Effect of IMC growth on thermal cycling reliability of micro solder bumps

Abstract

In this study, thermal cycling reliability tests and analysis for micro solder bump in 2.5D packaging were conducted. Finite element analysis modeling of micro bump with different intermetallic compound (IMC) thickness was developed to model the effect of IMC layer on fatigue behavior of micro bumps. It was found that stress in solder increased notably when the IMC layer is taken into consideration, and increased slightly as further increase in the IMC thickness. The thermal mechanical stress in IMC layer was also studied to assess the reliability of IMC integration. It was found that the stress decreased sharply with the increase of IMC thickness; when all the solder transformed into IMC, the thermal mechanical stress decreased to only 20% of the stress in 1μm IMC. Failure mode after thermal cycling tests was analyzed by cross section morphology. Cohesive cracks in the solder took place at the interface of tin solder and IMC layer, and heavier damage was found in bumps with thicker IMC thickness, which agreed well with the FEA results.