Keep The Heat Under Control

Thermal management can involve choosing optimum circuit materials, making measurements, and developing accurate models that can predict the effects of heat on a designs ultimate reliability.

Heat is an enemy to most electronic circuits. It can cause premature aging of active devices, deterioration of solder joints, and degradation of printed-circuit-board (PCB) performance. In power bipolar transistors, excess heat can even cause an undesirable effect known as “thermal runaway,” in which energy from the device is released to feed a temperature-rise loop that ends in the device’s destruction. Thermal management is a practical approach toward removing heat from electronic circuits, ideally without compromising electrical performance.

All electronic circuits generate heat to some degree, since efficiency levels are not close to 100%. The low efficiency of electronic circuits can be a particular problem at high power levels, such as in power amplifiers and the circuits that support them. The latter includes power combiners/dividers, filters, couplers, and terminations. Antenna circuits (on transmit) must also often handle high power levels. Managing the heat in an RF/microwave circuit requires a multipronged strategy that includes designing effective thermal paths to carry heat away from a circuit’s heat sources, such as power transistors; measuring the heat produced by the circuit and its circuit elements; and modeling the circuit for heat. Many companies proficient in thermal design will perform measurements and create models before and after a circuit is packaged, since the packaging plays an instrumental role in handling the heat.

One of the first steps in designing for effective thermal management of an RF/microwave circuit (or any type of circuit, for that matter) is to carefully choose a PCB substrate material. RF/microwave circuit designers must find a circuit-board material that is not only stable with time and temperature and capable of dissipating a required amount of heat, but also has the proper dielectric and electrical properties needed for a particular high-frequency circuit design. High-frequency circuit-board materials are characterized by a large number of mechanical and electrical parameters, with permittivity (relative dielectric constant) and dissipation factor two of the critical barometers of electrical performance, while thermal coefficient of dielectric constant, coefficient of thermal expansion, and thermal conductivity are the three main measures of a PCB material’s behavior with temperature.

The thermal coefficient of dielectric constant is a measure (typically in ppm/°C) of the variability of permittivity as a function of temperature in the z direction or thickness of a circuit material, usually referenced to a specific test frequency, such as 10 GHz. The coefficient of thermal expansion (also in ppm/°C) is also measured in the z direction and is used to gauge the reliability of plated through holes (PTHs) with the temperature swings experienced in normal material processing. The PTHs are used not only to connect circuits to ground planes, but as thermal pathways that help dissipate heat. The thermal conductivity is a reading (in W/m/K) of the amount of power that can be dissipated by a material for a give rise in temperature, also measured in the material’s z direction. By using circuit materials with enhanced thermal conductivity, for example, the temperatures at device junctions and solder joints can be minimized under high-power conditions.

While circuit materials based on fluoropolymers such as polytetrafluoroethylene (PTFE) are known for their low loss and general excellent electrical behavior, pure PTFE is subject to a great deal of expansion and contraction with changes in temperature. Thus, for use in PCBs, it is usually reinforced with some other material (such as glass fibers). In addition, some PCB material suppliers have developed circuit materials that combine PTFE and more thermally stable thermoset materials. For example, TMM® laminates from Rogers Corp. are ceramic thermoset polymer composites designed for applications in which thermal management may be a concern. Available with various values of relative dielectric constant, these materials exhibit a low thermal coefficient of dielectric constant, typically less than 30 ppm/°C, electrical stability with changes in temperature. The material offers twice the thermal conductivity of traditional PTFE/ceramic laminates, at 0.70 W/m/K, with isotropic thermal expansion properties that are closely matched to those of copper, so that stress is minimized on PTHs even during the temperature changes of material processing cycles. For even more demanding thermal applications, the firm’s RT/duroid® 6035HTC circuit material combines low loss with impressive thermal conductivity of 1.44 W/m/K to help manage heat flow away from a PCB’s solder joints and device junctions.

Power transistors are predictable sources of heat in a high-frequency circuit, especially as demands for higher power levels push the capabilities of newer transistor designs, including gallium arsenide (GaAs), gallium nitride (GaN), and silicon-carbide (SiC) devices. With higher power levels come higher power densities, and the need for mounting and packaging materials with extremely high thermal conductivities to dissipate the excess heat. Transistor carriers have been developed from a variety of materials capable of conducting heat, including copper, beryllium oxide (BeO), copper-tungsten, and even diamond. BeO, for example, has thermal conductivity of about 285 W/m-K at room temperature, which is outstanding—except when compared to some diamond heatsinks, with thermal conductivity values as high as 1800 W/m-K.

While many power transistor suppliers develop their own packaging based on thermal needs, some organizations, such as the Fraunhofer Institute for Manufacturing Technology and Advanced Materials (IFAM), specialize in developing custom materials and packages capable of dissipating a large amount of heat. The firm has developed heat sinks, packages, and even test fixtures (see figure) based on metal-phase-change-material (metal-PCM) composites which are designed to minimize the customary mismatch of CTE between different packaging materials for tailored thermal expansion behavior and enhanced packaging reliability at high power/temperature levels.

In addition to selection of packaging materials, the use of bonding films, such as organic adhesive films from American Standard Circuits (ASC) can help enhance the thermal conductivity of PCBs as well as packages. The company offers PCBs with customer-specific heat management requirements based on the use of PCBs with PTHs and its proprietary adhesive bonding films. ASC manufactures PCBs based on circuit materials from leading laminate suppliers, including Arlon Materials for Electronics Div., Nelco (Park Electrochemical Corp.), Taconic Advanced Dielectric Div., and Rogers Corp. Measuring the temperatures of circuit designs or even simulating the operating conditions that can bring thermal stress to a PCB requires specialized test equipment capable of accurately detecting wide temperature extremes and, in some cases, generating such temperature extremes. The line of ThermoStream® benchtop and portable systems from Temptronic, for example, are capable of creating and detecting temperatures from -90 to +225° using forced-air streams. Compact systems such as the model TPO4390A offer extremely fast temperature transition times for stress testing, with the capability of shifting from -55 to +125° in about seven seconds while maintaining 1°C accuracy. This system features a touchscreen for ease of control and four remote interface ports for system integration, including Ethernet, GPIB, and RS-232C ports. Khoury Industries is another supplier of thermal test equipment, along with test chambers and fixtures for thermal measurements. Microtek Laboratories is an outside test laboratory for RF/microwave designers in need of outside test services for an extensive range of PCB tests, including thermal stress, thermal shock, and thermal analysis.

Once a design has been tested and analyzed for thermal hotspots, a computer model can help understand the modifications needed to better dissipate the heat from those critical points in a PCB or package. HyperLynx Thermal software from Mentor Graphics, for example, can perform thermal modeling on double-sided, multilayer PCBs with as many as 3000 components on each side. The software provides precise calculation of junction temperatures for improved reliability predictions. Another model tool is the QoolPCB™ thermal modeling software from Advanced Thermal Solutions.

Docea Power recently released its AceThermalModeler™ (ATM) software for creating thermal models for PCBs, system-on-chip (SoC) designs, system-in-package (SiP) structures, and even three-dimensional (3D) integrated circuits (ICs). According to the firm’s Chief Executive Officer, Ghislain Kaisler, “With it, system architects can perform both thermal steady state or coupled power and thermal analysis for dynamic application profiles running on different architecture configurations.” The software’s models are meant to help designers understand thermal gradients across a circuit and develop packaging and integration solutions quickly and effectively.

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