Intermetallic compound formation and solderability for immersion tin Available to Purchase

To investigate the relationship between intermetallic compound (IMC) formation and solderability for immersion tin deposit under different number of reflows.

Scanning 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.

Auger 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.

This 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.

This paper provides valuable data that will assist PCB assemblers to optimise their solder reflow conditions when assembling boards that require multiple solder cycles.