Microstructural evolution of the intermetallic compounds in the high density solder interconnects with reduced stand-off heights

Purpose - This paper aims to investigate the microstructural evolution rules of the intermetallic compound (IMC) layers in high-density solder interconnects with reduced stand-off heights (SOH). Design/methodology/approach - Cu/Sn/Cu solder joints with 100, 50, 20 and 10 Am SOH were prepared by the same reflow process and isothermally aged at 150 degrees C. The IMC microstructural evolution was observed using scanning electron microscopy. Findings - The whole IMC layer (Cu3Sn + Cu6Sn5) grew faster in the solder joints with lower SOH because of the thinner IMC layer before aging. Also, the IMC proportion increased more rapidly in solder joints with the lower SOH. In all solder joints with different SOH, the growth rates of the Cu3Sn (epsilon) layers were similar, and slowed down with increasing aging time. The Cu6Sn5 (17) was consumed by the Cu3Sn (epsilon) growth at the beginning of the aging stage; while it turned to thickening after a period of aging. Finally, the Cu6Sn5 thickness was similar in all the solder joints. It is inferred that the thickness ratio of Cu3Sn to Cu6Sn5 would maintain a dynamic balance in the subsequent aging. Based on the diffusion flux ratio of Cu to Sn at the epsilon/eta interface, a model has been established to explain the microstructural evolution of IMC layers in high-density solder interconnects with reduced SOH. In the model, interfacial reactions are mainly supposed to occur at the epsilon/eta interface. Originality/value - The findings provide electronic packaging reliability engineers with an insight into IMC microstructural evolution in high-density solder interconnects with reduced SOH.