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Relative Humidity Dependence of Creep Corrosion on Organic-Acid Flux
Relative Humidity Dependence of Creep Corrosion on Organic-Acid Flux
This paper reports on the dependence of creep corrosion on humidity in the 15 to 80% relative humidity range on finished PCBs soldered with organic acid flux.
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Authored By:
Haley Fu - iNEMI, Shanghai, China
Prabjit Singh - IBM Corporation, Poughkeepsie, NY, USA
Dem Lee and Jeffrey Lee - iST-Integrated Service Technology, Inc., Taiwan
KarlosGuo and Julie Liu - Lenovo (Beijing) Limited Corporation, Beijing, China
Simon Lee and Geoffrey Tong - The Dow Chemical Company, Taipei and Hong Kong
Chen Xu - Nokia, Murray Hill, NJ, USA

Summary
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 electrically short circuit neighboring features on the PCB. The iNEMI technical subcommittee on creep corrosion has developed a flowers-of-sulfur (FOS) based test that is sufficiently well developed for consideration as an industry standard qualification test for creep corrosion. This paper will address the important question of how relative humidity affects creep corrosion.

A creep corrosion tendency that is inversely proportional to relative humidity may allow data center administrators to eliminate creep corrosion simply by controlling the relative humidity in the data center,thus, avoiding the high cost of gas-phase filtration of gaseous contamination. The creep corrosion relative humidity dependence will be studied using a modified version of the iNEMI FOS test chamber.

The design modification allows the achievement of relative humidity as low as 15% in the presence of the chlorine-releasing bleach aqueous solution. The paper will report on the dependence of creep corrosion on humidity in the 15 to 80% relative humidity range by testing ENIG (gold on electroless nickel), ImAg (immersion silver) and OSP (organic surface preservative) finished PCBs, soldered with organic acid flux.

Conclusions
The relative humidity in FOS chambers can be controlled at any humidity level in the 15-80% range in the presence of household bleach by throttling the flow of moisture evaporating off the household bleach and forcing the moisture to flow over a saturated salt solution so that the salt solution can dominate and control the relative humidity at its deliquescence relative humidity value.

The corrosion rates of copper and silver and the rate of formation of AgCl in a sealed FOS chamber, loaded with test PCBs, decrease with rising relative humidity most probably because of the absorption of the sulfur and chlorine gases by the moisture adsorbed on the test PCB and the chamber walls.

Test PCBs soldered with organic acid flux showed high propensity to creep corrosion at relative humidity levelsas low as 11-17%. This result provides the first direct experimental evidence of creep corrosion occurring on "dry" surfaces on PCBs that are likely contaminated with un-deactivated flux residues.

There is probably no common threshold relative humidity level below which creep corrosion will not occur.

Creep corrosion of OSP, ImAg and ENIG finished test PCBs soldered with rosin flux will be studied as a function of relative humidity and reported in the near future. Following this study, the iNEMI creep corrosion qualification test relative humidity condition may be revised.

Initially Published in the IPC Proceedings

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