A client contacted us for help – one of their normally reliable products was being returned to them as failures; some units tested as failed, some were NTFs. The returns were disproportionately coming from one geographic area – very disproportionately. New production units and the returned “failures” passed our client’s ROSE cleanliness testing. The returned units had significant corrosion of the silver finish.

What We Found

We locally extracted selected areas of a new production assembly with the C3 and performed IC analysis on the samples. We agreed with our client that their processes were resulting in clean assemblies. In addition to corrosion on the returned samples, there was a black residue in spots. SEM/EDS analysis indicated a conductive residue consisting of copper and silver (no surprise), plus sulfur. Repeating the extraction/IC analysis on the same areas of returned units resulted in a very high sulfur reading in all tested areas, including on the top surface of a BGA and in a reference board area away from components. 

What Is Causing This?

Our conclusion was that exposure to high concentrations of gaseous sulfur during use was resulting in creep corrosion. The absence of dendrites also indicated that this was the result of a warm, dry atmosphere. We had just described the area producing the majority of the returns – a warm, dry area suffering from air pollution.

Confirmation

As a follow-up experiment, we exposed a Foresite test board having immersion silver plated traces at 70oC in < 25% RH in a sealed bag with a sulfur-rich clay for 24 hours. These “flowers,” shown below, grew from a resulting copper sulfide residue.

In spite of well-controlled manufacturing processes, our client suffered failures of a product that had performed well in a variety of applications in the past.