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.