Tom,

I don't have any numbers for you, but this is typical from my
experience. In solderball-sensitive applications, I've had some
improvement by using the bow-tie aperture, and reducing the overall
aperture size to promote gasketing of the stencil to the pad.

I assume that you are not conformal coating the board? And that you
can't convince your customer that the solderballs are entrapped?

Ben Gumpert

-----Original Message-----
From: Steve Gregory [mailto:[log in to unmask]] 
Sent: Friday, October 12, 2007 4:17 PM
Subject: Re: 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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