Paco Solis will be presenting, "Metal Whiskers - Does Surface Contamination Have an Effect on Whisker Formation?", at SMTA AIMS Harsh Environment Electronics Symposium in San Diego, CA. His presentation is on Monday, October 5, at 1:30.

Creep Corrosion

Due to RoHS compliance, today's electronics manufacturers are selecting immersion silver as a viable board finish for protecting copper surfaces prior to assembly. Immersion silver provides a flat, plated surface, which allows for superior solderability and has the potential for a long shelf life. However, the exposure of the silver finish (from all vendors) to a sulfur-rich environment with moderate humidity can cause a very distinct chemical reaction identified as "Creep Corrosion."

Copper Sulfide (Cu₂S) crystals that form are not just a corrosion issue that can cause an open circuit but are conductive crystals capable of carrying enough current to short circuit the hardware. Can assemblies that have developed creep corrosion and electrical shorting be cleaned? After a period of research and development, we have been able to configure a cleaning process capable of removing the Cu₂S and the corroded silver. Our client, after cleaning rejected assemblies, applied conformal coating to them, put some through environmental testing and returned the remaining assemblies to the field. Test data and product performance have been positive. Utilizing conformal coating, the client was able to create a protective barrier that minimized moisture collecting on the surface of the assembly.

Summary: Creep corrosion failures can be cleaned; hardware recovered, and put back into service. It is important to clean and remove contaminants prior to the conformal coating process to ensure long-lasting product performance. Over time we expect to see more instances of creep corrosion failures due to more hardware being exposed to uncontrolled operating conditions where moisture and sulfur are present.

The current economic climate may well be dictating that product need be recovered rather than scrapped. Foresite has seen an increase in recovery cleaning of electronic hardware and we have gone to great lengths to educate ourselves regarding the best techniques for effective residue removal. Assemblies are not all created equal; size, spacing, types of components, and materials used to manufacture, all factor into the proper cleaning protocol. Cleaning in an in-line unit with saponifier and DI wash/rinse may be sufficient for one type of assembly, while another may require a pre-soak, in-line wash/rinse, secondary steam, and IPA dip. And, do you know if your cleaning process is "losing steam"? The ability to validate the consistency of your cleaning process is vital. Are the last assemblies through the in-line as clean as the first? We recommend monitoring your cleaning chemistry concentration, DI water quality, and the assemblies at the beginning, middle and end of the cleaning run. Ion chromatography (IC) allows you to monitor the process and quantify any ionic residues left behind after cleaning.

Things to consider:

• Saponifiers typically contain amines, which can be detrimental to product performance if not properly removed.
• Flux residues can become trapped under components (not easily removed with traditional cleaning processes) and cause field failures.
• Use of tap water in the wash or rinse process can increase ionic residues on an assembly.