Mike,

It is almost a given that some moisture will be adsorbed onto the surface prior to potting. Potting compounds and conformal coating, and really all polymers, are permeable to moisture.  The rate of permeation varies with the polymer and the thickness of the polymer.  It is my understanding, that over time, the moisture on the surface and in a potting material, or in a coating, will come to equilibrium with the external environment- assuming good adhesion to a clean surface.  The more important thing to worry about, in my understanding, is the cleanliness of the potted or coated items.  Contamination on the surface may prevent adhesion and there will be no equilibrium at that location as moisture will follow a path along the unbonded surface.   Additionally, if the contamination is water soluble, it will attract moisture to itself over time-  likely becoming a failure site.  This phenomena was called vesication by Carl Tautscher in his book "Contamination Effects on Electronic Products".  Carl described the water attraction of the water soluble contaminant as an "osmotic pump".  A search of Google Books will bring up passages from the book.  Separately, I will send you an old (1966) but good article on the permeability of polymers.

Good luck.
Mary Davis
(Semi-retired) Materials & Process Engineer

-----Original Message-----
From: TechNet <[log in to unmask]> On Behalf Of [log in to unmask]
Sent: Monday, April 1, 2019 8:39 AM
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Subject: [TN] risk of PCBA moisture uptake

Hello fellow designers,

One of our products uses 3 PCBA's made on our internal surface mount line. After the PCBA's are created, they are hand soldered together with through hole components and then put into a housing which is then filled with an Epoxy resin (Stycast 2075).

We are wondering about moisture update on the PCBA's between the surface mount process and our hand soldering process. There is some concern that any moisture absorbed would be trapped by the resin and therefore lead to premature electrical failure in harsh applications. Our factory is located in Ontario and is air conditioned in the warm months.

Option 1 (best case)
We would like to avoid any special drying or baking process on the PCBA's.
*       Does anyone think there would be a risk in doing this?

Option 2
We could put them in a dry cabinet. The one we have is a Dr. Storage T40W-600. It runs at 40C and <=5%RH.
*       Is this temperature of 40C enough to dry out the PCBA's?
*       How long should they be stored in there before use?

Option 3
We could buy a baking oven similar to what is used for components prior to re-flow.
*       What temperature and duration would be suitable?

I have some IPC standards (J-STD-033D, 1601A) but I'm not sure how to relate our PCBA's to the tables of drying/baking conditions.

With Best Regards,
Mike Messenger, P.Eng, PMP

Siemens Canada Limited
Manufacturing Engineer
PD PA PI LWT IE
1954 Technology Drive
Peterborough, ON K9J 7B1
705-740-7025
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