Investigation on Indium Thermal Interface Materials Fluxless Bonding Technology via In Situ Formed AgIn₂ Coating

Indium (In) is the widely adopted solder thermal interface materials (TIM1) for high-power central processing unit (CPU) chips, primarily because it offers improved heat dissipation performance. However, organic flux residues trapped within In solder can outgas and create a lot of voids (void rate ~35%) in In TIM1 during solder ball reflow, which limits its application in advanced ball grid array (BGA) package. In this article, to realize fluxless In reflow and obtain a low void rate In TIM1, a thin silver (Ag) layer is electroplating on a thick In TIM1 surface to form in situ generated AgIn2 coating, which can protect In from oxidation. Thus, flux is not required to remove the oxide layer of solder during reflow. After an In reflow and solder ball reflow for three times, a low void rate (4.2%) joint is produced confirmed by a scanning acoustic microscope (SAM). Better heat transfer capability and mechanical property (+11.4%) are also obtained. A novel AgIn2 coating decomposition mechanism during reflow is also found. During In reflow, AgIn2 would decompose into In and Ag atoms, and the Ag atoms could increase the wettability of solder and shear strength of the joint.