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Getting to the Root Cause - Solderability Defect Analysis

In this case study, the PCB fabricator investigates the root cause of solderability defects on ENIG-processed circuits. It was determined that the fabricator, due to process errors, caused hyper-corrosion within the nickel deposit, which led to the defects.    

Introduction

A PCB fabricator received a number of returned 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 solution to the problem.

Problem Identification

First, one must identify the problem or defect (Figure 1 and Figure 2).

Figure 1: Note area where solder did not completely wet-out.

I consider this a very minor issue considering the board design, reflow temperatures, and paste print could have a great deal to do with what is seen here.

Figure 2: Some additional dewetting.

Read the full column here.Editor's Note: This column originally appeared in the December 2014 issue of The PCB Magazine.