Adhesion strength of solder joints to Alloy 42 component leads

Low adhesion strength of Sn-Pb solder joints to Alloy 42, a widely used material for electronic component lead-frame, is a well known but little studied problem in the electronic packaging and assembly industry. In this study, the adhesion strength of two lead-frame materials, Cu and Alloy 42, soldered with 63Sn-37Pb were compared in push-off tests. It was found that the adhesion to Alloy 42 is weaker than to Cu. Higher reflow temperature and longer reflow time strengthen the adhesion, but the effect is not significant in the temperature and time ranges which are practical for electronic assembly. The failure analysis indicates that the solder joints on Cu failed within the solder, while the solder joints on Alloy 42 failed by a clean separation between solder and the lead. Similarly, poor adhesion to Alloy 42 was also observed with 43Pb-43Sn-14Bi, and the problem was more severe. The push-off strength of the 43Pb-43Sn-14Bi joints on Alloy 42 leads was only 50% of 63Sn-37Pb on Cu leads.