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32 SMT Magazine • July 2014 duced the problem of tin whisker growth which had been limited before by the addition of Pb. Whiskers are responsible for many system fail- ures in the military, medical and telecommuni- cation industries. There is no single, commonly accepted mod- el of whisker growth in the literature [2] . How- ever, most theories involve the role of compres- sive stress [2-5] , which may result from chemical, mechanical and thermal factors, with the whis- ker growth as a phenomenon of stress relief. The growth is affected by such factors as tem- perature, residual stress, mechanical force, the formation of intermetallic compounds (IMCs), broken oxide layer, electric field, etc. The higher the compressive stress, the greater the volume of Sn contained in whiskers [6] . In typical PCBs with a copper layer above a laminate substrate, the compressive stress gen- erated as a result of volume expansion during the formation of IMCs (a Cu-Sn alloy formed at the Sn/Cu interface), is generally regarded as the driving force for Sn whisker growth. Whisker formation during thermal stress is also induced to a great extent by compressive stress resulting from the thermal expansion coefficient (CTE) mismatch of different layers. It has also been observed that when a tin-alloy layer is deposited on copper and there is compressive stress in - duced by IMCs created at the interface between tin and copper, the compressive stress near the surface is lower for thicker films, which there- fore are more resistant for tin whisker formation [7,8] . Although full agreement has not yet been reached, it is suggested that when tin plating is over 5 µm [9] (or 8 µm [10] ) thick then the layer is more resistant to the whisker growth. Thick- nesses below 0.5 µm and above 20 µm retard growth even more [9] , although these very thin or thick plates may not be feasible in practice. This paper concerns the whisker growth on the surface of tin-rich, lead-free alloys soldered on a Cu layer above a non-conductive glass-ep- oxy laminate (FR-4) (i.e., with epoxy resin and woven fiberglass reinforcement), which is the most widely used PCB substrate material [11] . The glass-epoxy laminate is a mixture of two materi- als resin and glass fiber, with the resin filling the empty spaces between glass fibers. It was shown previously in [12] that the structure of the glass- epoxy laminate-surface has a fabric-like spatial non-uniformity caused by the regular structure of woven glass fibers and resin regions [12] . Pre- cise analysis of the glass-epoxy surface (before the deposition of the solder) showed that in a general planar view, a grid is apparent. The same grid pattern was visible on the surface of a tin alloy soldered over a glass-epoxy laminate after standard reliability tests of temperature cy- cling conditions (Figure 1). The structure of the laminate is responsible for forming the grid pattern visible on the sol- WHISKER GRoWTH IN TIN AlloyS oN GlASS-EPoxy lAMINATE continues fEATURE figure 1: (a) An SEM image of an alloy surface soldered on glass-epoxy laminate (with a Cu layer) with a visible grid of rectangular grid-field regions (with hillocks and whiskers) and surrounding them grid-frame regions (i.e., flat areas practically without surface roughness). image was taken after tilting the sample to attain better visibility of the grid. (b) Whiskers and hillocks visible within the region of grid field.