A 36" long board will cost plenty. But, there are workarounds.
I have need for a long flex cable (~36"). I sent for quotes, and they all came back as “no-bid.” Are long flex circuits really that much more difficult to build?
Long FPCs are more difficult to build. There are a lot of reasons for this. This month I will cover each, with possible workarounds.
Raw material size limitations. If your bids are from US-based manufacturers, they are probably getting their raw copper-clad materials in 24" x 36" sheets (unless special ordered). So even if the manufacturer makes its processing panel size 36" long, a 36"-long FPC would not fit unless it was run diagonally, which is not practical from a cost perspective. You may want to see if the fabricator is willing to purchase materials from Asia, which typically are delivered on long rolls. This would solve the raw material issue, but not any of the processing issues (covered later).
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.
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.