In a message dated 08/22/98 5:49:01 AM Pacific Daylight Time,
[log in to unmask] writes:

>       Thanks to everyone for the prompt responses, it validated my
>  concerns (I was right, nyah, nyah, nyah!). We're getting another stencil
>  burned right now with 12mil aps, the 1:1 stencil originally specd was a
>  disaster, 4-6 bridges every board.
>
>       Now to another topic: What are your  experiences with mask
>  dams/webbing and fine pitch? In previous lives, I tried to specify mask
>  between pads on 25mil and under. This seemed to help minimize bridging
>  potential. However, I am now dealing with people higher up that have had
>  bad experiences with this practice, and I want to bring about a dialog
>  to pursue some process experiments.
>
>       Step up to the pulpit and tell it like it is, brother (or
>  sister). Amen!
>
>  Mike McMonagle
>  PCA Process Engineering Supervisor
>  K*Tec Electronics
>  1111 Gillingham Lane
>  Sugar Land, TX  77478
>  (281) 243-5639 Phone
>  (281) 243-5539 Fax
>  [log in to unmask]
>
Hi Mike!

     Mask between the pads huh? I'll put it like this, I work at a contract
assembly company, and whenever we get a consigned kit that has fine pitch on
it, I grab one of the PCB's and one of the first things I look at is whether
or not the fine pitch has mask between the pads or not, because that tells me
what I'm going to do with the apertures on the stencil. If it has mask, I'll
do a reduction of the aperture width of 1-mil on each side (most of the time I
usually see 12-13 mil wide pads on 20-mil pitch) and use a 6 mil stencil. If
there's no mask, I'll do a 5-mil thick stencil, still do the same width
reduction, but lengthen the aperture on the toe and heel by around 5-10 mils
or so, that gives me some volume back that I took getting a thinner stencil to
reduce the bridging from no damn dams...the over printed solder will wick back
into the main fillet during reflow, and where it counts too, back at the heel.
I leave the apertures 1:1 for the rest of the 50 mil pitch stuff except for
the J-leaded parts which I lengthen too, since they need a good deal of solder
to form their fillets.

So, you can guess that I'm a proponent of mask between the pads...it gives you
a little wider process window, and ordering stencils isn't such a pain in the
buttocks.

I've talked with a few fab vendors about this subject, because I wanted to
know why it is I see boards with, and without it. It would seem to me that if
it was so beneficial to assembly, that I would see it on every board.

You can also pick up a magazine like Printed Circuit Fabrication and read
numerous advertisements from mask suppliers hi-lighting the fact that their
mask can achieve the kind of resolution to create dams between pads. Per
Enthone-OMI discussing their LPISM on their web page: "With fine-pitch boards,
high post-assembly yields can be obtained using solder dams between pads.
These dams, as narrow as 2 mils, help prevent solder movement between
component attachment sites and reduce the possibility of solder bridging and
shorting..."

So according to mask suppliers, the kind of material is out there to achieve
dams between fine pitch. But when you talk with Fab vendors about mask dams,
they seem to try and discourage the requirement as much as they can without
saying that they won't do it...the magic figure I hear a lot is 10-mils, fab
vendors seem a little more receptive doing 10-mil dams, but really don't want
to talk about anything less than that...so to say the least, having 14 mil
wide pads on 20 mil pitch is going to make you a pain in the butt customer to
your fab vendors if you want dams.

I wanted to learn what part of the PCB solder masking process is so difficult
that there is such a large gap between what the material supplier says it is
capable of, and what the fab vendor is comfortable with.

This by no means is the definitive word, but what I learned on a Sunday
afternoon Internet search is that it boils down to application methods. Many
fab vendors have curtain coating equipment or they screen masks on, the best
method from what I learned, electrostatic spray methods.
The following is from TeT Halco's web page talking about their DSA-2010 (a
spray coater they sell): "Controlled application assures that customer
dielectric requirements are achieved. Excess material is not applied due to
screen limitations between conductors, pads or inside holes. Coating weights
lighter than those typical of curtain application will also save on LPI cost
without sacrificing conductor coverage.

Solder mask dams between QFP pads are best established without high levels of
exposure and slow development times. The spray application, with a consistent
thickness in this area, allows manufacturers to achieve dams with a straight
sidewall and faster image and development times, again saving cost."

It allows you to apply a thinner, more precise coating of the mask, which
makes it easier to achieve the kind of dams that the mask suppliers say their
material can do. This equipment is fairly new, and not everyone has it, which
explains why many fab vendors shy away from doing fine pitch dams if they can.
It is possible to achieve fine pitch dams with equipment other than
electrostatic spray equipment, but everything during the process has to be
dead perfect to get acceptable results...it's just harder to do.

BUT, with having said all that, in my `net search today, I ran across an
article about having wide pads and no dams that was pretty interesting. It was
written by a Rick Lathrop and was on Heraeus's WEB page, I'll send you a copy
of it in a word document attached to another email I'll send since I can't
attach it here. It talks about the benefits of wide pads have on achieving
good HASL quality with fine pitch and having a bigger target to hit during
placement...so maybe your wide pads aren't a problem after all..

-Steve Gregory-

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