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The Rog Blog is contributed by John Coonrod and various other experts from Rogers Corporation, providing technical advice and information about RF/microwave materials.

Hybrid Multilayer PCBs Help Keep Vehicles Safe

April 2, 2018

Advanced automotive electronics systems have relied on the reflections from on-board vehicular radar systems for some time. Those radar systems are becoming more common in new car and truck models, helping drivers to avoid collisions with their millimeter-wave reflections at frequencies as high as 77 GHz. The vehicular radars are often fabricated on what are known as hybrid multilayer printed-circuit boards (PCBs). These are PCBs formed of different kinds of circuit-board materials, matching the characteristics of the different materials to the needs of the different circuit functions, from DC through 77 GHz.  

Many new drivers will be putting their safety in the hands of a technology—millimeter-wave signals—that they might have considered a bit mysterious a few years ago but that is rapidly becoming an essential electronic function in new-vehicle models. By bouncing short-wavelength signals at 77 GHz off potential “targets” front and rear, a vehicular radar system can provide a warning to a car and its driver for a possible obstruction in the path of the vehicle. To transmit and receive such high-frequency radar pulses, this type of commercial radar system requires PCB material with low loss and high stability, among other outstanding characteristics, such as the RO3003™ circuit laminates from Rogers Corp. with dielectric constant (Dk) of 3.0.

But a vehicular radar system also depends upon many other power and control circuits for proper operation, not just those millimeter-wave radar circuits at 77 GHz. It can perform repeatably and reliably in terms of power and control circuits with materials customarily used for those functions, such as good-quality FR-4 circuit material with high glass transition (Tg) temperature. Combining the various circuits required for an advanced electronic function such as a vehicular radar system leads to hybrid multilayer PCBs that employ circuit materials with the characteristics (and costs) best suited for many of the different functions needed under the modern vehicle hood.

Vehicular collision-avoidance radars are just one application for hybrid multilayer PCBs, of course, using something of a “systems-level” approach to the design and structure of electronic circuits. In many cases, multiple-function circuit designs can be realized as hybrid multilayer PCBs, using the different characteristics of several circuit materials to their greatest advantages. As an example, RO4835™ circuit material from Rogers Corp. provides stable, repeatable RF/microwave performance when used for high-frequency amplifier circuits in wireless base-station applications. It is a high-performance circuit material that is priced accordingly. The material’s laboratory-like RF performance is not required for amplifier supporting circuits, such as control and power-supply circuitry. It can make more sense to fabricate those supporting circuits using good FR-4 circuit material. Many wireless base-station amplifiers, such as in 4G LTE wireless infrastructure systems, take advantage of hybrid multilayer circuits to extract the best features from each type of circuit material. They may use a circuit material capable of good high-frequency performance, such as RO4835 laminate, for the RF/microwave circuitry and additional circuit material, such as high-Tg FR-4, for control circuits, power/bias circuits, and ground planes for the amplifier. The different circuit materials combine for a hybrid multilayer circuit that provides the RF electrical performance as needed but at reduced costs because of the substitution of lower-costing circuit materials for non-RF functions.

As more and more RF, microwave, and millimeter-wave circuits enter a vehicular operating environment, they face an increasingly hostile thermal operating environment. A material property known as thermal conductivity can make a significant difference in the behavior of circuit materials within that operating environment, especially where it is important to properly dissipate heat while handling significant amounts of electrical power.

A circuit material such as RT/duroid® 5880 laminate from Rogers Corp. features low circuit loss at RF/microwave frequencies for a wide range of applications, with a dissipation factor (Df) of only 0.0009 at 10 GHz. But this may not be the primary material of choice for an electronic circuit application which has thermal management concerns, since its thermal conductivity is only 0.22 W/m-K. However, by using RT/duroid 5880 in a hybrid multilayer PCB with a different circuit material that brings enhanced thermal conductivity to the combination, such as 92ML™ circuit material from Rogers Corp., the “systems-level” approach to circuit design makes it possible to combine the excellent 2.0-W/m-K thermal conductivity of the 92ML circuit material with the extremely low loss of the RT/duroid 5880 at microwave frequencies. The lower losses of the RT/duroid 5880 causes less heat to be generated by an applied RF power source and the overall circuit thermal conductivity is greatly improved by the 92ML materials, which combine to yield a multilayer circuit with significantly improved thermal properties.

This combining of the traits of multiple circuit materials can be advantageous in hybrid multilayer PCBs when balancing good electrical performance, such as low RF loss, with characteristics that may not be so good, such as that material’s high coefficient of thermal expansion (CTE). A high CTE will result in a large amount of material expansion with increasing temperature. Such expansion can be a concern for PCB reliability. In a base-station or vehicular operating environment in which a material’s excessive CTE may ordinarily limit its use, it is often possible to improve the reliability of that material by combining it in a hybrid multilayer circuit construction with additional circuit materials having better (lower) CTEs.

Similarly, hybrid multilayer circuits can be constructed from different circuit materials, such as RF/microwave materials and FR-4.  Differences in CTE values between circuit layers can lead to warping in a hybrid multilayer circuit. But by balancing layers in a multilayer assembly, such as top and bottom, with similar CTE characteristics, warping from CTE disparities can be minimized and manufacturing yields improved, even if the RF/microwave performance of the selected circuit material layers is not required for the functional purposes of those circuit layers.

Hybrid multilayer PCBs offer circuit designers an opportunity for creativity, by using circuit materials, for example with different values of dielectric constant (Dk) to realize a particular electrical function, such as a coupler. By using low-loss, low-Dk materials for some parts of the coupler, and higher-Dk material for other parts of the coupler, its performance and response can be tailored as needed with realistic fabrication tolerances, depending upon frequency and coupling value. In general, the use of hybrid multilayer PCBs allows for a certain amount of re-thinking of many designs, by using circuit material characteristics that best suit a particular design goal.

Do you have a design or fabrication question? Rogers Corporation’s experts are available to help. Log in to the Rogers Technology Support Hub and “Ask an Engineer” today.

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