In a message dated 10/30/2000 3:13:48 PM Central Standard Time,
[log in to unmask] writes:

> I am looking for some information on dry film soldermask vs. LPI
>  soldermask.
>
>  Manufacturing wants to change all boards to dry film mask and tent all via
>  holes so that when conformal coating is applied to the boards the coating
>  does not drip through the holes.
>
>  About 2-3 years ago I remember reading an article on problems with dry film
>  and alternatives to using dry film. Since I had only used dry film once or
>  twice a long time ago I did not save the information and can'
>
>  t find it in
>  any back issues of Printed Circuit Design or SMT that I have.
>
>  It is my understanding that our main supplier has the facilities to dry
>  film and is encouraging the dry film solution. (In my opinion more $ and
>  less Competition) Cost is not a big issue since we do not produce large
>  quantities of our products.
>
>  Any help would be greatly appreciated.
>
>  Rod Simon
>
>  [log in to unmask]   Flight Vision Inc.       Sugar Grove IL 60554
>  630-466-2119

Hi Rod!

If you go to:

http://www.merix.com/resourcecntr/tech/soldermaskapp.html

You'll find a fairly decent article that explains the big differences between
LPI's and Dry-films. Where I work, we have quite a few boards that use
dry-film soldermasks.

I've found a few problems with them. Most dry film masks are pretty thick,
and you really have to optimize your stencil thickness if you have fine pitch
SMT on the board, you can be printing a lot more paste than you want to if
you don't take the mask thickness into account.

I've also run into problems with certain dry-films not being able to take the
cleaning chemistry that we use here cleaning RMA flux residues...I've never
had a problem with LPI masks in that regard.

As far as the issue of tenting vias using an LPI goes, you can go back and
"Cap" the via's with something like a SR1000 after the main LPI has been
applied...I've seen that done before with good results..

My preference is LPI masks, they seem more durable, have better resolution,
and although I'm not a fab guy, they're probably easier to work with than
dry-films...(just a guess on my part).

Also one other thing, if your board has a high copper content (thick traces ,
I.E. 4-oz copper) it's very difficult to use dry-film mask and not have
"Soda-strawing" along the traces....

-Steve Gregory-

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