Intermetallic compound formation and solderability for immersion tin

Abstract

PurposeTo investigate the relationship between intermetallic compound (IMC) formation and solderability for immersion tin deposit under different number of reflows.Design/methodology/approachScanning Auger microscopy and X‐ray photoelectron spectrometer surface analysis techniques were used to study changes in immersion tin deposit layer when subjected to simulated solder reflow conditions.FindingsAuger analysis also showed that no three discrete uniform layers of pure tin, Cu6Sn5η‐phase and Cu3Sn ε‐phase can be observed after one reflow. Degradation in solderability performance after reflow was due to the formation of a Cu6Sn5 IMC at the surface. This IMC has inferior solder wetting properties compared to tin. As the number of reflow cycles increases the surface contains less tin rich regions and more IMC regions. Experiments showed that longer reflow times during the assembly process or use of a thicker tin layer can improve the solderability after three reflow cycles.Research limitations/implicationsThis work has shown that longer reflow times during the assembly process or use of a thicker tin layer can improve solderability after three reflow cycles. These two approaches are thus recommended when using immersion tin finishes on PCBs that require multiple lead‐free reflow cycles.Originality/valueThis paper provides valuable data that will assist PCB assemblers to optimise their solder reflow conditions when assembling boards that require multiple solder cycles.