The Elimination of Whiskers from Electroplated Tin
C. Uyemura & Co., Ltd.
Don Gudeczauskas, George Milad
Following the implementation of RoHS, the discontinued use of Pb-bearing products, and the introduction of lead-free (LF) solders, tin and its alloys have become the first choice as a tin lead replacement.
On the solder side, solutions have been implemented, including the SAC family of LF solders, for paste reflow, and tin/copper for HASL (hot air solder leveling). The industry is constantly progressing, adapting materials and processes to the higher reflow temperature profile for these LF solders. Today, there is a greater understanding of the types of solder joints, their reliability, and the type of IMC formed.
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 has shown a potential to form tin whiskers. Internal stresses in the deposit due to intermetallic formation or external stresses on the deposit are known initiators of whisker formation.
In a paper presented at the IPC Apex conference in 2011, two approaches were discussed to dissipate the stress. The first is to modify the substrate surface to control the growth in thickness and direction of propagation of the IMC formed. The 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 was presented showing that tin whiskers were not formed after 22000 hours post–plating after implementing these modifications. The stress causing tin whiskers is dissipated and tin whisker formation is inhibited.
Visit this page for product information on the Uyemura "22,000-hour solution" to tin whiskers.
Read "The Elimination of Whiskers from Electroplated Tin" from the Jan/Feb 2012 issue of Metal Finishing.