Characterization of key coating properties of immersion tin for PWB

In order to meet the requirement of RoHS, including the elimination of lead from electronic components, the printed circuit board industry is moving away from lead bearing hot air solder leveled final finishes to alternative materials. Immersion tin has been available on the market for some time, in spite of its high manufacturing cost and difficulties in processing, because it provides a thick, uniform metallic coating for improved ICT probe life, lubricity for press fit pins, and excellent solderability. Since the formation of intermetallic compounds of Cu₆Sn₅ and Cu₃Sn between the copper substrate and tin coating decreases the solderability and induces whiskers, various methods have been used in the formulation of immersion tin chemistries to retard the intermetallic formation process. This paper compares several immersion tin coatings through the investigation of the kinetics of intermetallic growth and phase transformation by using sequential electrochemical reduction analysis; and the associated solderability by wetting balance test. The intermetallic growth and whisker formation are related to coating compositions and crystal morphologies.