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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.

Taming Temperatures in Multilayer Circuits

May 4, 2016

Multilayer circuits provide the means for achieving high circuit densities in small volumes. The penalty for the compact circuit sizes can be the challenge of dissipating heat from active devices, such as power transistors. In a hybrid multilayer printed-circuit board (PCB), where layers may be devoted to RF/microwave circuitry as well as digital circuits, power conversion, and ground planes, heat is often channeled to a bulky heatsink which loses the advantages of small size provided by the original hybrid multilayer circuit design. Fortunately, there is a way to manage the heat without adding size, by distributing the heat throughout the multilayer circuit board, using a low-cost circuit material designed for that purpose, 92ML™ thermally conductive epoxy prepreg material.

Hybrid multilayer PCBs typically include different substrate materials for the different circuit functions, such as low-loss, high-performance materials for the RF/microwave circuitry and lower-cost materials, such as FR-4, for power and digital circuitry. The tradeoff for the lower cost of FR-4 is its higher loss and typically poorer thermal conductivity (TC) compared to materials designed for higher-frequency use, such as RT/duroid® 6035HTC from Rogers Corp.

When comparing circuit materials for thermal management, TC is a good starting place, since it provides a quick assessment of a material’s capability for heat flow. Higher values mean better heat flow through the material. RT/duroid 6035HTC, for example, has typical TC of 1.44 W/m-K in the z-axis, compared to FR-4, which has typical TC of 0.25 W/m-K in the z-axis. It is important to note that material suppliers often specify TC in the z-axis (through the thickness) of the material, whereas the TC in the (x-y) plane of the material can be somewhat higher.

The 92ML epoxy laminates and prepreg materials represent an alternative to FR-4 for hybrid multilayer circuit constructions, with a typical TC value that is many times that of FR-4, at 2.0 W/m-K in the z-axis (and 3.5 W/m-K in the x-y plane). It provides circuit designers with a laminate that is only slightly higher in cost than FR-4 but with better overall thermal characteristics, including TC that is typically eight times that of FR-4. That difference in TC translates into less of a rise in temperature from a thermal source mounted on one of the circuit boards of a hybrid multilayer circuit, such as a power supply or a power transistor. This can be an important difference between the two materials, especially where thermal management is a concern, such as in circuitry for power conversion, high intensity LED lighting, and automotive applications. In some cases, such as automotive applications, the heat may come from sources outside the hybrid circuitry, but effective material heat flow is vital to avoid thermal buildup and “hotspots” leading to compromises in reliability. For hybrid multilayer circuits requiring effective thermal management, the use of 92ML laminate and prepreg materials and their superior thermal conductivity rather than FR-4 can simplify a multilayer design, reducing or eliminating the need for thermal viaholes and/or heatsinks.

In either case of heat generated within a circuit board or outside of it, the thermal energy will flow through the PCB layers according to well-established heat flow models (for more on heating behavior in PCBs, see the earlier ROG Blog, “Probing Microwave PCB Heating Patterns, “from May 30, 2014), moving from areas of high temperature to areas of lower temperature. The composition of PCB materials can also influence the heat flow patterns, and understanding critical parameters related to thermal flow in a material such as 92ML can greatly benefit circuit designers faced with achieving effective thermal management in a compact hybrid multilayer design, especially for higher power levels.

In considering an alternative to FR-4, such as 92ML material, for non-microwave, heat-dissipating layers in hybrid multilayer circuits, a number of PCB parameters in addition to TC can be compared to provide a better understanding of how each material will fare when dealing with the heat, such as  dielectric constant (Dk) and dissipation factor (Df). A circuit material’s Dk has a great deal to do with the dimensions of transmission lines and other circuit structures formed for a given characteristic impedance at a desired frequency.

Higher Dk values result in smaller circuit dimensions with their higher power densities. Even for the material components of a PCB material with much higher thermal conductivity than the dielectric materials on which they are formed, such as copper conductors, heat will be generated at higher power levels and it must be channeled to areas of lower temperatures. Circuit materials with lower Dk values result in wider circuit dimensions with greater heat flow capacity. In the case of comparing FR-4 and 92ML circuit materials for thermal management in hybrid multilayer circuits, the Dk values are actually quite similar. FR-4 typically exhibits a Dk of 4.25 (no specified frequency) while 92ML laminate has a Dk value of 5.2 at 1 MHz and 4.9 at 1 GHz.

Dissipation factor is yet another circuit material parameter that can reveal a great deal about a material’s thermal dissipation qualities. Df is essentially the loss associated with the dielectric content of a PCB material, with smaller values indicating less loss and less heat produced by a source of power. For example, FR-4 has a Df value of typically 0.020 in the z-axis while 92ML material has a considerably lower Df value of 0.013 in the z-axis. In addition to its higher thermal conductivity, this lower Df performance contributes to the enhanced thermal performance of 92ML material, as a form of heat-spreading layer within a hybrid multilayer circuit.

In brief, 92ML ceramic-filled, thermally conductive epoxy laminate is halogen free, RoHS and WEEE compliant, and well suited for lead-free processing. For mechanical stability with temperature and excellent planar stability during processing, it features a coefficient of thermal expansion (CTE) that is very close to the CTE values for copper and aluminum, at 19 to 20 ppm/°C.  For high-power applications, it features a high maximum operating temperature capability of 150°C and a highvoltage breakdown rating of greater than 1000 V/mil. In addition, it can be supplied with metal backing (92ML™ StaCool™ laminates) for use in metal-clad PCBs.

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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