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October 2007

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Subject:
From:
Steve Gregory <[log in to unmask]>
Reply To:
TechNet E-Mail Forum <[log in to unmask]>, Steve Gregory <[log in to unmask]>
Date:
Mon, 15 Oct 2007 09:25:57 -0500
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Morning Tom,

One thing might help is to go to the largest mesh size solder paste that
you can use, small mesh sized solder paste tend to have more fines and
oxidation that sometimes create solder balls that don't coalesce back
into the main fillet.

Kind regards,

Steve Gregory

-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Steve Gregory
Sent: Friday, October 12, 2007 4:17 PM
To: [log in to unmask]
Subject: Re: [TN] Small solder balls

Hi Tom!

Got your picture posted. It's at:

http://stevezeva.homestead.com/files/1206_SolderBall.JPG

What kind of soldermask is on the board? LPI or dryfilm? Do you have the
mask relieved around the pads by a few mils?

I ask that because it almost looks like the solderball is caught right
at the edge of the mask sort of...

Steve Gregory

-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Gervascio, Thomas
Sent: Friday, October 12, 2007 1:38 PM
To: [log in to unmask]
Subject: [TN] Small solder balls

Has anyone ever established a baseline of the anticipated frequency of
solder balls under chip components? We are doing Space assembly work and
the specifications allow for no solder balls. We are using a ROL0 solder
paste and cleaning in an inline semi-aqueous cleaner with
saponification. What is being rejected are solder balls about a 1-2 mils
in diameter that are intermittently trapped under low profile chip caps
and resistors. I suspect it is an agglomeration of solder particles in
the paste occurring during reflow. I have tried all the usual things-
stencil aperture modifications: - homeplates; reverse homeplates(like a
bowtie design that reduced the printed pad by 25%); homeplates which are
offset 7 mils from the inside edge of the pad: tried reducing hot slump
of the paste (changed from 90sec soak between 150-170 sec (per vendor
recommendations) to ramp to spike; tried reducing placement force or
fudging part thickness.
 
What I am experience I believe is typical of the process but would like
some input from my peers.
 
Steve Gregory, can I post a photo on your website- stevezeva.com to show
what I am talking about?
 
Thanks
 
Tom Gervascio
 

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