Printed Circuit Design & Fab Online Magazine - Current Issue

Flexperts

Nick Koop Can via-in-pad be used on a flex or rigid-flex circuit with SMT parts?

SMT is successfully implemented on flex and rigid-flex every day. Standard through-hole constructions are the most cost-effective, but in some cases there is no room for through-holes and their larger pad diameters. Via-in-pad is a design strategy that may be required with very tight pitch BGA components and other SMT devices on flex and rigid-flex. The advantage of using a microvia is the hole size is quite small, and the associated pad is small as well. This provides more real estate for routing signals, especially out of BGA patterns. The rules-of-thumb and considerations are different, however, depending on whether the part is purely flex or rigid-flex.

For a pure flex, where all the material is flexible, consider a couple of things. Typically, via depths are much shallower than for a rigid or rigid-flex part. This means less solder may be consumed in the PTH. In some cases, the hole must be plugged, however.

Read more: Microvias on Flex Circuits

Mark Finstad 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.

Read more: Flex Layer Flipperoo

Nick Koop 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.

Read more: Comparing Rigid-Flex to Rigid Boards

Mark Finstad 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.

Read more: Foreign Material on White Polyimide