Fluxless flip-chip for multichip modules

The no-flux, Utilitarian Solder System (“No-FUSS”) flip-chip process is a versatile technique that can be used with virtually any die. The process combines Au ball bumping of the die with Pb/Sn-indium electroplating of the substrate. Previous work demonstrated that substrates electroplated with an 8 μm indium cap over 16 μm of 95Pb/5Sn had shown consistently reliable interconnects after thermal shock and thermal aging testing. Preliminary evaluations of samples with only 2 μm of indium over 22 μm of 95Pb/5Sn had shown promising results from a metallurgical viewpoint and further work was recommended. The current study compares the reliability and reproducibility of flip-chip interconnects prepared with a 2 μm indium cap with previous data from 8 μm samples. The results indicate that, although it is possible to produce low resistance, reliable interconnect using a 2 μm indium layer, the process is nor yet as reproducible as the 8 μm process and requires further development before it can be fully utilized