Effect of minor doping elements on lead free solder joint quality

During lead free transition, board level drop test reliability has become an important issue since current Sn3~4Ag0.5Cu lead free solder showed poorer reliability than SnPb solder. In order to improve the reliability, solder composition change to Sn1Ag0.8Cu is one of the potential solutions by preventing solder joint from brittle interface failure. In this study, four kind of minor doping elements, including Ni, Ge, Sb and Zn, in Sn1Ag0.8Cu lead free solder were used to confirm if the solder joint quality could be further improved by modification in bulk or interface properties. The properties checked in this study included DSC analysis, wetibility, IMC formation and ball pull test result on Ni/Au and OSP surface finish after multi-reflow and 150degC high temperature storage. The DSC result indicated that minor doping elements did not cause the significant difference in melting time so the impact to the reflow profile could be negligible. Sb doping caused highest undercooling and then refined the microstructure but Zn doping showed small undercooling and coarse microstructure. Regarding the joint quality on Ni/Au surface finish, only Zn doping could slightly reduce interfacial failure mode after multi-reflow or high temperature storage. As change to OSP surface finish, Ni and Zn doping were very effective to slow down Cu3Sn IMC and total IMC formation, respectively. However, the correlation between IMC thickness and interface failure rate was very poor. Ni and Zn doping could reduce the risk of interface failure after multi-reflow. After high temperature storage, the percentage of interface failure became higher even though the Cu3Sn IMC or total IMC still kept very thin by Ni or Zn doping