Consumer electronics and many commercial applications have demanded more be packed into thinner and lighter constructions; this puts a literal “squeeze” not only on the flexible circuits required but also the engineer who needs to satisfy the demand for thinner and thinner electronics. Adding layers is a good way to get more “action” within the same footprint but there are some considerations we must take into account if we want a reliable circuit.
We live by many design ‘rules’ but any engineer or PCB designer will admit to bending these rules on occasion. The key is to know how and when to bend these rules and the use of proper FMEA procedures to mitigate risk. I was thinking of these rules just the other day when I was reviewing a design for a very thick flex. Close attention to basic design rules becomes critical when we are asked to build a flex above six layers. At eight layers we need to pay close attention to every aspect of the design. Any application should be reviewed for basic electrical and mechanical requirements and a determination made as to the impact the material stack-up will have on both. Are we looking at a “formed” flex or is our circuit used in a dynamic application?
Let’s start with a quick review of what concerns us when we design a high layer-count flexible circuits, what potential failure modes are we likely to experience? Obviously as we add layers we add thickness. Each additional layer in turn requires an additional layer of adhesive. Our enemy here is Coefficient of Thermal Expansion (CTE), specifically the impact CTE has on our plated through holes. Though components with discrete leads requiring PTHs have become less common, there are still a large number of connectors and devices that require them (this is especially true in military applications). The more layers we add the more stresses we are placing on the PTH, or plated barrel during thermal excursions or thermal shock. In many cases it may not be as important to count the number of layers but to review the overall thickness and construction of the entire flex. This is where our “rules” may let us down.
IPC guidelines call for a minimum of 25um of copper plating on flex of 6 layers or less while requiring 35um on boards greater than 6 layers. This is regardless of actual board thickness. To be fair, these are indeed guidelines, no specification could possibly account for every possible variation within a circuit type. Regardless of the recommendations it is important for us as engineers and designers to review the stack-up carefully, examine the operating environment of the flexible circuits, and determine what we believe to be the best course to take. Remember these guidelines are minimums; it is up to us to determine if we require more for our particular application.
Building higher count multi-layers requires careful consideration beyond “general” guidelines. The IPC design rules are a great place to start, but when you “push the envelope” you need to find a flex provider that can assist you in reviewing not only the design for manufacture, but can also offer a true application review.