We observed similar conditions when the components are heavy enough to move,
almost heavy enough to break the surface tension holding them to the board.
We glued the components to the board. It greatly reduced changes in the
appearance of the solder fillets.

-----Original Message-----
From: Bob Perkins [mailto:[log in to unmask]]
Sent: Friday, March 17, 2000 2:33 PM
Subject: double sided reflow problems


Hi Technetters

        Recently on heavy double reflow boards we have had good reflow on
the first side but when we flip over the integrity of the solder joint which
is now on the bottom has been compromised after the second pass.  The
temperature in the oven is only slightly hotter (peak temperatures around
220C on both sides) to compensate for the added component load. On the
bottom however it seems like the heaver qfp's look like they are trying to
pull away, looking like poor wetting,  leaving a hallow or indentation on
side of the lead or back on the heal fillet.
The quick fix is to add flux to the bottom before reflowing again, but this
is time consuming. This may also require gluing of all these parts to hold
them in place, which is even more time consuming.
Could the answer be to increase the stencil thickness from a 4-mil to a
6-mil to add paste to the board to increase the surface tension helping the
board  hold the qfp's better?
Is this entirely an oven reflow problem?
Does adding flux increase quality all around, or is this a non value added
process?
Does anyone else have experience with boards being run though for a second
pass without flux?


Bob Perkins
Automation Technician/Manufacturing Engineer
[log in to unmask]

##############################################################
TechNet Mail List provided as a free service by IPC using LISTSERV 1.8c
##############################################################
To subscribe/unsubscribe, send a message to [log in to unmask] with following text in
the body:
To subscribe:   SUBSCRIBE TECHNET <your full name>
To unsubscribe:   SIGNOFF TECHNET
##############################################################
Please visit IPC web site (http://www.ipc.org/html/forum.htm) for additional
information.
If you need assistance - contact Keach Sasamori at [log in to unmask] or
847-509-9700 ext.5315
##############################################################