We have a customer who is designing, making and assembling a board for a
system.  Most of the parts on the board have gull wing leads.  They are
redesigning the board because the board is getting too hot.  They are
telling us that the board is too complex to put thermal vias under the
parts.  Their idea is to put multiple thermal vias in the surface mount pads
going through to the other side and mount a heat sink on the back.  The
holes are not only in the pads but will also be under some leads.  They are
also going to solder the parts on by hand to make up for solder losses in
the vias.  Their calculations says that if they get at least a 50% hole fill
they can remove enough heat to cool the board down to an acceptable
temperature.

This approach doesn't seem very good and I was wondering if anyone has done
or tried anything like this before.  I feel that removing heat from the
leads is not very efficient and that the solder in the holes will flow such
that there will be air gaps and most likely flux trapped in the holes.  I
don't know if the holes will cause more pad lifting or other problems.  I am
looking for some information, data or possible problems to convince me one
way or the other on the acceptability of this design.  Thanks for your time.

William May
NSWC Crane
812-854-3073

################################################################
TechNet E-Mail Forum 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's web site (http://www.ipc.org) "On-Line Services" section for additional information.
For technical support contact Hugo Scaramuzza at [log in to unmask] or 847-509-9700 ext.312
################################################################