My personal feeling is not to use a solder mask, at all, but to 
thoroughly clean the finished assembly to proven levels and to pot in a 
slow-curing two-component **flexibilised** epoxy, under vacuum. However, 
the most important point is that the board must be designed correctly in 
the first place to avoid local heating. Read D.J. Dean's book Thermal 
Design of Electronic Circuit Boards and Packages (Electrochemical 
Publications). IMHO, if you have salt and steam conditions, then 
protection of the finished assembly is a must, obligatory, mandatory if 
you wish to avoid spectacular firework displays with the current that is 
obviously available, although you don't mention the voltage. This is a 
case where the cost must be subordinate to the conditions of use.

I don't agree with making the board as a multilayer. 105 µm copper in 
innerlayers is a pig to manufacture for reliable operation and you 
cannot carry 20 A through single through-holes under normal conditions. 
This idea is a stick to break over your own back.

Avoid pumice or other abrasive scrubbing: it implants particles in the 
copper and these can actually weaken the adhesion, as they can pull away 
from the metal under strain.

Brian

John Parsons wrote:
> Greetings all,
> 
> We are a fab shop and are looking for a solution to a problem being
> experienced by one of our customers.  They have a board which resides in a
> somewhat hostile environment
>         - steam/salt water vapour
>         - board carries 20Amp current
>         - susceptible to multiple thermal excursions which put further stress on
> the unit
> The problem, as one might expect, is that the soldermask is not man enough
> for the job.  Some units have exhibited significant solder mask flaking from
> the conductors after as little as 4 hours of operation.  FYI, board is
> double sided with 3oz base Cu.
> 
> The product is currently only in the prototype stage but production volumes
> are expected to be in the 100,000-250,000 units annually.  We have proposed
> the obvious solution of utilizing a conformal coating post assy but that is
> an option they would initially prefer to avoid due to the inherent costs
> involved.  They would like us to first exhaust any possibility that the
> problem can be averted from the pcb fabrication level.  I should also
> mention that the expected lifespan of the unit is a maximum of 1000hrs and
> likely only several hundred hours.
> 
> Some options that have been discussed are; (in no particular order of
> likelihood to succeed of fail!)
>         - multiple coating of soldermask
>         - micro etch of copper prior to mask application (currently pumice scrub)
>         - conductor pacification with oxide alternative.  If this does not provide
> additional bite for the mask it may provide additional protection from
> corrosion once the mask has pealed off.
> 
> Any ideas from the lot of you?  Are there properties to specialty solder
> masks which might be beneficial in this application?  Any options to
> conformal coating??
> 
> Regards
> John Parsons
> 
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