A review of electrical and assembly costs and performance characteristics.

I often get asked to compare a rigid-flex concept with a rigid board and wiring concept. Among the key questions asked are: Is there a cost or a lead-time difference? Can rigid-flex handle performance requirements such as high-speed and low-loss? And will it be flexible enough? Let’s use a typical example where we have done this analysis.

To start, we need to satisfy the cost component. If the project is over budget, it is not going forward. So let’s dig into it. Per square inch, rigid-flex will cost more for a given area than a rigid board. This is due to higher-cost materials, as well as extra processing required for rigid-flex. Additionally, rigid-flex takes up more manufacturing area than a rigid board, as it not only has the board area but all the I/O interconnects. So, at first blush, rigid-flex is more expensive. But we can’t stop there; we need to consider the larger cost picture.

It's probably adhesive. Is it OK to leave it?

I have an LED application where I need my flex to have white solder mask or white polyimide. When I received my first prototypes, there seemed to be a lot of small specks of “stuff” on the outer surface. I have used flex circuits in the past, but they all used amber polyimide covers. I never noticed these specks before. Is there something different in processing white polyimide that would cause these spots?

Answer: I recently stopped at the drive-through of a local fast food restaurant and grabbed a sandwich, which I then attempted to eat as I drove. As I chomped into one side of the sandwich, BBQ sauce squirted out the other side and landed all over my jeans. After chastising myself for trying to eat something that messy without a table between the food and my pants, I pulled over and wiped up the mess. I was pleasantly surprised to see the spots on my dark jeans were barely noticeable. I can guarantee if I had been wearing white pants, those BBQ sauce spots would have stood out like a sore thumb.