials by mass percentage: Bi 53%, Sn 42%, In 5%, with a purity of ≥99.9% for each raw material;

[0010] (2) Place the raw material in a vacuum melting furnace, evacuate to 5×10⁻³Pa, then fill with high-purity argon to a protective atmosphere of 0.05MPa, and heat to 300-350℃ under the protective atmosphere for melting;

[0011] (3) The melting time is 20-30 minutes, and stirring is required during the process to ensure uniform composition;

[0012] (4) Cool to room temperature at a cooling rate of 5-10℃ / min to obtain an alloy ingot with a primary dendritic skeleton structure;

[0013] (5) Low-temperature stabilization treatment of alloy ingots;

[0014] (6) The treated alloy is machined to produce a thermal interface material of the required shape.

[0015] By adopting the above technical solution, the present invention obtains a continuous dendritic framework structure by controlling the cooling rate. The dendritic network can still maintain a certain three-dimensional connectivity in the semi-solid region, improve the liquid phase viscosity and structural stability, and effectively alleviate the pumping problem. The low-temperature stabilization treatment promotes solid-state diffusion, reduces component segregation and residual stress, further improves the stability of the dendritic network under semi-solid conditions, and enhances service reliability.

[0016] Preferably, the low-temperature stabilization treatment in step (5) is performed at a temperature of 100-110°C and the holding time is 4-6 hours.

[0017] By adopting the above technical solution, the present invention optimizes the temperature and time range of the low-temperature stabilization treatment, and performs the treatment in a completely solid state, which helps to homogenize the components and improve the interfacial bonding through solid-state diffusion.

[0018] Preferably, the cooling rate in step (4) is 5-10℃ / min, and a uniform dendritic framework structure is obtained under the laboratory conditions of furnace cooling of an ingot with a diameter of about 15mm, with a dendrite arm spacing of about 30-100μm.

[0019] By adopting the above technical solution, the present invention optimizes the cooling rate range to form a continuous dendritic skeleton.

[0020] Preferably, the low-temperature stabilization treatment in step (5) is performed at a temperature of 105°C for 5 hours.

[0021] By adopting the above technical solution, the present invention has selected the optimal low-temperature stabilization process parameters, which helps to achieve a balance between thermal conductivity and anti-pumping capability.

[0022] Preferably, the solidus temperature of the thermal interface material is about 116℃, the liquidus temperature is about 223-232℃, the recommended operating temperature range is 140-160℃, the thermal conductivity is 12-15W / (m·K) (the measured thermal conductivity is slightly higher than the estimated value of the simple mixing rule, which may be related to the dendrite orientation in small-sized samples), and the contact thermal resistance is ≤0.28cm²·K / W.

[0023] By adopting the above technical solution, the thermal interface material of the present invention has good comprehensive performance and can meet the heat dissipation requirements of medium and high temperature electronic devices. Detailed Implementation

[0024] The present invention will be further described in detail below with reference to specific embodiments. The raw materials involved in the present invention can all be obtained commercially and have a purity of ≥99.9%.

[0025] Example 1 (Optimization of Low-Temperature Stabilization Process Parameters)

[0026] This embodiment investigates the combination of low-temperature stabilization temperature (100-110℃) and holding time (4-6h) through multiple sets of experiments to determine suitable process parameters for improving interfacial bonding. The equipment used in the experiment was a vacuum induction melting furnace, the protective atmosphere was high-purity argon (99.999% purity), and the cooling method was in-furnace cooling.

[0027] The results of the screening of low-temperature stabilization process parameters are shown in the table below:

[0028] experimental group Stabilization temperature (°C) Insulation time (h) Thermal conductivity (W / (m·K)) Quality loss rate (%) in conclusion Preferred Group 105 5 14.8 2.8% Good overall performance A 100 4 15.2 4.0% Stability is average B 100 6 15.0 3.2% Good performance C 110 4 14.5 3.0% Thermal conductivity slightly decreased D 110 6 14.3 3.5% Organizational stability

[0029] Taking into account both thermal conductivity (≥14.5W / (m·K)) and mass loss rate (≤3.5%), the preferred group (105℃ / 5h) showed relatively better overall performance and was selected as the preferred process parameter.

[0030] Example 2 (Detailed process parameters and performance)

[0031] This embodiment provides a Bi-Sn-In system low-melting-point alloy thermal interface material with a dendritic skeleton structure and its performance parameters.

