EVALUATION OF LEAD-FREE SOLDER JOINTS IN ELECTRONIC ASSEMBLIES

The feasibility of printed circuit board assembly with lead-free solde r alloys was investigated. Studies were conducted with two baseline eu tectic binary alloys, SnBi and SnAg, and three new lead-free solder fo rmulations: (1) 91.8Sn-4.8Bi-3.4Ag (wt%) developed at Sandia Laborator ies, (2) 77.2Sn-20In-2.8Ag developed at Indium Corp. of America, and ( 3) 96.2Sn-2.5Ag-0.8Cu-0.5Sb provided by AIM Inc. The basic physical pr operties (melting temperature, wetting, mechanical strength) pertinent to each of the newly developed alloys are described. The feasibility of 0.4 mm pitch assembly was established with each of the lead-free so lder alloys investigated, although the processing windows were general ly found to be narrower. All solder joints exhibited good fillets, in accordance with the workmanship standards. Wetting of the lead-free so lders was significantly improved on immersion tin vs imidazole finishe d circuit boards. The laminates did not suffer thermal degradation eff ects, such as warpage, delamination, or severe discoloration (reflow w as performed under an inert atmosphere). It is thus concluded that the manufacturability performance of the new solder formulations is adequ ate for surface mount applications.