FR4

Buy flex rigid boards from a board shop. Seriously I think the time is at the original fabrication of the board. AFAIK, they use sheets of FR4 bond the parts that will remain to the flex and cut away the rest. It would not surprise me if they use a stencil and some kind of epoxy.

One time we were quoting a job to hotbar flex to a rigid board because flex rigid from the board shop was expensive. We never did get the job, I believe it was decided the fall out would be more expensive than getting it from the board shop in the first place. it had fine pitch where the two joined up.

Another time we had four different flex rigid boards from a customer. You could tell by the different styles that more than one engineer worked on layout of the boards. In other words there is flexibility on how it is done.

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