SMTA International Conference Proceedings


Author: Robert Veale
Company: Rockwell Automation
Date Published: 9/25/2005   Conference: SMTA International

Abstract: The RoHS initiative requires that PCBs use an alternate finish coating to replace the tin/lead HASL coating. The four candidates are immersion gold, immersion silver, immersion tin and an organic solderability preservative (OSP). While there has been much published regarding solderability and other general issues, the area of corrosion has not been adequately addressed. Industrial environments can and do produce corrosion of exposed metal surfaces and these corrosion products can lead to electrical failure.

The type of corrosion in the industrial environment is typically sulfur-driven corrosion and is found in steel, paper and tire manufacturing as well as other industrial processes. In this study, four alternate PCB finishes were subjected to an accelerated mixed flowing gas corrosion test and the results analyzed. The important findings of this study can be summarized as follows:

1) The immersion gold (ENIG) and immersion silver surface finishes failed early in the testing. These coatings are the most susceptible to corrosion failures and are expected to be much more susceptible than the traditional Sn/Pb HASL coatings. The use of these two coatings may make the PCB the weak link with regard to the robustness of the electronic module to corrosion.

2) The Instrument Society of America (ISA) has classified several levels of environmental severity for process and control systems: G1, G2, G3 and GX. G1 is benign and GX is open-ended and the most severe. [1] None of the coatings can be considered immune from failure in an ISA G3 environment. The immersion tin and OSP coatings could be expected to survive an ISA G2 environment.

3) The gold and silver coatings could not be expected to survive a mid to high G2 environment based on these test results.

As a side experiment, a tin/silver/copper and Sn/Cu/Ni solder alloys were applied and tested in this corrosive environment. There is no indication that the low silverbearing alloys will lead to corrosion failures due to silver migration. In fact, the solder coatings are protective against corrosion and if offered as a lead-free HASL PCB coating would offer superior corrosion protection.

Key words: PCB, corrosion, sulfur, ENIG, OSP, immersion silver, immersion tin. HASL

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