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September 1999

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Subject:
From:
"Stephen R. Gregory" <[log in to unmask]>
Reply To:
TechNet E-Mail Forum.
Date:
Wed, 22 Sep 1999 13:43:26 EDT
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In a message dated 09/22/1999 12:24:13 PM Central Daylight Time,
[log in to unmask] writes:

<< Technetters

 I have a situation where I need the advice from the Technetters. I have an
 assembly with HASL. In order to stay within the aspect ratio guidelines, I
 must use a stencil that has a .0085 aperature opening, a 20 mil spacing, and
 I must use a 6 mil stencil. I tried this and when the stencil pulled away,
 the surface tension pulled away the solder paste from the pad area. If I
 were to go to a 5 mil stencil (which is outside the Aspect Ratio
 guidelines), would this help me with keeping the solder paste on the pad and
 not on the stencil? Or is there another approach that I should consider
 instead of going to another stencil thickness? Any help you can give me
 would be appreciated

 Donn E Steffen
 Senior Quality Engineer
 VDO Control Systems
 Auburn, IN 46706
 Phone (219)925-8887
 Fax (219)925-8710
 Email: [log in to unmask] >>

Hi Donn!

The aspect ratio I've always went by was that the aperture width needed to be
1.5 times the stencil thickness, so in a 6-mil thick stencil the minimum
opening would need to be at least 9-mils wide (with nice sidewalls) But even
9-mils is pushing it. The minimum width that I always use is 10-mils wide for
a 6-mil stencil to avoid the problem you're having. If I go less than 10,
I'll start reducing the thickness of the stencil. Separation speed also plays
a big part whether the paste hangs up in the stencil from my
experience...it's like if you separate the foil from the board n-i-c-e and
s-l-o-w after the print stroke, the paste will ease out of the apertures and
give you a nice solder brick on the pad...

-Steve Gregory-

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