Dougal,
A little harshly stated perhaps, but this USA board fab agrees with your
tolerance comment. If used, I'd hope that tolerance would be limited to
specific components. Almost anything's doable, but +/- 1 would affect price
and yield at our shop.
Very little assembly experience here, so I can't respond to the soldermask
dams comment, except to say that most of our customers require them, and
sometimes relax registration tolerances rather than "ganging".
Regards,
Mark Hargreaves
-----Original Message-----
From: Dougal Stewart [SMTP:[log in to unmask]]
Sent: Saturday, June 22, 2002 7:30 AM
To: [log in to unmask]
Subject: Re: [TN] Registration of soldermask
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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