We were recently approached by a client who was having field failures on conformal-coated PCBAs with visible dendrites on SMT capacitors. In addition to the dendrites, a blue residue was observed. Similar components, used in other areas of the PCBA, were unaffected.

Through the use of localized extraction and ion chromatography analysis of the initial samples, coupled with on-site consultation, the root cause was identified and possible solutions recommended.

What Is The Problem?

In this particular process, SMT capacitors are mounted within a field of PTH pins that are subsequently wave soldered. A water-soluble, temporary mask is applied to the SMT capacitors to shield them from the wave. Then, a wash process follows to (hopefully) remove the water-soluble mask.

Initial C3 localized extraction and ion chromatography analysis indicated heavy ionic contamination in the areas with dendritic growth. Due to the location of the failures and the visible blue residue, the initial belief was that the water-soluble mask was not being fully removed, contributing to the formation of an electrochemical migration cell.

The ion chromatography data alone was not sufficient to determine whether the ionic content originated from the temporary mask or from one of the fluxes used on the PCBA. Follow-up testing of raw material found that the water-soluble, temporary mask had extremely high ionic content, making full removal vital.

Walk the Line

Foresite was invited to visit the contract manufacturer (CM) responsible for these assemblies. It was noted that a batch washer was being utilized for this particular product. Batch washers can produce inconsistent cleaning in certain situations, and this particular equipment was not in an optimized state.

In observing the full assembly process, it was also noted that subsequent hand soldering and localized cleaning processes were “not in statistical control,” to put it nicely.

Optimize the Process

After optimizing the wash and hand solder/clean processes, ninety-seven percent of the samples passed C3 localized testing with “CLEAN”/180sec. readings. The outlier is likely indicative of the inconsistent batch washer process. As a result, the client is currently exploring the option of transitioning to an in-line cleaning system, which in our opinion, should dramatically improve wash performance and consistency.

Conclusion

This particular failure was likely caused by a combination of factors, namely the incomplete removal of the water-soluble, temporary mask and the out-of-control hand solder/clean process further down the line. The CM had included several wash processes throughout the assembly process; however, as with any process, problems can arise when proper optimization, controls and verification are not in place. Once again, we are shown real-world proof of the importance of cleanliness to PCBA reliability.