[0032] The performance parameters of the thermal interface material prepared in this embodiment, after testing, are shown in the table below:

[0033] Performance parameters numerical values Test methods Test conditions solidus temperature 116℃ DSC Heating rate 10℃ / min, nitrogen atmosphere Liquidus temperature 223-232℃ DSC Heating rate 10℃ / min, nitrogen atmosphere eutectic phase melting temperature 116-152℃ DSC Heating rate 10℃ / min, nitrogen atmosphere Recommended operating temperature range 140-160℃ DSC / Metallographic semi-solid range Room temperature thermal conductivity Approximately 14.8 W / (m·K) LFA 25℃ Contact thermal resistance 0.25cm²·K / W steady state method 0.3MPa Dendrite arm spacing Approximately 30-100μm optical microscope 4% Nitric Acid Alcohol Corrosion Quality loss rate Approximately 2.8% Thermal cycling test 160℃, 0.2MPa, 500h (preliminary test)

[0034] DSC test results ( Figure 4 The data shows that the alloy exhibits typical two-stage melting characteristics:

[0035] (1) Low-temperature endothermic stage (115-152℃): corresponds to the melting of the low-melting-point eutectic phase between crystals, with a melting peak temperature of about 131.5-133.6℃ and a termination temperature of about 152.0℃;

[0036] (2) High temperature range (approximately 150-232℃): corresponds to the gradual melting of primary Bi-rich dendrites.

[0037] Combined with metallographic observation ( Figure 2 , Figure 3 The as-cast alloy has a continuous dendritic framework structure with dendrite arm spacing of approximately 30-100 μm. Within the operating range of 140-160 °C, the eutectic phase has essentially melted into a liquid state, while the primary Bi-rich dendritic network (whose significant melting begins at approximately 151 °C) can still maintain a certain three-dimensional interconnected structure in a partially melted state, forming a composite two-phase structure of "dendritic network + liquid eutectic phase", which helps to suppress the pumping out of the liquid phase.

[0038] Example 3 (Performance Comparison Test)

[0039] This embodiment compares the performance of the material of the present invention with that of a material that has not undergone low-temperature stabilization treatment:

[0040] (1) Sample of the present invention: thermal interface material prepared in Example 2 (low temperature stabilization treatment at 105℃ / 5h);

[0041] (2) Comparative sample: Alloy with the same composition, but without low-temperature stabilization treatment;

[0042] (3) Test conditions: temperature 160℃ (in the semi-solid range), pressure 0.1-0.2MPa (simulating typical electronic packaging conditions), time 500 hours (preliminary stability test under laboratory conditions).

[0043] Preliminary comparative tests conducted under laboratory conditions show that:

[0044] project Unprocessed Stabilization treatment at 105℃ for 5 hours Trend of change Thermal conductivity (W / (m·K)) 15.2 14.8 Slightly decreased Initial contact thermal resistance (cm²·K / W) 0.28 0.25 Some decrease Thermal resistance stability Poor better Significant improvement Quality loss rate Approximately 7% Approximately 2.8% Significantly reduced Micro-defect density higher lower Interface integration improvement

[0045] Note: After low-temperature stabilization treatment, the dendrite-eutectic interface of the material is improved by solid-state diffusion to homogenize the composition, reduce dendrite segregation, and release residual stress, thus exhibiting better structural stability in the semi-solid region and effectively alleviating the pumping phenomenon.

[0046] In summary, the present invention has the following beneficial effects:

[0047] 1. This invention obtains a continuous Bi-rich dendritic framework structure by controlling the cooling rate. The dendrite arm spacing is about 30-100μm. In the semi-solid range of 140-160℃, the dendritic network can maintain connectivity even in the partially molten state, providing a certain structural support and helping to suppress liquid phase pumping.

[0048] 2. This invention uses a low-temperature stabilization treatment at 105℃ to homogenize the composition through solid-state diffusion, reduce dendrite segregation, release residual stress, and improve the bonding at the dendrite-eutectic interface, effectively alleviating the pumping problem.

[0049] 3. The thermal interface material of the present invention has good comprehensive performance, with a solidus of about 116°C, a liquidus of about 223-232°C, a recommended operating temperature range of 140-160°C, a thermal conductivity of 12-15 W / (m·K), and a contact thermal resistance ≤0.28 cm²·K / W.

[0050] 4. After low-temperature stabilization treatment, the pumping tendency of the material in the semi-solid range is significantly reduced, the contact thermal resistance stability is improved, and the service reliability is enhanced;

[0051] 5. Referring to the Bi-Sn-In ternary phase diagram, adding 5% In can lower the melting point of the eutectic phase to about 116℃ while maintaining the Bi-rich primary phase as the main precipitate phase, thus achieving a reasonable separation between the framework melting temperature and the eutectic melting temperature.

[0052] 6. An inert gas protected melting process is adopted to suppress the volatilization and oxidation of low-melting-point Bi and In elements, reduce porosity and oxide inclusions in the alloy, and ensure the continuity of dendritic structure and interfacial bonding strength. Attached Figure Description

[0053] Figure 1 This is a flowchart illustrating the preparation process of the thermal interface material of the present invention.

[0054] Figure 2 , Figure 3 This is a metallographic micrograph of the thermal interface material prepared in Example 2 of the present invention. The metallographic sample was mounted, ground, and polished, then etched with a 4% nitric acid alcohol solution, and photographed in bright field mode under an optical microscope to observe the solidification structure of the cast alloy.

