Challenge: Maintaining data transmission integrity as signal speeds increase
Solution: DuPont Pyralux® AP all-polyimide laminate has excellent electrical and mechanical properties for minimizing signal loss at high speeds
In today’s fast paced electronics market, computing equipment is operating at ever increasing speeds. When designing flexible circuitry for this equipment, an important goal of the interconnect is to get the signals from point A to point B as fast as possible with a minimal amount of distortion.
At operating frequencies greater than around 1 GHz, signal attenuation of the circuit traces can pose a major problem. Attenuation, also known as loss, is a term that refers to any reduction in the strength of a signal and is a natural consequence of signal transmission. If the attenuation of a signal becomes too high, data transmission can fail. Consequently, designers must find ways to minimize the amount of energy lost by this phenomenon while maintaining the required speed.
An effective way to accomplish this is with the dielectric of the circuit material, which acts as an insulator between signals and return paths. Two important electrical properties of the insulator are dielectric constant (Dk) and dissipation factor (df).
Dielectric constant is a measure of a material’s insulating ability. The velocity of a signal is indirectly proportional to the material’s Dk. Thus, a lower Dk material allows signals to travel faster. Stability of the Dk over high frequencies is also important. If the dielectric constant of the material changes with respect to frequency, then the velocities of the fundamental and each harmonic will be different. This causes the signals to ‘arrive’ at different times and not add up in phase, resulting in a distorted square wave. A low Dk allows a high rate of speed while a stable Dk enables undistorted signal transmissions. Pyralux® AP all-polyimide laminate has a Dk of 3.4, compared to a Dk of around 4.2 for FR-4, and it remains very stable up to frequencies as high as 20 GHz or more.
Dissipation factor, or loss tangent, is a measure of an insulating material’s efficiency in minimizing energy loss and is directly proportional to signal attenuation. In materials with relatively high loss tangents, high frequency signals excite molecules in the dielectric causing it to absorb the energy of the signal and consequently inhibit its transmission. Every percentage decrease in dissipation factor results in a percentage increase in usable bandwidth. Pyralux® AP has a loss tangent of 0.002. This is roughly an order of magnitude better than FR-4.
In addition to these electrical properties, certain physical properties also play a key role in ensuring signal integrity, specifically by controlling it’s impedance. Tight impedance control ensures strong signal transmission from the source to the destination. A mismatch of impedance causes part of the signal to be reflected back to its source, thus causing distortion.
Two important material properties that influence impedance are the thickness and tolerance of the dielectric. For a given impedance target, a thicker dielectric allows the designer to use wider signal traces. Besides the added benefit of higher manufacturing yields, wider traces have lower percentage variation in cross-sectional area than narrower traces. This, along with a tight dielectric thickness tolerance, allows for overall improved impedance control. Pyralux® AP is the first all-polyimide flexible circuit material to be offered in dielectric thicknesses of 3 mils and greater, up to a maximum of 20 mils. Additionally, the thickness of the dielectric is tightly controlled, with a tolerance of +5%.
Another material property that influences signal loss is the roughness of the copper trace. As frequencies increase, the signal current tends to flow more on the surface of the conductors. As the skin depth (which is the effective depth of the trace that carries the high speed current) decreases, it approaches the roughness measure of the copper, resulting in increased transmission line resistance. A smooth copper surface reduces the amount of loss associated with this effect. Pyralux® AP is available with either rolled annealed (RA) copper or reverse treat electro-deposited (RTED) copper, both of which have much lower Ra and Rz values than traditional ED copper.
Materials Selected and Why
DuPont™ Pyralux® AP all-polyimide laminate is enabling the manufacture and use of flexible circuits in a growing number of high frequency applications. Flex designers are increasingly relying on the electrical and physical attributes of Pyralux® AP to meet their signal transmission requirements in today’s fast paced world.