Wetting performance vs. board finish and flux for several Pb-free solder alloys

The move to replace lead in electronic assemblies is rapidly gaining momentum. The driving factors are potential legislation, primarily in Europe, and global market pressures for more environmentally friendly products. This work examines the compatibility of lead free solders and printed wiring board (PWB) finishes. The solder alloys evaluated were Sn-3.5Ag-1.5In, Sn-3.4Ag-4.8Bi, Sn-3.8Ag-0.7Cu, Sn-3.5Ag and Sn-0.7Cu. The PWB surface finishes were Sn, Ag, Pd, Ni/Au and OSP. Eutectic Sn-37Pb solder was used for baseline comparisons. Surface finishes “as received” and after one reflow cycle in nitrogen with a peak temperature of 260°C were studied. Double-sided reflow is common and the ability of the finish to survive at least one reflow cycle prior to assembly is critical. The surface finishes (as-received and after one reflow cycle) were characterized by Rutherford backscattering (RBS), Auger electron spectroscopy (AES) and X-ray photoelectron spectroscopy (XPS). Wetting balance measurements were used to quantify solder wetting of each finish and alloy. Both a no-clean and a water soluble flux were used in the wetting balance tests