Authored By:
Masanobu Tsujimoto, Shigeo Hashimoto, Masayuki Kiso, Raihei Ikumoto,Toshikazu Kano and Genki Kanamori
C. Uyemura & Co., Ltd., Hirakata Japan
Don Gudeczauskas, George Milad
C. Uyemura & Co., Ltd., Southington CT
Summary
After the implementation of RoHS and the discontinued use of lead bearing products and the introduction of lead free (LF) solders, tin and its alloys have come to the forefront as the first choice of replacement to 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 types of solder joints that are formed; their reliability and the type of intermetallic compound (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 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, the first is to modify the substrate surface to control the growth in thickness and direction of propagation of the IMC and 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 will be presented to show the effect of each of the two approaches on the dissipation of the stress, resulting from IMC formation. As the stress is dissipated the primary cause of whisker formation is eliminated
Conclusions
In this study two distinct approaches were attempted to restrain whisker growth in tin deposits over copper. The first approach was to create a uniform IMC, by mico-roughening the copper substrate before tin deposition. A uniform IMC would eliminate high stress in localized areas. The second approach was to modify the grain, from a large columnar structure to a fine grained equiaxed structure, resembling the structure of tin-lead deposit. Tin deposit which had crystal structure similar to tin-lead deposit restrained tin whisker formation effectively. Crystal structure modification of the tin deposit was demonstrated to be a very effective way to restrain tin whisker formation.
Initially Published in the IPC Proceedings
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