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"Surface Mount Lead Corrosion"



2005

SITUATION:

I am a manufacturing engineer working for a government contractor that makes circuit cards for aircraft applications. In only one application, a live 3.6 VDC battery is installed on a particular circuit card prior to conformal coating, and prior to our in-line cleaning process.

The card with battery installed travels through the in-line cleaner for 2-5 minutes.

No corrosion is produced at the battery leads, as they are approximately 1/2" apart, and the cleaning solution may not be completely bridging the gap in the spray wash process.

However, there are only two printed wiring board pads connected by traces to the battery, and both exhibit severe corrosion (deplating of the soldered lead and pwb pad to bare copper, near total degradation of the solder joint beneath the lead). Of the two corroded pads in question, one is directly adjacent to a ground pad, and shows the worst corrosion of the two. (note: pad to pad distance is approximately 0.010").

A bare board had 3.6VDC applied by power supply and a drop of the cleaning solution applied between the "hot" pad with adjacent ground pad, and produced severe corrosion within 7 minutes, drawing a peak current of 165 microamps, nominal 100 microamps.

We are still investigating the cleaning solution chemistry (no PH readings, yet), but estimate resistance to be 33K ohms, based on the voltage applied and current measured.

MY QUESTION:

At a constant voltage, how does distance relate to a general deplating/corrosion equation (and is there a standard equation available based on solution resistance and not PH)?

Erik Cotton
Aerospace - Ocala, Florida, USA
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