Matthias,

Hot bar soldering has been used successfully on millions of flex over
the years. However, I was involved in the redesign of one notable
failure a few years ago.

The problem was that the original design had a long row of fine pitch
pads. They were having a great deal of trouble with pad registration
during assembly. In order to get a short cycle time they had the bar
temperature quite high to help drive the heat through the flex. They
were also limited in the clamping pressure they could apply because of
the fragile nature of the substrate. Calculating the differential
thermal expansions made it clear that the flex was growing so much that
some pads would miss their partners and a few could even be displaced
enough to be soldered to the wrong pad. The solution was to break the
row up into shorter segments with expansion slots between them. There
were also some locators added to help constrain the movement.

I wrote a program a few years back to help calculate the expansion of
the flex circuit composite structure. The Flex Circuit
Thermal/Mechanical Analysis Program is available at:

http://www.geocities.com/CapeCanaveral/Launchpad/9641/program.html
or,
http://www.rogers-corp.com/cmu/software.htm

Andy Magee
Flex Guru - Consultant
[log in to unmask]
1-937-435-3629

ps. Choose your flex materials wisely if you expect high reliability
when continuously operating under the hood at 150C.

Matthias wrote,

I'm working on my first design with flex on ceramic. A semi-rigid
board with a lot of SMD stuff (two copper layers) has to be TAB-like
hot bar soldered with its one-layer-flex-end to an Aluminiumoxide
ceramic substrate with AgPd traces (good old thick film technology)
and two wirebonded chips (one of them a pressure sensor) on it.
I need a not too expensive but reliable technology, because it will
be a mass product for automotive application under the hood (about
150° C max.). The whole assembly will be "folded" in a sealed
housing.

My question is, where can I find good design rules for both sides of
the solder connection? Solder depot on one or both sides? Solder
paste printing/dispensing? Which surface finish on the flex side?
HASL, chemical tin, NiAu?
Any experiences about the reliability of these constructions?

Thanks in advance, regards
Matthias Mansfeld
-----------------------------------------------
Matthias Mansfeld Elektronik
* Printed Circuit Board Design and Assembly
Am Langhoelzl 11, D-85540 Haar, GERMANY
Phone: +49-89-4620 0937, Fax: +49-89-4620 0938
E-Mail: [log in to unmask]
Internet: http://www.mansfeld-elektronik.de

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