Wetting force of lead-free solders on bare copper and Ni-Pd coated copper

A push toward lead-free solder alternatives has been established to protect the environment and to avert any impending government restrictions on the use of lead. From a materials science standpoint, the microelectronics industry would also push toward a lead-free solder if its properties surpass that of eutectic tin-lead solder. Before the industry is ready to accept a lead-free solder, the solder should at least be comparable to the properties of tin-lead solder. One of these properties is solderability. The wetting force, a measure of how quickly and how well a soldering action can occur, plays an important role in the attachment of microelectronic component leads to associated land patterns. However, little data are available on the wetting characteristics of lead-free solders. The objective of this study was to determine the wetting force of three Pb-free solders as a function of flux type and surface coating Sn-37Pb was also tested for purposes of comparison. The Pb-free solders evaluated were Sn-3.5Ag, Sn-58Bi, and Sn-9Zn. The fluxes used were "aqueous" and "no-clean." The copper coupons were of two types: bare copper and NiPd coated. A wetting force balance was used to determine the wetting characteristics with bath temperatures at 240 degreesC and at 62 degreesC above the melting (solidus) temperatures of each solder alloy. Metallography and SEM/EDX were used to examine the solder/surface interfacial interactions. The wetting test results revealed that Sn-3.5Ag was comparable to the eutectic Sn-37Pb alloy. The Sn-9Zn alloy was found to have extremely poor wetting characteristics.