They couldn’t understand it. Their new Immersion Silver Printed Circuit Board Assemblies (PCBAs) were experiencing corrosion, but the electronics company had examined its own materials and methods and could not find anything wrong. The boards couldn’t be placed into service until the cause was determined, and in the meantime, they were losing money. So they turned to Foresite.
Though no detrimental levels of ionic contamination were detected, Foresite determined via Scanning Electron Microscopy / Energy Dispersive X-Ray Spectroscopy (SEM/EDS) that sulphur was causing creep corrosion of the PCBAs. Cardboard packaging was believed to be the source of the sulphur that was causing the corrosion. As follow-up work, Foresite conducted an experimental study to evaluate additional materials for the client.
Foresite designed the following experiment:
- Panels from three suppliers (A, B, and C) were shipped to Foresite in normal cardboard packaging. Panels A and B featured Immersion Silver finish, while Panel C was Electroless nickel immersion gold (ENIG).
- A PCB from each panel was placed in a new Kapak-equivalent bag along with a 2” x 3” piece of associated cardboard.
- 6 mL of DI water was added to each of the Kapak-equivalent bags.
- A control PCB from each panel was placed in a new Kapak-equivalent bag alone.
- A 2” x 3” piece of cardboard (inner box) from each of the suppliers packaging was placed in a new Kapak-equivalent bag.
- 6 mL of DI water was added to each of the bags.
- All nine bags were folded over and placed in an oven at 60 oC for fifteen days.
Following the exposure, the samples were analyzed with the C3/C.I. system followed by ion chromatography. Below is the relevant subset of IC data:
All PCBs exhibited low, acceptable levels of ionic and organic contamination with good C3 results after exposure. Visual inspection of the PCBs showed minimal signs of plating degradation on supplier A’s PCB, primarily in the form of small black dots across some of the pads. These were likely visual evidence of minimal, acceptable levels of hyper-corrosion. Supplier B’s PCB exhibited heavy plating degradation and areas of creep corrosion. Supplier C’s PCB exhibited no significant signs of degradation after exposure (as expected with an ENIG finish). It is worth noting that the cardboard sample from Supplier B readily degraded in the presence of moisture, likely enhancing the transfer of detrimental compounds.
Surprisingly, significant differences in physical and chemical composition were noted in the variety of cardboard packaging materials evaluated in this experiment. We recommend a thorough review and understanding of packaging specifications to ensure your product is protected and not subjected to additional contamination. Electronic assemblies should also be packed suitably (e.g. plastic bag, ESD bag) prior to being placed in cardboard packaging for additional protection. They should never be packaged in direct contact with cardboard.
Figure 1 - Creep corrosion after exposure to cardboard packaging and moisture
About the author
Tony Musall, who was previously with Delphi Electronics, has been with Foresite for almost seven years as a project engineer. He specializes in failure analysis techniques, utilizing ion chromatography, FTIR, XRF and X-ray. He advocates problem solving specific to client issues.
Have questions? We would love to help. Contact Foresite, Inc. today to discuss how we can help you figure out your electronics reliability issue.