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Advice on conformal coating

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daqq:
Hi guys,

We will be doing a conformal coating of a PCB. The PCB contains lots of stuff, but the iffy part are several diodes in a MINIBLOC package ( similar to http://ixapps.ixys.com/datasheet/dsei2x30-04c_dsei2x31-04c_dsei2x30-06c_dsei2x31-06c.pdf ).

We can use two approaches:
 - spray/dip coat the fully assembled PCB with the diodes screwed on.
 - spray/dip coat the PCB without the diodes screwed on.

Either way we shall be masking a few parts of the PCB since there are contacts that must remain exposed either way.

I'm for dip coating the PCB without the diodes screwed in and screwing them in later. A colleague is for the other option.

What is the proper way to do this? A PCB with screw on power parts?

Thanks,

David

tszaboo:
I'm 90% sure the thermal expansion of those terminals is going to break apart the conformal coating after a while.
I used these in the past, It is not even that simple to just scew it in and done.

daqq:
Thanks for the info. The conformal coating is fairly flexible (Electrolube HPA https://www.electrolube.com/pdf/tds/044/HPA.pdf ), so cracking should not be that much of an issue.

tszaboo:
Wait, you are not applying the coating on the screw then? Might have misunderstood that.
I would not dip the SOT227 parts into the coating in any case, since the bottom of it has to be thermally conductive to the heatsink below it.

daqq:

--- Quote ---Wait, you are not applying the coating on the screw then?
--- End quote ---
I would prefer not to. My colleague has a different opinion. My preferred option would be to mask all the PCB, coat the PCB, remove the masks and assemble the diodes.


--- Quote ---I would not dip the SOT227 parts into the coating in any case, since the bottom of it has to be thermally conductive to the heatsink below it.
--- End quote ---
The diodes won't be needing a heatsink at all, given that they are only used for their pulsed characteristics (the average power on them will be less than 1W (the peak pulse power will be much higher).

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