Flexible printed circuits are quickly becoming the preferred printed circuit board. Unlike other boards and interconnects, flex circuits are lightweight, easy to install, durable, compact, and—yes—flexible. The range of motion can include over 360 degrees, making it the perfect choice for nearly any situation. You can use flexible circuits in “bend to install” or dynamic applications, where the circuits are continuously in motion. Flexible circuits are also advantageous for design packages where space is a primary concern.
Flexible printed circuit boards have several advantages over rigid PCBs. They include, but are not limited to, ease of form, fit and function.
Ease of use:
1) Design constraints are minimal. PCBs can be designed to fit any device shape.
2) Range of motion allows PCBs to suit nearly any application.
3) Less mass reduces risk in environments with regular vibrations.
4) Reduces errors found in standard PCB assemblies.
5) Reduces weight through the elimination of additional wires, cables and connectors.
Reducing PCB size:
1) Thinner and more lightweight than their rigid alternatives.
2) Durable against motion and bending.
3) HDI allows for the miniaturization of devices.
Rigid-flex PCBs:
1) Blends flexible and rigid PCBs. Commonly formed with flexible circuits connecting several rigid flex boards.
HDI for flex:
1) Smaller package size increases the need of HDI.
2) Allows additional space for other features on the PCB.
Cost reduction:
1) Total cost of installation is reduced.
2) Flex circuits eliminate several steps within the production process, shortening overall turntime and reducing cost.
The bend radius is the minimum amount the flex area can bend. Knowing the amount of times your flex PCB will bend is crucial to your design. If a PCB is bent more times than the design allows for, the copper will begin to stretch and crack.
One of the most common methods is to reduce the copper thickness of the traces and more over the thickness of the plane layer. One way of reducing copper, on a plane layer, is by cross hatching the plane.
Typically we recommend .015” wide signals with .025” spacing for the cross hatched plane layers. Ground and power planes are usually cross hatched in flexible printed circuit boards in order to maintain or increase the flexibility of a circuit board.
Single-sided, double-sided, and multilayered flex circuits can be stiffened in specific areas by adding localized rigid material. This material can add support for mounting components, increasing strength, thickness and rigidity.
The thickness of flex legs can be adjusted for component needs as well, such as the “Ziff” end of a flexible circuit. Kapton and FR4 materials are commonly used for stiffeners, this material can be attached with thermally cured acrylic adhesive or pressure sensitive adhesive. Stiffeners should overlap bared coverlay by .030” to relieve stress.
Vias are at greater risk for peeling on flex designs. To reduce this risk, make annular rings as large as possible and consider having your vias teardropped. Adding tabs or anchors to vias will also help prevent peeling.
For rigid-flex designs, hole to flex distance is important. That is, the distance between vias and the rigid-flex transition area. Avoid going below 50 mils for high reliability applications. Keep in mind the rule most broken in rigid-flex designs: most manufacturers will not allow less than 30 mils for commercial applications.
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