Dear all,

We have received few MLBs from a contract supplier. The board was inspected and accepted for wiring. The assembly is for high reliability applications. We conformal coated, tested,thermal cycled and closed the package. We opened the package for some rework, we found corrosion under solder mask on some tracks. 

The corrosion is in the form of  blisters and localized. Center of the tracks or edge or localized and not covering the entire width of the tracks. 

What could be the reason for corrosion under solder mask ?. How to diagnosed for root cause analysis ? regards,

R.Saravanan