AgIn2 thickness on void rate, microstructure, IMC growth, thermal and mechanical properties of fluxless In@AgIn2 joint

Abstract

Indium (In) has been extensively used as a thermal interface material (TIM1) between the die and lid in high-power central processing units (CPUs) to enhance heat dissipation. However, organic flux residues trapped within the In solder during indium reflow process can outgas during subsequent solder ball reflow cycles, leading to the formation of significant voids (up to 35% void fraction) in the In TIM1. This issue limits the application of In in advanced ball grid array (BGA) packages. In this study, for the first time, varying thicknesses Ag coatings were electroplated onto the surfaces of thick In TIM1 to form a non-oxidizing AgIn2 layer (In@xAgIn2, where x = 0.4, 1, 3, 6 μm) to protects the inner In from oxidation and enables fluxless reflow. Joints prepared with In or In@xAgIn2 underwent indium reflow and three solder ball reflow cycles to simulate the reflow processes typical of BGA packages. A clear AgIn2 thickness effect on solder wettability, microstructure, intermetallic compound (IMC) growth, joint thermal and mechanical properties were found. The results showed that [email protected]₂; had a contact angle of 26.2°, which was 2.6° lower than that of pure In solder. Joints prepared with [email protected]₂; also exhibited the lowest void fraction (≤2%), which contributed to better heat dissipation. During reflow, the Ag atoms from the AgIn2 protective layer altered the morphology and reduced the thickness of the Ni3In7 IMC layer. After reflow, the Ag atoms either solubilized in In or formed AgIn2 IMC with distinct distribution characteristics in the solder layer, which increased the shear strength of the joints by 81.5%. The fracture mode of the joints also changed from ductile to ductile-brittle, and ultimately to brittle.