Because of their position in the worldwide printed circuit market, flexible circuits are still a mystery to a lot of electronic product developers. Questions are brought up frequently about their electrical capabilities. Can a flexible circuit handle several amps of current? Can I use heavy copper on a flexible circuit? What about impedance control? Can it be done on a flexible circuit? These are questions that are not a concern to designers of traditional printed circuit boards because of their widespread use and decades of popularity.
Traditional “rigid” printed circuit boards have been widely used in the United States since the 1940s. They showed up in consumer products the next decade and have been produced in very large scales ever since. The electronics industry is full of experts on the design, manufacture, and assembly of printed circuit boards (PCBs).
Flexible circuits were introduced in a very limited way in the 1950s, but have not enjoyed the widespread use that its rigid cousin has. Although you will find flexible circuits in many consumer products today, including cellular telephones, laptop computers etc. they still make up only about 15% of the worldwide market for printed circuits. Reports for 2014 show, that the rigid printed circuits market worldwide will be about $65B, while the same reports for flexible circuits will be about $12B. For that reason, there aren’t as many experts on the design, manufacture, and assembly of flexible circuits as there are for traditional rigid printed circuits.
There’s good news! Flexible circuits and rigid printed circuits are manufactured in much the same way. The materials are different to be sure, but the main processes are similar. The same print and etch methods used for printed circuit boards are also used to manufacture flexible circuits. Both begin with full coverage of copper and the unwanted areas are etched away from the pattern that was designed on your CAD system. Both utilize plating to create through holes and vias that allow multiple conductive layers. A note of caution however, flexible circuits are much more tedious at high layer counts than their rigid equivalent. Both circuits have insulation layers albeit different materials and processes. The point is, when it comes to electrical considerations, you can count on flexible circuits as well as you count on printed circuit boards.
Let’s look at few examples of questions that are frequently asked.
- Can a flexible circuit handle several amps of current?
Yes! The same “Current Carrying Capacities for Copper Traces” tables used for printed circuit boards are valid for flexible circuits also. Because the raw material, in this case copper, is used in both, the current carrying capacities are the same as well as the temperature rise above ambient. Flexible circuits can be manufactured with the same copper thicknesses available for PCBs. Typical thicknesses used are ½ oz, 1 oz, 2 oz etc.
- Can I design and achieve impedance control on a flexible circuit?
Again, yes! The calculations are the same as for printed circuit boards with the exception of dielectric constant and material thicknesses. We still must concern ourselves with trace width and thickness, space between traces, distance between layers, and dielectric constant. Once those calculations are made and confirmed the flexible circuit performs as well as a rigid PCB.
- Can I provide EMI shielding on a flexible circuit?
Yes! The same design methods used for a printed circuit board will work for a flexible circuit. Ground layers and loops may be used in the same manner to keep noise in or out of your sensitive areas.
When faced with the design of a flexible circuit and its electrical characteristics it’s best to consult with an experienced manufacturer. Remember though that the design in this respect will differ little from the expertise you already have as a printed circuit board designer.