Pete

I couldn't lead you to a single publication about this item but I try to give some answers from my experience.
I believe the most important thought that has to be kept in mind is the fact that any polymer is permeable to humidity. There are variations in the amount of water a polymer can absorb until it is saturated and there are variations how fast the water diffuses through the polymer until it reaches an interface but this is a natural process in polymersthat can not be avoided. On this thought I'd like to look at your questions:

- Yes, component encapsulations and PCB materials will absorb humidity until an equilibrium of the water content of the polymer ant the surrounding atmosphere is reached. This is not a stationery condition but a dynamic process that depends on the changing environment. How fast the water contents of a polymer changes depends on the material properties, on the difference in the water contents of the polymer and the surrounding media and on the temperature. A water splash will not significantly enhance the contents of moisture due to the fairly low diffusion coefficients of the polymers used.

- Yes there are studies about the influence of moisture on the performance of components in the field on IC's. Humidity combined with the ionic impurities that are  present in IC packages are an important root cause for corrosion on the surface of chips.

- If moisture is trapped under a conformal coating it will be
1) absorbed by the coating
2) absorbed by the components encapsulation
3) absorbet by the PCB
4) diffuse through the coating
These effects will lead to a varying contents of humidity in the encapsulation of the component depending on the climatic conditions around an assembly. 
Problems might arise if a coated assembly is standing in a warm humid environment for a while and then undergo a fairly rapid change to low temperature. Then the relative humidity in air pockets below the coating will rise, eventually above the dew point. This can cause all sorts of things like shorts and corrosion.
 
- Additionally,  hygroscopic residues below the coating will , like polymers, absorb moisture. The source of this moisture is the surrounding medium in this case the coating. This means, the coating is getting drier thus absorbing moisture from the environment with  the effect that a transport of water through the coating takes place. The unfortunate thing is that the equilibrium of moisture absorption depends on the water pressure in the materials. And this pressure can be quite high it can lead to blisters below coatings when the volume of the contaminant is increasing dramatically during absorption ( As it is the case when salts are absorbing so much water that a saturated water/salt solution forms).

Did that help you? I hope I didn't tell you any old cheese as we say in Switzerland.

Have a great day

Guenter

Guenter Grossmann

Swiss Federal Institute for Materials Testing and Research EMPA
Centre for Reliability
8600 Duebendorf
Switzerland

Phone: xx41 1 823 4279
Fax :      xx41 1823 4054
mail:     [log in to unmask]

---------------------------------------------------------------------------------
Technet Mail List provided as a free service by IPC using LISTSERV 1.8d
To unsubscribe, send a message to [log in to unmask] with following text in
the BODY (NOT the subject field): SIGNOFF Technet
To temporarily halt delivery of Technet send the following message: SET Technet NOMAIL
Search previous postings at: www.ipc.org > On-Line Resources & Databases > E-mail Archives
Please visit IPC web site (http://www.ipc.org/html/forum.htm) for additional
information, or contact Keach Sasamori at [log in to unmask] or 847-509-9700 ext.5315
---------------------------------------------------------------------------------