Properties of a Newly Developed Immersion Tin Coating

Gradually diminishing solderability and potential risk of whisker growth are two major concerns in using immersion tin as a final finish for printed wiring boards. Both phenomena are attributable to the spontaneous formation of intermetallic compounds (IMC) of Cu₆Sn₅ and Cu₃Sn between the tin coating and copper substrate. The formation of IMC consumes the free tin in the coating that is essential for good solderability. It also builds up compressive stress within the tin coating that induces whisker growth. The formation of IMC is known to be affected by the microstructure of the tin coating, as evidenced by the difference in whisker propensity observed between bright and matte tin coatings. Various methods have been used in the formulation of immersion tin chemistries to alter the microstructure of the tin deposit and to retard the intermetallic formation process, yet there is no direct experimental data to link solderability and whisker propensity to the physical properties of the tin deposit. The objective of this study was to investigate such a link through the evaluation of a newly developed immersion tin chemistry. The propensity to form whiskers was evaluated by aging in an ambient environment for 3000 hours and accelerated thermal cycling tests. Solderability was evaluated by wetting balance tests after simulated reflow thermal excursions. The distribution of IMCs was investigated by sequential electrochemical reduction analysis and cross examined by focused ion beam.