A client recently contacted us with a problem of chip resistors failing with a higher-than-acceptable resistance on conformal coated PCBAs inside a sealed enclosure. Upon visual inspection, a black residue was observed at the end terminations of suspect components under the conformal coating.  

Through a combination of analytical techniques, including ion chromatography, XRF, and cross-sectioning, the failure mode was identified, the root cause was determined and a solution was recommended.

What Is the Problem?

The client was experiencing a small number of field returns of an automotive electro-mechanical assembly. Chip resistors were found to have out-of-tolerance (high) values or, in some cases, were completely open circuit. The client had noted a small amount of visible residue adjacent to the end cap metallization of suspect chip resistors. Ion chromatography and XRF identified the residue as silver sulfide, caused by the reaction of the silver base metal with environmental sulfur.  Corrosion of the silver base metallization affected the resistance of the devices.

This presented a few additional questions that needed to be answered.

• How is the base metallization (which shouldn’t be exposed) reacting with sulfur?

• What is the origin of the sulfur in the sealed enclosure?

• Why wasn’t the resistor protected by the conformal coating?

Why Is This Occurring?

Let’s answer, to the best of our knowledge, the three questions above.

How is the base metallization (which shouldn’t be exposed) reacting with sulfur? Cross-sectioning was employed. SEM imaging revealed gaps between the end cap metallization and the protective epoxy topcoat, which left small areas of exposed silver that could react with environmental sulfur.  

What is the origin of the sulfur in the sealed enclosure? Ion chromatography was conducted on other components of the overall assembly. It was determined that a black foam dampener used elsewhere in the sealed enclosure was the source of the soluble sulfur. A follow-up experiment was conducted with bare copper traces and suspect resistors exposed to, but not in contact with, the black foam material.  

Why wasn’t the resistor protected by the conformal coating? At elevated temperatures, with no bias, resistors exhibited the same failure mode within about (8) weeks (visible corrosion product and value drift) - the copper traces were heavily corroded as well.

Why Didn’t Conformal Coating Prevent This Failure?

The conformal coating used on this particular PCBA was silicone-based.

Silicone-based coating materials (and to an extent, all coating with the exception of vapor-phase deposited parylene), generally provide good chemical and salt spray resistance, but DO NOT provide a barrier that is completely impervious to moisture or gas.

Moisture and environmental contaminants will eventually permeate the coating material and reach the board surface if constantly exposed.

Conclusion

No matter the care that goes into PCBA design and manufacturing, a single chip resistor that does not meet quality requirements can cause headaches, including field returns. Even clean, coated, well-assembled PCBAs are at risk when placed in service in a difficult environment.

PCBA reliability needs to be a concern for both the electrical and mechanical disciplines, lest the harsh environment be self-induced. In this case, removing the sulfur source (black foam dampener) from the assembly will resolve the issue.