AN ALTERNATIVE SURFACE FINISH FOR TIN/LEAD SOLDERS: PURE TIN
Author: Y. Zhang Company: Lucent Technologies Date Published: 9/10/1996
Surface Mount International
Abstract: Electroplated pure tin, bright or matte, has been used in the electronic industry as an alternative for tin/lead finishes for its excellent solderability and corrosion resistance. However, notorious whisker formation often associated with electroplated thin coatings of pure tin has limited its applications, especially in high density semiconductor packages because of the potential danger of bridging. A novel tin electrodeposition chemistry and process which appears to overcome the problems of whisker growth and dewetting in reflow applications has been developed. This process produces smooth, satin bright tin deposits which have stable, large grain structures. Deposit property characterizations show that the finish is very low in organics, has excellent ductility, solderability and reflowability. The chemistry is capable of operating over a wide range of current densities, thus applicable to rack, barrel and reel to reel operations. The chemistry is fully analyzable with conventional analytical methods. Extensive bath life study showed that the deposit’s appearance and properties, including grain structures, are stable in relation to the age of the electroplating chemistry. In addition, the grain refiners used in this chemistry are highly stable, and have few breakdown products as the chemistry ages. All these features imply a robust process for manufacturing environments. Detailed characterizations of the chemistry and the deposits are presented.