PCB007 Magazine

PCB-Dec2014

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52 The PCB Magazine • December 2014 true. However, further analysis is required be- fore one can accurately draw any conclusion. It was suggested that cross-sectional analy- sis be performed in order to look at the inter- face of the solderjoint and pad on those areas that appeared dewetted or non-wetted (Figure 3). Note in Figure 3 the very non-uniform electroless nickel deposit. In addition there appears to be corrosion spikes of the immer- sion gold into the nickel deposit. Now with this information there are a few critical ques- tions the troubleshooter needs to ask and find answers to: 1. What gold thickness is being deposited on the nickel? 2. What is the phosphorous content of the nickel being deposited? 3. How long of an immersion time is used for the gold process? With additional analysis performed, a num- ber of these questions are answered (Figure 4). A closer look at the cross-sections in Figure 4 provides evidence of corrosion spikes in the nickel after the deposition of the immersion gold. What accounts for such a stark contrast? These are both of the same design. The only dif- ference was that the board represented by the coupon on the left was processed down the ENIG line by a member of the tech team. In this case, that individual adhered to strict proce- dures as to dwell times, operating temperatures in each process tank and checked the analysis of each process step to ensure all of the key ad- ditives and concentrations were within the pre- scribed ranges. With this information in hand, the team found out that due to a customer request for a thicker gold deposit (4–5 µin as opposed to that called out in the IPC Spec 4552), the PCB fabricator processed those parts in the immer- sion gold for a longer period of time (essen- tially 19-minute dwell time as opposed to the 10-minute dwell time recommended by the supplier). The idea for the longer dwell time GETTING TO THE ROOT CAUSE: SOLDERABILITy DEFECT ANALySIS continues Figure 4: normal nickel structure on test coupon performed by trouble shooting team (left). on right, coupon from board processed by the Pcb fabricator. Figure 3: after assembly.

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