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Tarnish Occuring after Washing???

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#64682

Tarnish Occuring after Washing??? | 21 July, 2011

I am having an recent issue occuring that after I have washed our PCBs using only high pressure DI water heated to approx 150 degrees and then being dried at 120 degrees, the once shiny metallic finish is now becoming a dull tarnished color, it does not come off after spot cleaning either anyone have suggestion or can offer help would be greatly appreciated.

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#64683

Tarnish Occuring after Washing??? | 21 July, 2011

what exactly is tarnishing parts, pads, solder joint?

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#64684

Tarnish Occuring after Washing??? | 21 July, 2011

What is the metal finish? Silver, HASL, Tin??

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#64690

Tarnish Occuring after Washing??? | 21 July, 2011

We have seen this before. We switched to a cold water wash and the issue went away. The hot wash changed the plating brightness somehow.

`hege

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#64699

Tarnish Occuring after Washing??? | 24 July, 2011

As a shot in the dark response to a hopelessly vague question ....

Back in the old days, we'd see corrosion of metal parts after DI water washing when our water was on the high-end of the resistivity range, usually after we received fresh tanks from our supplier. Starting out, fresh tanks would have a very high resistivity, maybe around 18 megaohm. Sometimes, it would turn some PTH crystal cans the color of tea.

Face it, DI water is corrosive. Prove this to yourself by tossing a copper penny into your rinse tank, fishing it out in a couple days, and reporting back to us about the changes on the appearance of the coin. That's one of the reasons that you have plastic and don't have copper piping from your DI water source.

Typically, DI water generating equipment tends to produce water with a resistivity of 2 to 5 megaohm. Water with resistivity of greater than 10 megaohm can cause leaching of metals, which appears as corrosion issues.

If your DI water process controls don't include resistivity (or conductivity) upper and lower limit monitoring, you might consider CARing your machine to attempt to assure something gets done to improve that element of your process control regime.

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