Hi Jim,

are you describing the ubiquitous 'mid-chip ball' or 'squeeze ball'?

If so, I worked on this when I was developing solder paste. My take on it
is, it is caused too much paste followed by paste slump, cold slump would
exacerbate the issue, but hot slump will cause the paste (paste flux) to
'flow' out before reflow occurs and the surface tension of the metal becomes
dominant.

Either change your paste to one that doesn't cold slump or hot slump, they
exist through careful selection and careful incorporation of appropriate
solvents or spend time modifying your reflow profile to dry the paste faster
at the start of the profile and reduce the flow characteristics at higher
temperatures.

It's also worth considering re-designing the stencil to reduce the paste
printed in the culprit areas.

If you want to carry out some tests, find a hot-stage microscope and run
some pastes at different profiles, you'll be able to see the flow-out and
other interesting phenomenon. With a modern system you'll be able to
programme real time profiles and record the data and video on a PC.


For what it's worth,

Andrew Hoggan
BBA Associates Ltd
www.bba-associates.com

-----Original Message-----
From: TechNet [mailto:[log in to unmask]]On Behalf Of Marsico, James
Sent: 14 August 2001 20:34
To: [log in to unmask]
Subject: [TN] soldr balls


Here's a relatively basic SMT question, but we've haven't experienced this
before.  We're seeing solder balls under (way under, in the center) chip
components.  Any recommendations?

Jim Marsico
Senior Engineer
Production Engineering
EDO Electronics Systems Group
[log in to unmask] <mailto:[log in to unmask]>
631-595-5879

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