Wetting properties of lead-free solders

The wetting and spreading behavior of different lead-free solder materials on Cu substrates was determined by a sessile drop method under different atmospheres. After these measurements, the solidified samples were cut in halves. One part was used for push-off tests in order to determined the shear strength of the solder/substrate couple whereas the other part could be used for various metallographic investigations. Up to now, lead-free solders of the Sn-Ag-Cu, the Sn-Ag-In, and the Sn-Ag-Bi type have been investigated on Cu substrates, and the push-off tests were performed both immediately after solidification and after thermal cycling. The effect of alloying elements and testing conditions on the relationship between wetting and mechanical properties of solder/ Cu model joints are discussed. It can be shown that thermal cycling changes the shear strength considerably in some instances. All the results are compared with those obtained for standard Sn-Pb solders.