《Investigation on Indium Thermal Interface Materials Fluxless Bonding Technology via In Situ Formed AgIn2 Coating》
Title: Investigation on Indium Thermal Interface Materials Fluxless Bonding Technology via In Situ Formed AgIn2 Coating
Author: Jing Wen, Yi Fan, Guoliao Sun, Jinyang Su, Linzheng Fu, Zhuo Chen, and Wenhui Zhu, Senior Member, IEEE
Publication time: 2025
DOI : 10.1109/TCPMT.2024.3522254
Journal IEEE TRANSACTIONS ON COMPONENTS, PACKAGING AND MANUFACTURING TECHNOLOGY
Abstract: Indium (In) is widely used as a solder-based thermal interface material (TIM1) in high-power central processing unit (CPU) chips, primarily because it enhances thermal performance. However, residual organic fluxes in indium solder can release gases during the solder ball reflow process, leading to a high void fraction—approximately 35%—in indium TIM1. This limitation restricts its application in advanced ball grid array (BGA) packaging. In this study, to achieve flux-free indium reflow and obtain indium TIM1 with a low void fraction, a thin silver (Ag) layer was electroplated onto the surface of thick indium TIM1, enabling the in-situ formation of an AgIn₂ coating that effectively prevents indium oxidation. Consequently, no flux is required during the reflow process to remove the oxide layer from the solder. After one indium reflow and three solder ball reflows, scanning acoustic microscopy (SAM) confirmed the formation of joints with a low void fraction (4.2%). Moreover, the resulting joints exhibited improved thermal conductivity and mechanical performance, showing an enhancement of 11.4%. Additionally, a novel decomposition mechanism of the AgIn₂ coating during reflow was identified: during indium reflow, AgIn₂ decomposes into indium atoms and silver atoms. The silver atoms enhance the wettability of the solder and improve the shear strength of the joint.
Conclusion: In this study, a low-vacancy-rate indium (In) TIM1 was prepared using a solderless bonding technique with an AgIn2 protective layer. The formation and decomposition mechanisms of AgIn2 on thick indium layers were investigated, as well as the influence of silver (Ag) in AgIn2 on the performance of the thick indium layer and the joint properties. The main conclusions are as follows:
1. After the silver-plating process, an in-situ AgIn2 intermetallic compound (IMC) protective layer forms on the indium surface. This protective layer prevents oxidation of the underlying indium, enabling solderless bonding and achieving joints with a void rate as low as 4.2%. This process holds significant value in ball grid array (BGA) applications.
2. During the indium reflow process, AgIn2 on the thick indium layer decomposes into indium atoms and silver atoms. The silver atoms inhibit the lateral growth of the Ni3In7 intermetallic compound, reducing the contact angle by 23.5%, enhancing the wettability of the solder, and increasing the shear strength of the joint by 11.4%.
3. After three reflows using lead-free solder (SAC), the average chip temperature of InAg joints decreased by 1.2%, the overall maximum temperature dropped by 2.7%, and the shear strength increased by 38.0%, thanks to the low void rate.
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About the Author: The innovation of this study lies in:
1. A silver-plated AgIn₂ protective layer is in-situ generated on the surface of the indium-based thermal interface material, enabling solderless reflow and addressing the issue of numerous voids caused by residual flux in ball grid array (BGA) packaging. This approach achieves a low void rate (4.2%) for the solder joints.
2. The decomposition mechanism of AgIn₂ on a thick indium layer has been identified. The silver atoms generated during this decomposition can inhibit the lateral growth of the Ni₃In₇ intermetallic compound and reduce the contact angle, thereby improving wettability and significantly enhancing the shear strength of the joint.
3. After three reflow cycles, the In@Ag joint exhibits a lower average chip temperature and a lower global maximum temperature, while maintaining higher shear strength, thereby further confirming its excellent thermo-mechanical reliability.