Should you add a strain relief bead to your rigid-flex circuit?

You are putting the finishing touches on your new design and feeling pretty good – you have made both the mechanical and electrical teams happy. At least mostly happy. One nagging issue remains: Should you add a strain relief fillet or bead to the interface between the rigid and flex portions of your circuit?

IPC-2223 discusses strain relief and considerations for and against. An estimate may be that about 50% of all designs include stain relief, and the rest do not. Keep in mind that the guidance below applies to both rigid flex and also to flex with FR4 or metal stiffeners. How do you decide if yours needs it?

First, let's discuss the role of the strain relief bead. At its core, the goal is to avoid a very sharp bend of the flex right at the rigid/flex interface. Some are concerned resin may flow out from the rigid material that could create a sharp edge, and the flex may be bent sharp enough to be damaged by the sharp rigid or resin edge.

Voltage, heater size and material costs all factor into the decision process.

I would like to use a flexible heater in a product I am designing, but I am concerned about cost. Flexible heaters typically seem a lot more expensive than regular (copper) flexible circuits. What is the advantage to using a resistive metal? Why not just use copper for all flexible heaters?

Copper can certainly be used for the heating element material in a flexible heater in many applications. When a design permits copper to be used for the heater, that will usually be the most cost-effective way to go. But how do you know if copper is a good choice for an application?

Cost impacts of copper heating element. The first hurdle to clear to ensure copper is a good cost-effective solution for a heater is the electrical resistance requirement. The biggest problem with using copper as the heating element is that it has very low coefficient of resistance. To get any appreciable resistance in the heating area, very thin copper must be used and element traces made very narrow in order to pack as many lineal inches into the heating area as possible. Both these will work against your goal of reducing costs. Also, base laminates that are clad with less than 0.5oz copper thickness are typically more expensive due to their fragile nature, and industry use of materials this thin is far less than use of 0.5oz or 1oz copper thicknesses. Another drawback of narrow traces is that the manufacturer's yields will be less optimal, which in turn equates to a higher selling price of the end-product. Another downside to incorporating very narrow traces that many designers fail to take into account is the trace width variations from part to part due to etch tolerances.