The Elimination Of Whiskers From Electroplated Tin
Authors: Masanobu Tsujimoto, Shigeo Hashimoto, Masayuki Kiso, Raihei Ikumoto,Toshikazu Kano and Genki Kanamori Company: C. Uyemura & Co., Ltd. Date Published: 5/14/2013
ICSR (Soldering and Reliability)
Abstract: As RoHS lead free regulations began to take hold globally, tin and its alloys were the first choice as an alternative to eutectic tin/lead. On the solder side the transition has moved forward and solutions have been implemented, like the SAC family of LF solders, for paste reflow and tin/copper for HASL (hot air solder leveling). The industry is constantly making progress adapting its materials and processes to the higher reflow temperature profile for these LF solders. Today there is a much better understanding of the nature of the intermetallic (IMC) bond as well as the reliability of LF solder joint. On the surface finish side, replacing tin/lead has posed greater challenges. Component leads and connector finishes were being converted to tin as an obvious alternative. This works well as a soldering surface, however any part of the lead or the connection surface that is not soldered to, has shown a potential to form tin whiskers over the life of the part. Internal stresses in the deposit due to IMC formation or external stresses on the deposit are known to initiate whisker formation. In this paper two approaches are implemented to dissipate the stress that is formed. First is to modify the substrate surface to control the growth in thickness and direction of propagation of the IMC. Second is to modify the large columnar tin deposit crystal structure to mimic the fine equiaxed structure of tin/lead solder. The former is achieved thru controlled micro roughening of the substrate and the latter by the use of additives to the plating bath. Data will be presented showing that by implementing these two modifications that the stress causing tin whiskers is dissipated and tin whisker formation is inhibited.