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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