Can switching the top and signal layers cause irreparable problems?
I have a two-layer flexible circuit that has worked well in my application for over a year. The top layer has signal lines, and the bottom layer is a plane. The flex is bent a few hundred times during its service life. Due to a recent change to other components in the device, I flipped the flex layers, so the plane is on the top layer and signals on the bottom. Everything else stayed the same. Now we are seeing cracks in the signal traces. Would just flipping the layers cause these conductor cracks when we never had issues before?
A lot of things could be going on here, so let’s examine the possibilities. First, it is important to understand what happens when a flex circuit is flexed. When a circuit is bent, there will be compression forces on the inside of the bend, and tension (stretching) forces on the outside. This is true on any material or laminate, whether it is a 1" thick plate of steel or a 0.005" flexible circuit. The thicker the material or laminate, the more extreme these forces.
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.
To avoid post-soldering delam, keep parts dry, and use plenty of adhesive.
I am experiencing flex circuit delamination. What is causing it?
Delamination occurs for a few different reasons. Very often, it happens during solder reflow during assembly. Usually this is due to moisture absorbed into the part prior to the soldering operation. Flexible materials absorb moisture, and during soldering that moisture can rapidly heat and expand, resulting in delamination. The first line of defense is prebaking prior to soldering. In a standard oven, just getting above the boiling point of water is good enough, so 225° to 250°F is plenty. Bake time can vary depending on circuit thickness, but two hours is a good start. Thick rigid-flex parts may need four to six hours. If parts are stacked in the oven, time may need to be added to ensure the parts in the middle of the stack dry out.