Relative humidity dependence of creep corrosion on printed circuit boards

Creep corrosion on printed circuit boards (PCBs) is the corrosion of copper metallization and the spreading of the copper corrosion products across the PCB surfaces to the extent that they may merge and electrically short circuit neighboring features on the PCBs. The iNEMI technical subcommittee on creep corrosion has developed a flowers-of-sulfur (FOS) test chamber in which the relative humidity can be controlled anywhere in the range 15 to 80% using a saturated salt solution with the deliquescence relative humidity of the salt equal to the desired relative humidity in the chamber. In addition to the sulfur vapors, the chamber has a specially designed setup to allow the release of chlorine gas off household bleach in a somewhat controlled and repeatable manner. The chamber had been used earlier to study the effect of relative humidity on creep corrosion on PCBs soldered using organic acid (OR) fluxes. This paper extends the study by including PCBs soldered with rosin (RO) flux. Preliminary finds are that both the fluxes are similar in their propensity to cause creep corrosion. The study confirms the observation that creep corrosion on OSP (organic surface preservative) and ImAg (immersion silver) finished PCBs occurs only in the low relative humidity range of 15 to 50%; whereas, creep corrosion on ENIG (gold on electroless nickel) finished PCBs can occur both in low and high relative humidity environments contaminated with sulfur and chlorine gases. Evidence of the initiation of creep corrosion at the solder mask overlap over copper metallization is presented.