Hi Luis,

Just a quick reply.

>>I don't understand your question with regards to water.  Your board should
>>not be exposed to any water until the assembly process has completed (both
>>sides and wave solder) and you are washing the board.
>
>The question is: Can I use OSP to add a water (humidity) protection to the
>circuit after assembly and during its operational life ?.

No, it will be burned off after the solder process.

>>From your fine explanation of the process guess that this is just an
>intermediate process and not a final work in the PCB manufacturing.
>
>Have tested the solderable varnish to achieve this water protection and it
>works very well. The varnish Covers all the PCB, IC pins, etc and if you
>have any component that can be damaged if covered by varnish (connectors,
>jumper pins, etc) you can solder them after varnish application. Then of
>course will be exposed to oxidation but you can change them there after.
>
>Didn't have so much luck with liquid epoxy. For mechanicaly complex circuits
>you need to pot the circuit in a mould, something hard to manufacture and
>most of the time needing "hand trimming" the resultant body to make it fit
>correctly in its working place. If you use a more dense material (a kind of
>paste, silicone, etc) there will be a lot of cavitys between components wich
>will fill with liquid if exposed to any humidity/water. Water takes longer
>to evaporate from there and oxidation will cause problems.

For your application, you're going to have to be careful of a few things:

1.  Your boards MUST be EXTREMELY clean and dry before applying the varnish.
You might want to bake the cards at 40C for forty eight hours or so before
applying the varnish.  If you trap flux and water under the varnish, you're
asking for problems with dendritic growths.

The formula to get metallic growths (dendrites) is Flux + Water + Voltage Bias.

2.  If this is for a product, you might want to look into a commercial
conformal coating process (where the coating is a gas and is allowed to
precipitate upon and harden all around the card and components).  For
connectors, they are protected while in the chamber.

Conformal coatings are typically available as an offering to suppliers of
military electronics although you will find a lot of commercial suppliers
using it when there is a problem with moisture sensitive components on their
product.
Good luck!

myke

"I was well aware that the processes of puberty are often fatal to psychic
power."
- Sir Arthur Conan Doyle