[0055] Figure 2 (Scale bar 100μm, high magnification) This is a local magnification of the low-magnification structure, clearly showing the details of dendrites and intergranular structures. The black area represents the primary Bi-rich dendrites that have been etched and recessed, while the light-colored / off-white area represents the low-melting-point eutectic structure in the intergranular space. The dendrite trunk edges are clear, and details of secondary and tertiary branches are visible. The interdendritic filling phase is distributed in a network and blocky pattern, with some areas showing fine eutectic phase aggregation. There are no obvious casting defects such as porosity or inclusions.

[0056] Figure 3 (Scale bar 200μm, low magnification) The field of view is wide, allowing observation of the overall distribution and orientation of the dendrites. The dendrites exhibit distinct directional growth, with branches interlacing to form a continuous black network framework. The dendrite arms are well-developed, and secondary dendrites are clearly visible. The dendrite spacing is approximately 30-100μm, and the spaces between the dendrites are filled with light golden eutectic tissue. The overall microstructure is highly homogeneous, with intact dendrite morphology and no defects such as overheating, oxidation, or severe segregation.

[0057] Figure 4 The DSC differential scanning calorimetry curve (heating rate 10℃ / min, nitrogen atmosphere, NETZSCH equipment) of the thermal interface material prepared in Example 2 of this invention is shown. The vertical axis represents the DSC signal (mW / mg), with the upward direction indicating heat release; the horizontal axis represents temperature / time.

[0058] The curve exhibits typical two-stage melting characteristics:

[0059] (1) Low-temperature endothermic stage (115-152℃): corresponds to the melting of the low-melting-point eutectic phase between crystals. The test results of three parallel samples show that the initial melting temperature of the eutectic phase is in the range of 115.9-127.3℃ (with slight differences due to the influence of local compositional homogeneity), the termination temperature is stable at 152.0-152.1℃, and the melting peak temperature is between 131.5-133.6℃.

[0060] (2) High-temperature endothermic / exothermic stage (150-232℃): corresponding to the gradual melting of the primary Bi-rich dendrites. After the endothermic peak ends, the curve begins to show an exothermic signal at about 150℃ (corresponding to the delayed melting and rearrangement of some dendritic skeletons), and the exothermic termination temperature is between 223.7-232.6℃.

Claims

1. A Bi-Sn-In based low-melting-point alloy thermal interface material, characterized in that, The material is composed of Bi 53%, Sn 42%, and In 5% by mass, and has a composite structure of primary Bi-rich dendritic framework and intergranular low-melting-point eutectic phase.

2. The thermal interface material according to claim 1, characterized in that, The solidus temperature of the material is approximately 116°C, the liquidus temperature is approximately 223-232°C, the eutectic phase melting temperature is 116-152°C, and the primary Bi-rich dendrites begin to melt at around 151°C.

3. The thermal interface material according to claim 1, characterized in that, The recommended operating temperature range of the material is 140-160℃, the thermal conductivity is 12-15W / (m·K), and the contact thermal resistance is ≤0.28cm²·K / W.

4. The thermal interface material according to claim 1, characterized in that, Within the operating temperature range of 140-160℃, the low-melting-point eutectic phase between crystals has basically melted into a liquid state, while the nascent Bi-rich dendritic network can still maintain a certain degree of connectivity and framework structure, forming a composite structure.

5. The thermal interface material according to claim 1, characterized in that, The dendrite arm spacing is approximately 30-100 μm.

6. A method for preparing the Bi-Sn-In system low-melting-point alloy thermal interface material according to claim 1, characterized in that, Includes the following steps: (1) Weigh the raw materials by mass percentage: Bi 53%, Sn 42%, In 5%, with a purity of ≥99.9% for each raw material; (2) Place the raw material in a vacuum melting furnace, evacuate to 5×10⁻³Pa, then fill with high-purity argon gas to a protective atmosphere of 0.05 MPa, and heat to 300-350℃ under the protective atmosphere for melting; (3) The melting time is 20-30 minutes; (4) Cool to room temperature at a cooling rate of 5-10℃ / min to obtain an alloy ingot with a primary dendritic skeleton structure; (5) Low-temperature stabilization treatment of alloy ingots; (6) The treated alloy is machined to produce a thermal interface material of the required shape.

7. The preparation method according to claim 6, characterized in that, The low-temperature stabilization treatment in step (5) is performed at a temperature of 100-110℃ and a holding time of 4-6 hours.

8. The preparation method according to claim 7, characterized in that, The low-temperature stabilization treatment in step (5) is performed at a temperature of 105°C for 5 hours.

9. The preparation method according to claim 6, characterized in that, The cooling rate described in step (4) is 5-10℃ / min. Under the laboratory conditions of furnace cooling of an ingot with a diameter of about 15mm, a uniform dendritic framework structure is obtained with a dendrite arm spacing of about 30-100μm.

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