Jorge
> 1) Prior conformal coating I need wash the board, but there is 6 SMT
> photo sensor and more than 20 LED , what cleaning method should I use in
> order to not damage or affect performance of this components ?
It MAY not be necessary to clean the boards, but you MUST get your process
under good control.
If you do feel the need to clean - which is undoubtedly the safer option,
then I would need to know the volume of assemblies you have to process
before giving you an answer as to the cleaning process you should consider.
> 2) What level of DRY the board should be before coating, the
> LED are heat
> sensitive and could not be exposed to high temperatures for baking ( more
> than 125 degree C ), what baking set up should work ?
If you have a good cleaner, then it should not be necessary to bake the
assemblies before coating. If you need to, then 6 to 8 hours at 80DegC
should be fine.
> 3) What kind of conformal coating should I use for spray application (
> acrylic, epoxy,silicone or polyurethane ) , the board environment will be
> exposed to 74 degree Celsius in a sunny day ( air temperature inside the
> box )
First, I should inform you that the most likely temperature inside a box
exposed to the sun will be nearer 95DegC.
The easiest / most forgiving material you could use would be an acrylic
coating - and it will give you all the protection you could require UNLESS
there is something you have not told me?
Application would always be better by dipping. However, I assume that you
have lots of components all over the assembly that you do not want to coat
so spraying will be cheaper (less labour for masking and de-masking).
Selective coating will be good, but your investment will be around $40K.
> 4) The photo sensor and LED should not be conformal coated, what kind of
> masking should I use, for spray coating.
Use a peelable latex temporary masking compound. Better results will come
from a natural latex than a synthetic.
> 5) In the back side of the board ( there is no component in
> this side ) a
> heat sink will be attached, and no conformal coating should be
> applied since
> this heat sink will be in contact with a sheet metal, but the heat sink
> covers about 30% of area in the bottom side, It is OK to leave the bottom
> side without conformal coating since there is no components at
> this side, or
> should I mask the conformal coating and then attach the heat sink after
> conformal coating was applied ?
Now you tell me! Should have read this message first before replying!
Well, first off the coating if applied correctly, will not be a good thermal
insulator - only around 0.002" or 50 microns thick. Maybe this will be OK
with no masking?
However, with no components, and some masking on the top-side, I would
strongly recommend spraying top-side only.
> Thanks for any advice at any question.
>
> Jorge Santana
You are welcome, I hope this helps. Visit my web site for much more info.
Regards, Graham Naisbitt
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