Evaluation of Lead- Free Solder Joints in Electronic Assemblies

The feasibility of printed circuit board assembly with lead-free solder alloys was investigated. Studies were conducted with two baseline eutectic binary alloys, SnBi and SnAg, and three new lead-free solder formulations: (1) 91.8Sn-4.8Bi-3.4Ag (wt%) developed at Sandia Laboratories, (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 properties (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 solder alloys investigated, although the processing windows were generally found to be narrower. All solder joints exhibited good fillets, in accordance with the workmanship standards. Wetting of the lead-free solders was significantly improved on immersion tin vs imidazole finished circuit boards. The laminates did not suffer thermal degradation effects, such as warpage, delamination, or severe discoloration (reflow was performed under an inert atmosphere). It is thus concluded that the manufacturability performance of the new solder formulations is adequate for surface mount applications.