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60 SMT Magazine • August 2015 PCB Nickel Foot Over the last few years we have seen more of this phenomenon, which indicates very poor process control in PCB manufacture. The nickel has extended onto the surface of the laminate and in one case the side wall of the solder mask. This has allowed the gold to also coat the surface (Figure 7). This could be an overactive plating process of copper needles left in the surface of the laminate after the etching process, and can impact the electrical performance and also increase the number of solder shorts. Copper Dendrites The three examples all show copper den- drites formed on the surface of printed circuit board assemblies. In each case they lead to in- termittent failures in the field. They will oc- cur with a tin/lead and lead-free process and need to be analysed to find the root cause of the problem. The examples show the formation of a copper dendrite/fern between two conduc- tors. This fault may occur when flux residues remain on the board surface and are then sub- jected to high temperature and humidity. A circuit with a voltage applied of 5–10 volts can figure 6: The silver layer between the resistive element and the termination was being corroded in au- tomotive and medical applications. figure 7: example of nickel foot. FeAture CASE STuDY: DETECTING PROCESS DEFECTS continues

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