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50 The PCB Magazine • December 2014 by Michael Carano omg electronic chemicalS Getting to the Root Cause: Solderability Defect Analysis c o l u m n trouble iN your taNk In this case study, the PCB fabricator inves- tigates the root cause of solderability defects on ENIG-processed circuits. It was determined that the fabricator, due to process errors, caused hy- per-corrosion within the nickel deposit, which led to the defects. Introduction A PCB fabricator received a number of re- turned PCBs from one of its assembly customers. The issue, according to the assembly team, was dewetting, and in some cases, non-wetting of surface mount pads. In addition, some of those components failed to make a reliable bond to the pad, leading to significant failures. And as so often is the case, someone invokes the ghost of black pad as the culprit. Certainly while such an anomaly is a possibility with respect to the root cause failure mechanism, this case requires significant study in order to properly reach a so- lution to the problem. Problem Identification First, one must identify the problem or defect (Figure 1 and Figure 2). I consider this a very minor issue con- sidering the board de- sign, reflow tempera- tures, and paste print could have a great deal to do with what is seen here. Now a few other points to consider. The surface finish used for these parts is elec- troless nickel-immer- sion gold (ENIG). As is often the case, the first thing one does when seeing any type of solderability/assem- bly defect is to suggest ENIG is the issue and the root cause is black pad. Sometimes that is Figure 1: note area where solder did not completely wet-out. Figure 2: Some additional dewetting.