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Cleaning moisture sensitive components?
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Cleaning moisture sensitive components?

by jseaber on Fri Jun 28, 2013 1:47 pm

We're considering a low volume SMT line for our shop. I'm confident we can quickly learn to stencil, program a pick-and-place, and figure out a thermal profile for an oven.

So far, I've found little information about recommended cleaning processes. I'd rather not invest in a $40k+ defluxer at our volumes. One SMT equipment re-seller says a dishwasher works great, as long as you adhere to local water regulations. Seriously? :shock:

Several of our boards require 0.1% thin film resistors. DigiKey briefly explains:

In addition, thin film resistors (except for the RNCS and HDM) are susceptible to moisture if operated under extremely humid conditions at low power loading.

In the few instances I've had to wash these boards by hand, compressed air was not sufficient to dry them. Every resistor must be subjected to 250C+ heat from a hot air rework station. That's fine when we're handling 1 or 2 boards. Drying a batch of 500 boards this way could take all day...

Has anyone dealt with similar moisture sensitive parts before? Could we simply bake the boards again in the oven, at a lower temperature (after cleaning)?
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Re: Cleaning moisture sensitive components?

by adafruit_support_mike on Fri Jun 28, 2013 4:34 pm

WRT tihn-film resistors, moisture is a long-term issue rather than a short-term issue. The nichrome resistive layer can corrode over time, changing the properties of the film. Having a voltage gradient across the resistor can make corrosion happen faster, since oxidation is one of the major electrochemical reactions.

As far as washing the flux off the boards goes, don't worry about it. Chuck the boards in a toaster oven, set it to 125C (2560F), and let the set for an hour. That will bake off any moisture.

Make sure your water doesn't leave mineral deposits behind when it evaporates though. That could possibly create a whole new set of problems.
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Please be positive and constructive with your questions and comments.