Tuesday, July 06, 2010 | C. Xu, W. Reents, J. Franey, J. Yaemsiri and J. Devaney, Alcatel-Lucent
With increasing adoption of lead-free PWB surface finishes, along with increasing product deployments in more corrosive environments, the electronics industry is observing increased occurrences of corrosion-induced product failures. Particularly, creep corrosion on immersion silver has been observed to cause product failures after very short service periods in G2 and worse environments, in some cases less than one year. In our previous work (APEX 2009) , we demonstrated that creep corrosion of ImAg can be correlated to the presence of certain types of surface contamination (for instance residues left behind by organic acid fluxes). In this work, creep corrosion observed on OSP finished circuit boards will be reported. The effect of post-reflow cleaning process on creep corrosion will be discussed. A laboratory MFG test will also be discussed that replicates field creep corrosion. Comparison of creep corrosion susceptibility between OSP and ImAg PWB surface finishes will also be made.
Creep corrosion is the mass transport process in which solid corrosion products (typically sulfide and chloride) migrate over a surface without the influence of an electric field. It was first reported by Egan and Mendizza for Ag2S on an Au surface . In their experiment, a silver substrate was used and one half of the Ag surface was coated with a thick Au layer (see Figure 1). When the sample was exposed to a sulfur containing environment, silver sulfide was formed on exposed Ag. Furthermore, it was also observed that silver sulfide spilled over onto the Au surface and migrated away from the Ag-Au interface. This migration was called creep corrosion . In Figure 1, one can clearly see the leading edge of the creeping silver sulfide. Interestingly, when the experiment was repeated with a Ag-Rh or Ag-Pd interface, Ag2S did not creep onto either the Rh or the Pd surface, even though heavy silver sulfide was formed on the exposed silver surface in both cases. Apparently, Ag2S creep corrosion is highly surface specific and only happens on the Au surface but not on the Rh and Pd surfaces. This is a very important characteristic of creep corrosion. First, a site is required for creating corrosion product, for example an exposed Ag substrate in this case. Additionally, a surface is needed to support the creep of corrosion products. In this case, it is the Au surface. Rh and Pd, on the other hand, would not support the creep of silver sulfide.
Figure 1: Creep corrosion of silver sulfide on Au surface .