Leadframes, finish

Tin whisker growth is a well-known surface relief phenomenon (Ref 1-9). The subject of Sn whisker growth has often been reviewed. One of the latest reviews (Ref 10), covers most of the early work and the proposed mechanisms of Sn whisker growth and will not be repeated here. In this chapter, we limit our review based on our own hndings of spontaneous Sn whisker growth at room temperature on leadframes finished by eutectic Sn-Cu and pure Sn (Ref 11-14). A three-dimensional nonlinear stress analysis on the initiation of Sn whisker has been reported... 

Pig, 2 SEM image of Sn whiskers grown on eutectic Sn-Cu finish on a leadframe, showing that a whisker shorts two of the leadframe legs. 

In Fig. 5(c), a cross-sectional SEM image, prepared by focused ion beam, of pure Sn finish on Cu leadframe is shown. While the layer of CugSns compound can be seen between file Cu and Sn, there is almost no Cu6Sn5 precipitates in the grain boundaries of Sn. The grain size in the Sn finish is also approximately several microns. The lack of grain boundary Cu6Sn5 precipitates is the most important difference between the eutectic Sn-Cu and the pure Sn finish. 

Fig. 5 whiskers formed on SnCu and Cu leadframes. (a) Cross-sectional SEM images of a leg of the Sn-Cu finished leadframe. The rectangular core of the Cu is surrounded by an Sn-Cu finish approximately 15 urn thick, (b) An enlarged magnification of the interface between the Sn-Cu finish and the Cu leadframe. A scallop-type CueSns compound can be seen, (c) A cross-sectional SEM image, prepared by focused ion beam, of pure Sn finish on Cu leadframe. 

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