Study on the Generation of the Intermetallic Compound and the Occurrence of the Whiskers Observed in the Double-Layer Plating System （Sn/Cu）

Many research studies have been conducted so far about the occurrence of the whiskers in Sn-plating. However, most of them have paid attention to the generation mechanism of the whiskers, and to the time variations of both the number and lengths of the whiskers. Very few research studies have been conducted on the correlation between the generation of the intermetallic compound and the occurrence of the whiskers. In the present study, double-layer plating(Sn/Cu thin films)was electrodeposited on the 7-3 brass substrate to investigate quantitatively the time variation of the amount of the intermetallic compound generated in the Sn plating film. It was clarified that the relation between the amount of the intermetallic compound and the elapsed time can be expressed by the two different straight lines on the log-log graph. In the early stage until the first seven days after the plating processing, the rate of the generation amount of the intermetallic compound was faster than that in the next stage. When the amount of the intermetallic compound per area is less than 5% , the whisker density increased with an increase in the amount of the intermetallic compound per area. On the other hand, in more than 5% of domains of the intermetallic compound per area, a clear correlation between them was not observed. It is considered that the internal compressive stresses increase in the inside of the Sn plating with an increase in the amount of the intermetallic compound. It is inferred that the whiskers arise on the Sn-plating film to release the above internal compressive stress. Seven days later after the plating processing, the generation rate of the amount of the intermetallic compound decreased because of the change in the diffusion mechanism, i.e. from the grain boundary diffusion to the volume one. Corresponding to the rate of the amount of the intermetallic compound, the rate of the whiskers formation was also suppressed. [doi:10.2320/jinstmet.J2016055]