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February 2007

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Subject:
From:
David Greig <[log in to unmask]>
Reply To:
TechNet E-Mail Forum <[log in to unmask]>, David Greig <[log in to unmask]>
Date:
Fri, 16 Feb 2007 15:16:55 -0000
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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
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Dunfermline KY12 0RN
United Kingdom
t: +44 (0)1383 624 975
www.gigadyne.co.uk
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-----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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