Scott Gaskins asked:

>I am working on a quality team to improve our surface mount
>assembly processes and was critiquing some SMD boards from other
>manufacturers and was wondering why some of them are putting a
>clear over-coating on fine pitch (.025" and smaller) parts after
>they are attached to the board.
>
>It seems to me that this would limit re-work (if required) unless
>there is some way to remove the coating.  It would also mean
>that no probing of the parts could be done in a troubleshoot area.
>
>There must be some good reasons for this clear coating applied to
>the leads.  Please let me know of any reasons all of you have!

What you describe sounds like a resin remaining from the flux.

Many of the so called no-clean fluxes, especially those which are
more properly called "leave-on" will have a considerable residual
which might appear as a "coating". This would of course be
concentrated in areas where the paste was applied in the assembly
process rather than covering the whole board.

If this is the case, you would likely find that when you apply heat,
the resin will liquefy and not impede removal or re-soldering.

As far as probing the leads, this would depend on how much force is applied,
and the shape of the probe tip. It may be a problem. If you have test
point pads or vias where no paste was applied, you could probe the net there...

Ask your supplier what type flux is applied, this should quickly verify
my theory.


cheers


Jerry Cupples
Interphase Corporation
Dallas, TX USA
http://www.iphase.com


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