Dendritic growth – metallic short circuits – can be eliminated.

A failure mechanism that shows up for investigation every month is metallic short (dendrite) formation around contaminated areas, bias voltage and a fluid media (requirements for a corrosion cell). The combined conditions are present on the failed assemblies and are very repeatable.

Voltage (3V)+hand solder residues+dirty bare boards+moisture at 65% RH
DC                not completely heat-activated  HASL flux       noncondensing    
 = Dendrite (metallic short)

In this case study, the electronics hardware shows poor performance (85% failure rate) in ESS testing and the field. Problems occur on the bottom-side of a soldered assembly at a hand-soldered location, for which extra liquid flux was used and a thick-cored solder. Chloride levels were 9.5 µg/in² and weak organic acid (WOA) levels were 179.23 µg/in². The hand soldering operation was a no-clean process.  

When comparing product that fails in ESS-biased humidity testing to units that do not fail, the data show a clear difference. Good-performing assemblies have chloride levels of 2.5 µg/in² (cleaned bare boards) and no additional liquid flux with WOA levels at 132.11 µg/in² (when hand-solder and wave-solder flux residues are properly heated). The combined conditions present create good product performance and perform well under humidity testing and field exposure.

Voltage (3V)+hand solder residues+clean bare boards+moisture at 65%RH
DC         completely heat-activated  saponified DI water noncondensing     
= Good performance (no short)

Knowing the residues present at this location, predicting the field performance when subject to high humidity conditions is straightforward. The partially heat-activated WOA residues (moisture-absorbing) were, by themselves, not corrosive; however, combined with high chloride residues from a HASL flux (marginal cleaning during board fabrication) the moisture absorbed by the flux was enough to establish the conditions for a corrosion cell. To eliminate the corrosion cell, we must understand what residues are in this specific area.

Because all boards are biased and will always be exposed to varying levels of moisture, all we can control is the process residue type and levels in these localized contamination sites. If fully heat-activated, flux will not absorb moisture, and by cleaning the bare boards, we would eliminate corrosive moisture-absorbing residues of the corrosion cell and pass ESS testing conditions. The chemical conditions that cause dendritic shorts eliminated, good field performance can be ensured.

Terry Munson is with Foresite Inc. (residues.com); tm_foresite@residues.com. His column appears monthly.

• Prev
• Next