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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.

Bonding Materials Minimize Loss in Multilayer Millimeter-Wave PCBs

May 7, 2021

Circuit loss increases with increasing frequencies, a circuit material characteristic that has limited the use of PCBs for millimeter-wave (mmWave) applications in the past. But as the demand grows for circuits serving available frequency spectrum in the mmWave range, circuit designers are meeting those demands using newer low loss circuit materials or hybrid circuits composed of multiple layers of dissimilar circuit materials, such as very low loss circuit materials for higher-speed, higher-frequency circuits and more cost effective circuit materials such as FR-4 for functions such as ground planes, power planes, and control lines not requiring such low loss circuit material. Applications such as Fifth Generation (5G) New Radio (NR) cellular wireless base stations and their short range, high speed data links and automotive radar systems are driving the demand for mmWave circuits and low loss circuit materials capable of supporting them. Fortunately, several low loss circuit materials are available for mmWave circuits. In addition, a critical ingredient for multilayer mmWave PCBs—prepreg materials—are available with the performance needed to minimize circuit losses well into the mmWave frequency range.

A prepreg provides electrical insulation between circuit materials but also is a binding material to hold together circuit layers in a multilayer PCB. It can be characterized by its electrical and mechanical behavior although in mmWave circuits, its electrical loss will be a primary concern. Prepreg materials based on polytetrafluoroethylene (PTFE) exhibit very low loss, in concert with PTFE circuit laminates, providing excellent low loss performance in multilayer PCBs. However, assembly of a multilayer circuit with PTFE based prepreg requires a PCB fabricator to perform fusion bonding to adhere the layers together, and the number of PCB fabricators with the capabilities and experience to fusion bond PTFE based prepregs in a multilayer PCB is limited. PTFE based prepreg provides excellent electrical performance but can be challenging to handle in large scale PCB fabrication processes. This tradeoff between electrical performance and practical fabrication processes using PTFE based prepreg materials must be weighed carefully.

For manufacturing high volume PCBs for mmWave applications, those tradeoffs can help guide the choice of an optimal prepreg. As an example of a well-known and often used prepreg, RO4450F™ prepreg from Rogers Corp. is quite friendly to the PCB fabrication process, although it may not be the first choice for multilayer mmWave PCBs. The thermoset material is very friendly to the PCB fabrication process, even at high volumes, and has a dissipation factor (Df) or loss of 0.004 at 10 GHz. While acceptable for many RF and microwave applications, that loss is considered marginal for signals reaching 24 GHz or higher into the mmWave frequency range (where signal power is more limited). Although RO4450F prepreg has been used in mmWave circuit applications where loss performance was less critical, circuit applications with more demanding loss performance require a prepreg with lower Df.

In comparison, Rogers 2929 material is a bonding material which is like a prepreg, used to hold together the different layers of a multilayer PCB. It has a lower Df than RO4450F prepreg, 0.003 at 10 GHz, which translates into less circuit loss at higher frequencies. When this bonding material is combined with very low loss circuit laminates, multilayer PCBs with very low loss through the mmWave frequency range are possible.

Because signal power is limited at mmWave frequencies, such as in high resolution radar applications, low loss materials with even lower than 0.003 Df values are essential for circuits in these applications. Rogers SpeedWave™ 300P prepreg features a low Df of 0.002 at 10 GHz, making it an excellent candidate for many emerging mmWave circuit applications requiring low loss circuit materials. It is available in different thicknesses and glass styles, including spread-glass and open-weave versions, allowing designers of multilayer circuits some flexibility. The choices in thickness and glass styles have little impact on the prepreg’s Df, which ranges from 0.0019 to 0.0022 at 10 GHz depending upon options. Similarly, the Dk ranges from 3.0 to 3.3 in the z-axis (thickness) at 10 GHz for all thicknesses and glass styles. To minimize the glass-weave effect at mmWave frequencies, prepreg with spread glass should be used, and SpeedWave 300P prepreg in 1035 (0.0020 or 0.0025 in. thickness) and 1078 (0.0030 or 0.0035 in. thickness) variations should be considered.

To better understand how the combination of a prepreg material and a circuit laminate behave in terms of loss at higher frequencies, CLTE-MW™ circuit laminates from Rogers Corp. were teamed with SpeedWave 300P prepreg in the fabrication of stripline test circuits. CLTE-MW glass-reinforced laminate with spread glass has low loss, with a Df of 0.0015 at 10 GHz. The combination of prepreg and laminate was characterized with high performance test equipment capable of measuring signal frequency, phase, and amplitude well into the mmWave frequency range and found to exhibit insertion loss of 2.25 dB/in. at 77 GHz. This is very low loss for stripline at 77 GHz, a part of the spectrum used for mmWave radars in Advanced Driver Assistance System (ADAS) automotive safety applications. This transmission-line insertion loss compares favorably with current 77 GHz radar applications, which are largely based on microstrip transmission lines for the high frequency portions of hybrid multilayer circuits. Microstrip exhibits low loss due to having some of its electromagnetic (EM) fields in air compared to stripline’s dielectrically encapsulated transmission lines. However, stripline is being used in many newer, high resolution 77 GHz radars being fabricated as part of multilayer hybrid PCBs, and the prepreg Df plays an important role in the overall loss of stripline hybrid circuits at mmWave frequencies.

At the higher, mmWave signal frequencies employed in newer, high resolution radar circuits, impedance matching of transmission lines and other circuit structures is critical for maintaining low loss and minimal signal reflections. Minimal impedance variations can be achieved by closely matching the Dk value of the prepreg to the Dk of the low loss laminate containing the mmWave circuitry. SpeedWave 300P prepreg teams well with CLTE-MW laminates because the prepreg has a typical Dk of 3.16 in the z-axis at 10 GHz while CLTE-MW laminates have a typical Dk of 2.94 to 3.02 in the z-axis at 10 GHz.

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