Hi Christina,
What would you be basing the yields off of? Is it solder defect, component placement, etc....? I have worked for a flex house for quite a few years. You are right. The Polyimide is very dimentionally unstable. (even more so on humid days) I would imagine that processing very small parts would be a problem especially on a very large flex. Also, we have had some problems printing fine pitch QFP's due to the gross bared insulation around the pads. This caused the stencil not gasketing to the pad. So in answer to your question yes, the yields would not be as high for flex compared to ridgid PCB's especially if design considerations for SMT manufacture are not realized. Extra precautions need to be taken to maximize accuracy and minimize defects. I would be happy to share any of our resolutions for some common problems that we encountered. Please let me know if you are interested and I will get a hold of you.
Good Luck
Chris
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