Jim , to me this could indicate one inadequate joint to begin with :
check the first pass - 'm sure(?) you'll find something (eccentricity on p&p
?).
When you say fine for first time - that may be, just, by a whisker,
the second pass definitely exposes any deficiencies .
Look at the 2's under 40x mag, there could be something to begin with .
Found the uneven wetting/placement usually the cause .
Rotate the boards just in case of shade somewhere,
as you said - heat = one side does reflows
(yes, on bottom - near via?, being small)
and cooler/bigger other side joint pulls upon cooling.
If you find you need more paste ;
just print over pad onto mask, it'll wicks back ;
preferably on out axis (watch SIR).
With steady hand you can adjust the stencil aperture with dental tips on
flex shaft.

cu  paul

-----Original Message-----
From: Marsico, James [mailto:[log in to unmask]]
Sent: Wednesday, 12 April 2000 21:41
To: [log in to unmask]
Subject: [TN] DOUBLE SIDED SMT REFLOW


Hello TechNet:

We've just ran our first production board designed with double sided SMT
components.  Everything worked well (bottom side components actually stayed
on!) except for one thing.  During the second reflow operation, some of the
0402s on the bottom tombstoned.  These all soldered fine the first time
around.  My first thought is to raise the heat slightly, but I thought I'd
check with you guys/girls first.  Any recommendations?

Thanks,
Jim M.

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