Brittle Failure and Effect of Thermal Aging on Pb-free Solder Flip Chip Using Electroless Ni-P UBM

Electroless Ni plating is a cost effective process for manufacturing the under bump metallurgy (UBM) in a solder bump flip chip. However, Pb-free solder joints using electroless Ni UBM are prone to brittle failure during reflow and underfill processes. Hence, application of electroless Ni UBM to a Pb-free solder flip chip is restricted by low assembly yield. In order to analyze the causes of brittle failure, thermal cycling tests were performed with various solder flip chips. In the thermal cycling tests, a eutectic PbSn solder flip chip before underfill shows no brittle failure, even after 5 cycles. On the other hand, most Pb-free solder flip chips before underfill failed less than 5 cycles because of brittle failure such as Si cratering and delamination on the electroless Ni-P/Al interface. Consequently, the flip chip assembly using Pb-free solder and electroless Ni-P UBM shows low yield problem during the flip chip assembly cooling process due to the thermo-mechanical stress build at the UBM/solder joint. To reduce stress on the solder joints, the effects of additional aging after reflowing were investigated. SnAgCu solder flip chips before underfilling were aged for various times and temperatures immediately after the solder reflow process. The results showed that the yield of the assembly process was increased by about 30 percent by additional aging. Furthermore, SnAgCu solder joints aged for longer time showed better reliability in thermal cycling tests before underfilling. Measurement of the flip chip warpage reveals that the additional aging processes cause creep deformation in the solder bumps and result in smaller flip chip warpage in comparison with directly air-cooled flip chips.