Effect of bath contamination on electroplated solder bumps

Today's solder bumping technologies commonly require electroplating multiple metallurgies. Chip manufacturers often process these multi-metal bumping wafers on the same equipment. Handling multiple metal baths on the same equipment inherently carries some risks of bath cross-contamination due to drag-out. Understanding the effect of bath cross-contamination can help chip manufacturers assess production risks in events of inadvertent and significant contamination, due to processing, equipment, or human errors. This paper summarizes the effect of cross-contamination on the on-wafer performance of electroplated solder bumps. Plating baths were deliberately contaminated by copper and nickel metals at levels of 0, 10, 100, 500 ppm. CuSnAg and NiSnAg bumps were plated in those contaminated baths. The bumps were then evaluated based on parameters such as metal content in contaminated films, bump height non-uniformity and coplanarity, bump morphology/defects, Ag% in solder bumps, shear strength, bump adhesion, and intermetallic compounds (IMC) formation. The results showed detrimental effect by Cu-contaminated Ni at levels above 100 ppm due to degradation of Ni crystal structure. In case of Ni-contaminated SnAg, there was essentially no effect, not even at 500 ppm, because Ni was not found incorporated into the SnAg films. For Cu-contaminated SnAg, there appeared to be some effect on shear strength at 100 ppm, and surface appearance at 500 ppm.