Microwave Journal

Low-Loss PCBs Enable MM-Wave Auto Electronics

May 16, 2012

Evolution of the electronic systems in automobiles and other vehicles is exciting to watch, and many technologies once associated with the military, such as radar systems, are becoming available to average drivers. As highlighted in the previous ROG blog, 24-GHz short-range-radar (SRR) systems are being offered more and more in car models around the world. But vehicle designers and manufacturers are also looking ahead to the greater resolution possible with 77- and 79-GHz automotive radar systems. And for that evolution in automotive electronic systems to truly take place, printed-circuit-board (PCB) materials are important building blocks that will enable the potentially much safer automobiles of the future.

Compared to 24-GHz automotive radars, systems at 77 and 79 GHz with their smaller wavelengths can operate with considerably smaller antennas. Because the Doppler shifts are more significant at millimeter-wave frequencies than at 24 GHz, these higher-frequency systems also more precisely determine distances and relative speeds between vehicles and other objects. The high resolution possible at 77 and 79 GHz also enables radar systems that can detect dangerous road-surface conditions, including the presence of ice.

Circuit materials for these higher-frequency automotive radar systems must meet a similar set of requirements as detailed in the previous Blog about 24-GHz automotive radar systems, but perhaps with even tighter tolerances for systems operating at 77 and 79 GHz. The consistency of relative dielectric constant (εr) across a circuit board, for example, is particularly critical at 77 GHz, where variations in dielectric constant (Dk) can translate into changes in the impedance of transmission lines, and changes in frequency. Such variations in frequency can result in wrong readings in an automotive radar system that can compromise the safety of the system. In general, variations in a circuit material’s Dk can cause variations in the impedance of a transmission line, which result in higher reflected energy, higher return loss, and higher insertion loss.

The benefits of RO4000® PCB materials from Rogers Corp. (www.rogerscorp.com) for 24-GHz automotive radar systems were highlighted in the last Blog. For 77- and 79-GHz automotive radar applications, Rogers RO3000® circuit materials bring their own favorable attributes for these millimeter-wave circuits. RO3003™ high-frequency

laminate, for example, has been used to fabricate antennas in automotive adaptive cruise control (ACC) circuits at 77 GHz, where its tight Dk tolerance contributes to stable frequency operation even this high in the spectrum. RO3003 laminates exhibit a Dk of 3.00 at 10 GHz with Dk tolerance within ±0.04. Minimizing loss at these millimeter-wave frequencies is also important, due to limited available transmit and receive power at 77 and 79 GHz. Antenna-grade RO3003 laminates are characterized by a very low dissipation factor of 0.0013 at 10 GHz, indicating that dielectric losses will be low even at 77 and 79 GHz.

Because any change in a circuit material’s Dk can affect the performance of a millimeter-wave automotive radar system, another important material parameter to consider at these frequencies is the temperature coefficient of dielectric constant, or TCDk. This property describes how much the material’s dielectric constant will change with changes in temperature, when tested over a set range of temperatures in a short time period. And since a typical commercial vehicle may be subject to a wide range of operating temperatures, this is an important parameter for projecting the stability of a 77- or 79-GHz automotive system with changes in temperature. Some laminates, for example, can exhibit TCDk values in excess of +200 ppm/°C at certain temperatures and frequencies, resulting in large swings in the value of relative dielectric constant with temperature. The RO3003 material, which is engineered for higher-frequency antennas and other circuits, has a typical TCDk value of + 11 ppm/°C at 10 GHz and for temperatures from -50 to +150°C. This last part is important to note when comparing materials, since TCDk must be referenced to a range of test temperatures to be meaningful.

Since an automotive radar system must endure a wide range of operating conditions, mechanical stability with temperature is also important for maintaining reliability, especially in high-resolution 77- and 79-GHz systems. Rogers RO3000 PCB materials such as RO3003 laminates are ceramic-filled polytetrafluoroethylene (PTFE) composites engineered for high electrical performance but also excellent mechanical stability over changing environmental conditions. The RO3003 laminates, for example, have a coefficient of thermal expansion (CTE) in the x and y plane of 17 ppm/°C that is closely matched to that of copper for excellent dimensional stability over a wide range of temperatures (-55 to +288°C). Through the material, in the z direction, the CTE is 24 ppm/°C to ensure high reliability of plated through holes (PTHs).

Another material parameter to consider for automotive millimeter-wave electronic applications, including 77- and 79-GHz radar systems, is good thermal conductivity. Although the power levels of higher-frequency circuits tend to be relatively low, any increase in the thermal conductivity of a PCB is to be recommended, since it will mean a reduction in the maximum temperature of a circuit board for a given amount of power handled by the PCB. Good thermal conductivity in the PCB material can also improve the thermal stability of the dielectric constant, since heat will be better distributed across the PCB material while minimizing any hot spots on the circuit board.

Although Rogers has developed other PCB materials that can achieve the electrical performance levels required by 77- and 79-GHz automotive radar electronics, such as RT/duroid® 5880 laminate, the RO3000 materials combine outstanding electrical and mechanical characteristics with low cost, three key parameters needed to expand the emerging market for 77- and 79-GHz millimeter-wave automotive electronic systems. The RO3000 materials can also be processed using standard PCB methods developed for PTFE-based circuit materials, to minimize processing costs even at these high millimeter-wave frequencies.

Do you have a design or fabrication question? John Coonrod and Joe Davis are available to help. Log in to the Rogers Technology Support Hub and “Ask an Engineer” today.