Wetting behaviour of SAC305 solder on different substrates in high vacuum and inert atmosphere

The wettability between solder and substrate is a very important issue in reliability of soldering process. The contact angle theta is used to measure the degree of wetting. The contact angle of lead-free alloy Sn-3 % Ag-0.5 % Cu (wt%) was measured, as a function of temperature, for three different commercial surface finish substrates used in printed circuit boards (PCB): Sn, NiAu and Organic Solderability Preservative (OSP). The measurements were performed by the sessile drop method in two different atmospheres: high vacuum and inert gas. In high vacuum the results showed that on substrates of NiAu and OSP the solder started spreading suddenly at 225 degrees C and in Sn substrate the contact angle decreases slightly with temperature. In inert gas atmosphere the results showed different behaviours: the contact angle between molten solder in OSP and NiAu substrates is sensitive to temperature; and, in Sn substrate, the contact angle does not change with temperature. The NiAu and OSP substrates showed a better degree of wettability than Sn substrate, in inert atmosphere.