Very astute of you, Steve.
Mask adhesion problems _are_ much more likely to be a surface prep problem
than a curing problem, in general. In this case, though, it may be a curing
problem, but not that the mask is undercured, which could be corrected, as
you said, by more curing. (Whew- six commas in that sentence!)
With small vias plugged with LPISM what can happen is the outer surface of
the mask cures before the inside does, trapping solvents inside. The result
is kind of like popcorn...the solvents vaporize at high temps and the
pressure forces the mask off the top of the via. Pre-baking at a lower
temperature helps by removing the solvents slowly, from the inside, before
complete curing.
If it's important to you or the customer to have those vias covered, these
boards are scrap. If not, you probably don't have to worry about uncured
mask somewhere else.

Timothy Reeves
ECD Circuit Board Division
13626 South Freeman Road
Mulino, OR 97042
[log in to unmask]
(503) 829-9108 (800) 228-8198  FAX (503) 829-5482


> ----------
> From:         Stephen R. Gregory
> Sent:         Friday, February 11, 2000 6:12 AM
> Subject:      Solder mask failing tape test..
>
> Mornin' all!!
>
> This morning I got a email from one of our buyers telling me that one of
> the
> boards at one of our fab shops is being held-up because it doesn't pass
> their
> solder mask tape test. My buyer said that they told him that the mask is
> only
> coming off the via's, and it was because of uncured mask. The mask is a
> LPI
> and the via's aren't meant to be tented. Also the board doesn't go through
> wave solder as the only through-hole is hand soldered because it goes up
> through the board from the bottomside.
>
> My buyer is asking me if we should accept the boards. I have a few
> concerns.
> One thing is that if there is uncured mask at the via's, what guarantees
> are
> there that there isn't more at other places on the board? Shouldn't
> uncured
> mask be a simple thing to fix by either baking the PCB (if it's a thermal
> cure), or exposing it to more UV (if it's a UV cure)?
>
> Further still, does uncured mask come off in a tape test? Or is mask
> coming
> off from a tape test more an indication of an adhesion problem from
> contamination?
>
> Thanks in advance for all replies,
>
> -Steve Gregory-
>

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