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June 2002

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Subject:
From:
Dougal Stewart <[log in to unmask]>
Reply To:
Dougal Stewart <[log in to unmask]>
Date:
Sat, 22 Jun 2002 12:29:30 +0100
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Well, I'm impressed and I see why you USA board fabs stay in business ! Even
the best automatic printers with optical registration would find it hard to
achieve 0.001" positional tolerance - the heat from the lamps exposing the
artwork will make the artwork shift by more than this..... So, if this is
what you specify, you will have to pay the price, which is either scrap or
rework at the fab house, which they should add onto your board price, or is
absorbed by the fabricator, who then goes out of business trying to achieve
the physically near impossible. After that you will go to the far east for
your supply where they will ignore your requirement (sorry, not understand)
and you are back to having ganged openings or reduced pad width.
I have only come across one soldermask capable of achieving a soldermask dam
of less than 0.004", as normally the undercut will remove the dam if the
artwork is less than 0.006". To achieve dams on a 20 mil pitch device (which
is actually 19.685 mil), use a 10 mil pad, and just state that you require
solder dams.
Why don't you allow ganged openings - what part of your assembly process
can't you control? Is it solderpasting, position of paste screen, volume of
paste, or positioning of components?

Dougal Stewart

email:  [log in to unmask]
phone: +44 1896 822204
mob:    +44 7984 629667
----- Original Message -----
From: "Greg Scott" <[log in to unmask]>
To: <[log in to unmask]>
Sent: Friday, June 21, 2002 11:19 PM
Subject: Re: [TN] Registration of soldermask


> Loan,
>
> What we do is design with 2mil per side opening with a solder mask note:
>      No finished solder mask over pad edge.  Annular ring of exposed board
material around pad edge to
> be .001 min, .003 max.
>      Maintain  a solder mask bridge of .004 min. wide between fine pitch
solder pads.
>
> Also the pad can not be wider then .012 with this approach.  Do not allow
for ganged soldermask
> relief.  We had all
> rework problems with that approach.
> Have not had problems since this note.
>
> Greg Scott
> Cray Inc.
> Seattle, WA.
>
> "Tempea, Ioan" wrote:
>
> > Hi technos,
> >
> > what would be a reasonable requirement for the registration of the
solder
> > mask on boards with 20 mils fine pitch having solder mask dams. We asked
for
> > 0.002" and it almost comes over the edges of the pads.
> >
> > Is 0.001" doable, with no impact on the price?
> >
> > Thanks,
> > Ioan
> >
>
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