Round Robin Testing of Creep Corrosion Dependence on Relative Humidity
Authors: Prabjit Singh, Larry Palmer, Haley Fu, Dem Lee, Jeffrey Lee, Karlos Guo, Jane Li, Simon Lee and Geoffrey Tong, Chen Xu Company: IBM Corporation, International Electronics Manufacturing Initiative (iNEMI), iST-Integrated Service Technology, Inc., Lenovo (Beijing) Limited Corporation, The Dow Chemical Company, Nokia Date Published: 9/17/2017
Abstract: Creep corrosion on printed circuit boards (PCBs) is the corrosion of copper metallization and the creeping of the copper corrosion products across the PCB surfaces to the extent that they may electrically short circuit neighboring features on the PCBs. The paper will report the results of the iNEMI technical subcommittee round robin testing of the effect of relative humidity on creep corrosion on PCBs. The PCBs tested had three different finishes: immersion silver (ImAg), gold on electroless nickel (ENIG) and organic surface preservative (OSP). The PCBs were soldered using two different fluxes: organic acid (OR) and rosin (RO) fluxes. The round robin test used the iNEMI developed flowers of sulfur (FOS) chamber in which the temperature is maintained at 50oC and the relative humidity can be kept constant at various values in the 15 to 80% range. The source of gaseous sulfur is a bed of sulfur and the source of chlorine gas is household bleach in a petri dish. The relative humidity is controlled using a specially designed setup that throttles the release of moisture from the household bleach using a saturated salt solution chosen such that its deliquescence relative humidity is equal to the desired humidity in the chamber. The round robin tests showed the surprising result that in environments high in sulfur and chlorine gaseous contamination, creep corrosion can occur in relative humidity as low as 15%. Since no clear relative humidity threshold may exist below which creep corrosion will not occur, relative humidity control alone may not be enough to eliminate creep corrosion. Unfortunately, this finding suggests that unless the contamination can be controlled at the source, direct control of gaseous contamination by gas-phase filtration may be the only practical means to eliminate creep. The paper showed that the iNEMI creep corrosion test is now sufficiently well developed for consideration as an industry standard qualification test for creep corrosion. The paper also presents the corrosion rates of copper and silver, in environment high in sulfur and chlorine gases, as a function of relative humidity.