PCB007 Magazine

PCB-June2014

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60 The PCB Magazine • June 2014 Actual sections showing the levels of hyper- corrosion are shown in Figure 1. Due to extensive wetting balance and sol- der spread testing (along with BGA shear test- ing), Level 1 and Level 2 conditions did not re- sult in failures. Wetting forces and solder joint strengths continued to meet criteria for accept- ability. Certainly, Levels 3–5 are cause for con- cern. Although Level 3 spikes do not lead to wetting failures, this condition would be an in- dicator for future reference. So again, be vigilant with respect to categorically rejecting parts for very minor imperfections. If that was the case, the entire IPC-600 document would be re-writ- ten where even the most minor imperfections would fall in the non-conformance category. So what would you do with the following? Figure 2 shows Level 2 corrosion spikes (left) and on the right a section with Level 3 spikes. Note there is no issue with solder wettability in either of the soldered sections. Again, it is important to recognize that on occasion there will be minor corrosion spikes visible within the nickel deposit. And one should not rush to judgment and categorically reject the circuit boards without first studying solderability data. Regardless, there are numer- ous strategies and process control techniques that can be employed to eliminate any of these issues. Some of the most critical controlling the phosphorous and gold contents. Control the Phosphorous Content Yet it stands to reason that the lower the phosphorous content in the nickel deposit, the greater the possibility of corrosion. So one may decide to (by design) utilize an EN process that tends to co-deposit a higher phosphorous con- tent upwards 10–11% by weight. While this lev- el of phosphorous will certainly minimize the chance of hyper-corrosion, there are also two negative impacts. The first is the possibility of a thinner gold deposit. This stands to reason since the gold deposition mechanism relies on the electromotive force to drive the reaction. The more corrosion resistant the nickel, the thinner the immersion gold deposit. Thus the IPC ENIG spec could easily be violated. The second con- cern is that the higher phosphorous content of the nickel will negate proper solderability of the nickel (reduced wetting). I suggest that in order to optimize the thick- ness of gold (within spec) and enhance solder- ablity without excessive corrosion, engineers Figure 1: degrees of hyper-corrosion. THE DEgREES oF NICKEL HyPER-CoRRoSIoN AND MITIgATIoN STRATEgIES continues

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