Lead-free soldering assessment using different board finishes

A lead-free surface mounting process was assessed and compared to a conventional leaded one. The ternary lead-free Sn-3.0Ag-0.5Cu solder paste was compared to Sn-36Pb-2Ag. Two different board finishes were used, the first one being a lead-free Matte tin (100% Sn) and the second a Sn-37Pb. The boards were aged in a pressure cooker under the influence of 103ºC, 100% RH for a time period of 30 days. The shear strength of some solder joints was measured as a function of aging and production parameters used. The void content was analysed as a function of board metallization, solder paste used and component pitch (large and small pitch). The solderability results were also analysed, where factors such as wetting, solder balls, bridges, and solder joint shape were compared. The results show that Matte tin is the preferred board finish when using the lead free solder paste. This is based on the fact that the combination lead-free solder and Matte tin resulted in much lower void content compared to the combination lead-free solder and Sn-37Pb metallization. The shear test results show that the un-aged boards attained a higher shear strength compared to the aged ones. Two failure modes were however observed during the shear testing. The majority of the aged samples showed complete pad lift-off while un-aged samples showed a fracture through the solder joint. Regarding the solderability results, the lead free process depicted comparable quality to the leaded process