Understand Electrical Specifications for Better Flexible Circuit Design

Understand Electrical Specifications for Better Flexible Circuit Design

November 3, 2015 in Design Tips

Understand Electrical Specifications for Better Flexible Circuit Design

Electricity is fundamental for every flexible circuit, which is why accurate electrical specifications are critical to designing a circuit that meets application needs. These specifications include current carrying capacity, impedance control, EMI shields, and embedded resistive and inductive patterns.

Printed circuit boards (PCBs) set the standard for how electrical considerations are specified. Using PCBs as a reference, we’ll explore design techniques for flexible circuits along with some of the tools available to assist in accurate design the first time.

Current Carrying Capacity of Flexible Circuits

 Carrying high currents through a flexible circuit can be a point of concern for designers, but it doesn’t have to be. Flexible circuits use copper conductors just as their rigid cousins do. To carry more current, increase the trace width, thickness or both. Keep in mind that when current travels through a copper conductor, the energy expelled is heat. Most charts or trace calculators will advise how much the temperature will rise above ambient for each trace width, to allow a confident design with minimal effect on surrounding components.

Impedance Control on Flexible Circuits

 Impedance controlled signals are necessary in today’s high-speed circuits. Whether a design requires embedded microstrip, stripline or simple matched signal lengths, flexible circuit design uses the same methods and formulas as PCBs. The differences are the dielectric constant of the materials and the inherent decreased distance between copper layers. Flexible circuits require a little more attention around trace width and available material thicknesses.

EMI Shields on Flexible Circuits

 Some circuit designs require protection for sensitive signals from external and internal “noise” sources. Similar to printed circuit boards, ground layers and loops ensure noise does not affect performance of flexible circuits. The ground layers or shield layers may use a cross hatch design if frequency allows, or may be solid planes similar to traditional PCBs. One advantage that flexible circuits have in this regard is they can be completely surrounded with conductive foil to provide shielding even on the edges. This method is sometimes used to emulate shielded cable.

Embedded Resistive and Inductive Patterns on Flexible Circuits

 Embedded resistive patterns may be used for signal delay and inductive patterns for consistency in transmitter or receiver circuits. Both are used only after compromise has been made with available circuit real estate vs. cost of passive components. The same design methods used to create these patterns on PCBs may be used for flexible circuits. There are some specific flexible circuit design considerations to take into account.

The one drawback to inductive patterns on flexible circuit material would be how many layers of copper coils can be used. It’s simple to manufacture a PCB with 6, 8 or 10 layers of copper. However, it’s much more difficult to achieve those layer counts on flexible circuits without significant increase in cost. Resistive patterns, on the other hand, pose no design problems and are achieved as easily on polyimide (flexible) material as they can on FR4 materials.

By better understanding electrical considerations, designers and engineers can collaborate to ensure flexible circuits are designed right for application the first time. Learn more about Tramonto’s expertise of flexible circuits and design here.

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