How to Design for Flex Circuit Boards

31 Mar 2017
How to Design for Flex Circuit Boards

People have been designing Printed Circuit Boards (PCBs) for a long time. The process has passed from entirely manual steps to the highly-sophisticated use of Computer Aided Design (CAD) methods.

Although much of the design process is now automated, a good designer still has to make some crucial decisions to turn the design into a successful product. Moreover, as flex circuit boards offer a wide variety of advantages through all the three dimensions of space, it is highly imperative the designer understands the basic design principles, especially those pertaining to flexible circuits, to avoid early failure.

Taking a Holistic Approach

A flexible circuit, apart from addressing electrical concerns, works to fulfill some mechanical aspects as well. As the flex circuit can be bent and formed, it fills a three-dimensional space, unlike a rigid PCB that has to be necessarily placed in a two dimensional area. This requires addressing with care the areas of the flex circuit that will flex continuously in operation or will be folded or bent while installing.

The best approach here is to first make a paper or plastic model of the proposed flex PCB and deploy it physically in the available space to check for proper operational functionality. Such physical models of paper mockups will also help to address the ergonomics of assembly and concerns of access during field repairs.

Bias for Copper

Unless there are conflicts in actual use, a design of the flex PCB should favor a bias for leaving as much copper as possible on the board. Maintaining extra copper helps enhance the dimensional stability of the circuit, especially in single-layer flex circuit designs.

Setting Tolerances

As the base material is flexible and prone to distortion, applying the largest practical tolerance to all features and locations facilitates the manufacturer. If the flex PCB is rather large, it helps to have more than one datum to compensate, especially as tolerances have a tendency to accumulate. For instance, features needing high accuracy, such as placing BGA footprints, will need individual local datums relative to the feature. It helps to define one as the primary or master datum and others as secondary.

Staggering the Length

Outer metal layers of a flex PCB, when bent, suffer tensor strain leading to buckling of the inner layers. Designers stagger the length of conductors to ease the strain. They add slightly to the length of each succeeding flex layer, moving outwards from the bend radius.

Conductor Size

The conductor size (width and thickness) primarily depends on the combination of requirements of the current it has to carry, the voltage drop allowed, and the characteristic impedance the control needs. However, allowing for the maximum flexibility of a flex circuit in dynamic application, use of the thinnest possible copper helps. That leaves the designer to opt for a wider trace rather than a thicker one to accommodate the electrical needs.

Placement of Vias

Although the flex PCB is useful both in static and dynamic applications, placement of vias needs the designer’s discretion. As vias are not flexible, designers can avoid premature failures by placing them away from the region of the PCB that undergoes continuous flexing in operation.

Looking for advice?

For any inquiries on how to design your flex PCB’s do not hesitate to contact the team at Synergy Electronics for advice, along with a rapid and competitive quote. Further information on rigid flex PCB’s can be found in our industry news under ‘Rigid Flex PCB Manufacture’.

 

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