Sometimes it can be of interest to reduce the purity of your nitrogen.  The
nitrogen increases the wetting action and decreases the surface tension of your
solder.  When such a "light" component is even slightly one sided vs. the other,
the solder will pull one side with a little more grasp than the other, hence you
have tombstoning.  What purity are you using.  I have noticed that some lower
the purity by introducing O(2) into the spike zone.  I would also check my
placements not only for skew, but for exact alignment prior to reflow.  Also,
check the innerwiring density as one side of the pad location may be getting
hotter than the other if this is a reoccuring problem.  If all else fails,
thermocouple it and see if you get a temp differential!

Jason Smith
Process Materials Engineer
Lexmark Electronics



[log in to unmask] on 05/25/99 08:28:51 AM

Please respond to [log in to unmask]; Please respond to
      [log in to unmask]

To:   [log in to unmask]
cc:    (bcc: Jason Smith/Lex/Lexmark)
Subject:  [TN] Tombstoning of 0402's




Good Day Everyone,

We have been running some experiments with different pad sizes / shapes for
reflow of 0402 type components. There are definitely differences in the
amount of tombstoning seen, but in all cases we experienced an unacceptable
level of defect. We run a BTU VIP98N oven with nitrogen inert and a fairly
standard ramp / soak / spike type profile. The paste is an eutectic no clean
that has been on the market for several years. Has anyone any suggestions in
reducing the number of tombstones on 0402's? We have run 0603 successfully
for quite awhile. Feel free to answer me directly (by email or phone) or via
the TechNet if you deem it of general interest.

Thanks,
             Robert Furrow
             SMT Process Engineer
             978-960-3224
             [log in to unmask]

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