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Water cleaning and Moisture Entrapment

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Bill Childs


Water cleaning and Moisture Entrapment | 24 December, 2008

I can't seem to locate any information/concerns about moisture entrapment in IC packages on the promary side in a double sided SMT assembly using water soluable solder paste when soldering the second side. A typical process would assemble/reflow/clean side 1 then assemble/reflow/clean side 2. We bake the assemblies after the first cleaning process at 220F for 2 hours but recently had a failure of a device that looks very much like a moisture induced failure that raised my suspician the these parts may be absorbing water from the first wash. These parts (SO14 package)were not classified as moisture sensitive but were 5 years old and had not been stored in a sealed container. For the most part every one of these parts failed, visibly showing a crack in the body. They were the only device on the assembly that failed or showed any evidence of cracking.

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Water cleaning and Moisture Entrapment | 29 December, 2008

It takes time for the moisture to penetrate into the body. I think the 5 years of ambient would be more the culprit than washing the boards. Especially since you are baking them after wash.

Get a few of the ICs and put them loose on a board and put them through at reflow temperatures and see if they crack without the washing step.

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Bill Childs


Water cleaning and Moisture Entrapment | 2 January, 2009

Thanks for the reply Stephen. The time factor sounds logical for moisture invasion during cleaning, probably why there's no articles showing concern about the process. I did try experimenting with some samples and believe it or not could not duplicate the failure; I even tried cleaning them and reflowing the second side and still no failure. Mysterious stuff....... Guess I'll chaulk it up to the 5 year cycle but still a little puzzling.



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Water cleaning and Moisture Entrapment | 6 January, 2009

The easiest way to prove out the absorbtion theory is to weigh the parts before wash, after wash, and after baking. You probably want to do this with the bare parts in isolation as the fab will pick up considerable moisture.

Differences in weight show absorbtion or dissipation of moisture.

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Water cleaning and Moisture Entrapment | 12 January, 2009

Hi Bill, You should try pre-baking any such old devices prior to using on SMT Reflow. Refering to MSL-type3 guidelines could help in your case.

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