Flex and Rigid Flex Circuitry
At times, due to an application’s requirements, the conventional rigid PCB cannot be used and in steps flex or rigid flex circuitry. As shown in Fig. 1 (below), a flex circuit is used in applications or products that have moving or mechanical parts. Flex circuitry is usually made of very thin 3 mil Kapton material.
Sometimes a board with a rigid and flex portion is required, Fig. 2 (below). Called a rigid/flex board, the rigid part remains stationary, while the flex section can move up and down or back and forth. Medical devices in particular use rigid/flex components, and some consumer items such as printer ribbon cables.
Like its rigid PCB counterparts, flex circuitry is available in single through multi-layer versions. It can be 18 to 20 layers. However, as the thickness increases, flexibility is compromised as the number of layers increases.
Aside from this limitation, flex circuitry also poses a specific challenge, especially as today’s products and sub-assemblies are becoming increasingly smaller with greater electronics functionality packed in. This challenge deals with populating these flex circuits with high-end, highly advanced components like BGAs, micro QFNs, and micro LGAs. As you can imagine, since this circuitry is not flat, but bends and moves, connections are subjected to this constant movement and thus become weaker over a short time span. Or they crack or break creating product failures.
It’s best to use as few as possible of these advanced packages, and when they are used, it is highly recommended to use those with higher pitch such as 0.5mm and above.
If the component going on the flex is thick and heavy, then a stiffener needs to be attached on one side of the flex with the component going on the other side. That stiffener provides surface flatness and rigidity needed to assemble the component. Plus it provides support for the weight of the component.
At NexLogic, we design the stiffener around the components at the layout stage to assure proper assembly. Also at design layout, we make sure there are certain limitations on the flex circuitry in terms of length. There are limitations for keeping the board straight when it comes to all the processes like routing, plating, and etching because a flex circuit can be very long. Plus, we carefully monitor and understand the fab house’s restrictions in terms of length.
Our experienced PCB designers are also careful about designing trace widths because flex circuitry and Kapton behave considerably different — in terms of etching — than regular FR4, which is a regular epoxy-based material. Along this line, our designers are in sync with flex circuitry requirements. For example, NexLogic designers factor in these designs the angle and frequency calculations associated with the bend of a curve. They also design traces to be no more than six mils, but more toward four mils at the most.
Moreover, our designers are extremely careful about component placement to avoid hindering the curvature of the board material and limit components to only one side or one section of the board to maintain natural bending and flexing. Careful and meticulous design steps like these avoid placing a component like a BGA at the wrong place. Then with bending and flexing, a BGA ball may crack or a ball joint may eventually break in the field. In extreme cases, a BGA device can break off from the flex circuit, thereby causing unnecessary rework.