Effect of IMC Thickness on the Mechanical Properties of Microbumps

Abstract

With the continuous development of electronic products towards high density and small size, the size of interconnecting solder joints also continues to decrease. Under normal service conditions, micro-solder joints will be subjected to multiple loads at the same time, which in turn will cause many reliability problems in the package structure. This paper focuses on the growth pattern of IMC (Intermetallic compounds) during hot-press bonding and analyses its effect on the shear strength and creep properties of copper pillar micro-bumps. Firstly, the solder joints with different IMC thicknesses were obtained by adjusting the bonding parameters during the thermal compression bonding process, and the growth pattern of IMC during the thermal compression bonding process was analyzed. Secondly, shear experiments were conducted on solder joints with different IMC thicknesses, and the effects of IMC on the shear strength and shear fracture mode of solder joints were analyzed. Finally, creep experiments were performed on cylinder-shaped solder joints with different IMC thicknesses under different stress conditions. The results show that throughout the bonding process, copper atoms migrate under the combined effect of temperature gradient (JTM) and concentration gradient (JCM), and IMC shows asymmetric growth, and the growth rate of IMC gradually decreases with the increase of bonding holding time. Under the same temperature conditions, the shear strength of the solder joint shows a trend of increasing and then decreasing with the increase of holding time. In addition, with the increase of IMC thickness, the shear fracture mode of solder joints changes from ductile fracture in the initial stage to tough-brittle combination fracture and finally evolves to brittle fracture. With the continuous increase of stress, the steady-state creep strain rate of all solder joints increases, and the fracture location tends to be closer to the intersection of the solder and the copper pillar from inside the solder joint. With the increase of IMC thickness, the steady-state creep strain rate of solder joints shows a trend of decreasing and then increasing.