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February 2007

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Subject:
From:
"Stadem, Richard D." <[log in to unmask]>
Reply To:
TechNet E-Mail Forum <[log in to unmask]>, Stadem, Richard D.
Date:
Fri, 16 Feb 2007 12:17:20 -0600
Content-Type:
text/plain
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text/plain (233 lines)
You got it Dewey, shorts that give you wedgies when you walk. 

-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Whittaker, Dewey
(EHCOE)
Sent: Friday, February 16, 2007 10:03 AM
To: [log in to unmask]
Subject: Re: [TN] LPI soldermask resolution

What are cross shorts? Ones that give you wedge voids or base layer
exposure?
Dewey

-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Lee parker
Sent: Friday, February 16, 2007 8:57 AM
To: [log in to unmask]
Subject: Re: [TN] LPI soldermask resolution

Dave

Back in the good old days when 40 mil pitch was all the rage, Bell
Laboratories carried out several studies to ascertain the benefit of
soldermask dams. It was found that the dams cut cross shorts between
pads by several factors.


Best regards

Lee

J. Lee Parker, Ph.D.
JLP Consultants LLC
804 779 3389



----- Original Message -----
From: "Steve Gregory" <[log in to unmask]>
To: <[log in to unmask]>
Sent: Friday, February 16, 2007 10:36 AM
Subject: Re: [TN] LPI soldermask resolution


Hi David,

It's been my experience that having mask between the pads does help to
prevent solder bridges. I think it helps the solder to coalesce back to
each pad when it is liquidous during reflow, and helps with gasketing of
the stencil during solder paste printing. That doesn't mean that it's
absolutely necessary, but it does help in my opinion. That's why the
term "solder mask dam" is sometimes used to describe the strip of mask
between the pads.

If an LPI mask is used, I would prefer to see mask between the pads...

-Steve Gregory-

-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of David Greig
Sent: Friday, February 16, 2007 9:17 AM
To: [log in to unmask]
Subject: Re: [TN] LPI soldermask resolution

Hi list,

I want to ask what may well be a silly question -

Why is having a strip of soldermask between the pads of fine pitch
(~20mil) IC's so important?


Best Regards

David Greig
______________________________
GigaDyne Ltd
5 Albany Business Centre
Gardeners Street
Dunfermline KY12 0RN
United Kingdom
t: +44 (0)1383 624 975
www.gigadyne.co.uk
______________________________

-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Phillip Bavaro
Sent: 16 February 2007 00:45
To: [log in to unmask]
Subject: Re: [TN] LPI soldermask resolution

Hi Steve,

I am accustomed to thinking of LPI soldermask as a +/- .003"
material....and for that reason we try to discourage designers from
trying to have dams less than .006" wide.

Having said that, I will admit that it is my preference is to not use
soldermask on certain 20 mil pitch components such as BCC48, yet we do
use soldermask on BGAs at the same pitch.

I will assume you are talking about stripes (dams) between the pads for
a gull wing QFP leads, and yes, the resolution of LPI allows any good
pwb fabricator to put a 6 mil wide or larger dam between the pads.

Now if your pads are equal or greater than 14 mils, then the yield at
the fabricators will be impacted.

Hope this helps,

Phil


12/2007 -0600, Steve Gregory wrote:
>Mornin' All!
>
>What kind of resolution should a LPI be able to give you? Specifically,

>LPI's should be able to give you a solder mask dam between 20-mil pitch

>leads if you have 8-mils of space between the pads shouldn't it?
>
>Thanks!
>
>-Steve Gregory-
>
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