April 1, 2015
by Jim Eatman
Flex Circuits are Superior for High Frequencies
Flex Circuits work better at high frequencies than FR-4 boards do
Flex circuits make better transmission lines than FR-4 rigid boards do. This is because flex circuit material is homogeneous.
Flex material does not contain glass fillers, and the thickness of flex material is tightly controlled by the manufacturer. This improves the consistency of high-speed performance from board-to-board. Special, low dielectric constant flex material is available for extremely high-speed applications.
Inductance, Capacitance, and Resistance
Traces on any PC board or flex circuit have inductance, capacitance, and resistance. As such they act like transmission lines. A microstrip design is a common form of transmission line implemented on PC boards and flex circuits as seen below. A stripline is another form of transmission line design.
Transmission line theory was developed in the mid to late 1800s to model the performance of long wires and cables for the telegraph industry. One of the problems that the telegraph industry had was, the longer the wire, the less signal was present at the far end. This property of transmission lines is called attenuation.
In addition, discontinuities in PC board or flex circuit traces can cause reflection of the signals. Examples of discontinuities include changes in trace width or shape, vias, changes in the dielectric constant or thickness of the dielectric materials, and proximity of other traces. These can all result in signal integrity problems. A PC board or flex circuit trace that is not terminated in its characteristic impedance will reflect signals at the termination.
Dk- Dielectric Constant
PC board and Flex Circuit traces do not propagate signals at the speed of light. The speed of the signal on any transmission line depends upon the dielectric constant (Dk) of the materials surrounding the line. The speed at which signals travel down a PC board or flex circuit trace is important in the design of digital circuits.
All of these issues become more critical as signal rise and fall times get shorter, and as the traces get longer. What might not be a problem at 100 MHz could easily be a problem at 1 GHz.
In addition to attenuating and reflecting signals, PC board and flex circuit transmission lines are subject to crosstalk. Because PC board and flex circuit traces are capacitive and inductive, signals on one trace can couple to another trace. This causes interference. Crosstalk can be reduced through design – by shielding traces with copper planes, increasing the spacing between traces, routing traces orthogonal to each other, or using differential pairs.
PFC has years of experience in high speed flex solutions and general signal integrity. PFC is constantly testing and qualifying new low loss materials. In addition to being able to assist with the design of high speed circuits, PFC can test the performance of transmission lines on production boards to assure quality. PFC’s fine line and space capability also gives designers more and better options when designing high speed circuits on flex.
About the author: Jim Eatman has a 38-year career in Engineering and Sales which includes stints at Texas Instruments, and in the hybrid microcircuit industry. Currently, Jim owns Eatman Associates and is PFC’s Manufacturer’s Representative in the Southwest USA. Jim Eatman has a BSEE from Texas A&M University. You can contact Jim through his website: www.eatman.com