We had a "Spring loaded" fixture that pulled the Flex tight and used loads of Board supports. There were pins that went through the Connectors holes and held the flex firm. We only had a few components so it was no problem. Lucky for us because the fixture going through the reflow process acted like a big old heat sponge.
We also did not have fiducials so we used the through-hole pads as fiducials. We taught the location as a unique fiducial.
NOTE: Our success with this was for pure R&D and was marginal. I could not imagine going into full production.
Cal Driscoll Communications Test Design, Inc. www.ctdi.com
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