Remaining at the forefront of technological advancements in the printed wiring industry requires a proven combination of process methodology, state-of-the-art
processing equipment, and a clear-cut understanding of customer base and customer application parameters. As the design hurdles related to the electronics packaging market are continuously being raised, the primary focus remains that of increasing density, enhanced performance, and improved reliability. Moreover, as component densities increase, so increases the difficulty in maintaining those performance and reliability factors. We are rapidly approaching a point in the evolution of electronics where thermal management ultimately becomes a design engineer's top priority.
It is widely accepted that increased power density, implementation of higher wattage components, and the upward spiral in switching frequencies have become the primary drivers in the search for more efficient and cost effective thermal management. In this article, we shall address some of the industry issues and solutions related to thermal management in RF designs.
Thermal Management
To address these and other thermal management concerns, we must take a closer look at the epoxy and glass materials that make up a printed wiring circuit board, and more specifically, the thermal impedance of these materials. Simply stated, thermal impedance is a material's inherent resistance to heat transfer, and this thermal impedance is typically the sum of the base material impedance, imperfections within theses base materials as well as imperfections found at the interface between the base materials and conductive laminations (Figures 1 and 2).
Figure 1
The higher the thermal impedance of a given board material, the lower the ability of that particular board material to draw heat away from component junctions as well as impeding the transfer of component junction heat to any ancillary sinking materials that may be used.
Figure 2: Simplified heat transfer example : Click for larger image
A good demonstration of the importance of thermal management is the Arrhenius Chart (Figure 3), which basically states that for each 10 degree Centigrade rise in component junction temperature, component junction life expectancies will be halved.
Figure 3: RF power field effect transistor
Typical methods used in transferring heat away from component junctions have included metal back planes, thermal vias, thermal coins, heat spreaders, heat risers, thermally conductive adhesives, forced air, and water cooling (Figure 4). Though widely applied within the electronics industry, the above-mentioned cooling methods are often accompanied by negative design factors which typically include increased costs, weight, and size.
Figure 4
Digital and LED designs
With respect to digital and LED junction temperature reductions, one appropriate alternative may be the use of substrate materials exhibiting a higher thermal conductivity. One good example of such a thermally conductive material is Arlon's 91ML, as the 91ML aids in limiting the peak temperature of a component junction by disbursing and dissipating heat 'in plane.' This unique approach also possesses the ability to transfer heat more evenly and more rapidly when used in conjunction with ancillary heat-sinking systems. The advantages include reductions in cost, size, and weight over conventional heat sink requirements.
Thermal
imaging (Figure 5) illustrates that materials having 'in plane' thermal conductivities of 2 to 4 W/mk will increase a board's ability to remove component-generated heat by a factor of 10 to 20 times over that of conventional epoxy board materials, which typically have a thermal conductivity of 0.2 W/mk. It soon becomes evident that the use of thinner materials and higher strength dielectrics in conjunction with a lower thermal impedance is advantageous for use in all high power and high-density applications.
Figure 5: Thermal management in electronics
RF designs
As the importance of good thermal management applies to high-power digital designs, proper thermal management is equally important in the RF arena, and in particular, to RF power amplifier applications. Often, the use of conventional heat-sinking in high-power RF circuits creates a host of design issues, and, therefore, it becomes increasingly necessary to incorporate unique or novel heat-sinking methods within the active RF realm. Unfortunately, this typically incurs additional costs for engineering, prototyping, and manufacturing.
As RF frequencies and amplifier power demands increase, the need for more efficient heat dissipation becomes the main priority. In addition to the heat-sinking dilemma, the thermal stability of a board material also plays a crucial role in maintaining consistent and reliable operation over a wide temperature range. (Figure 6).
Figure 6: Practical Application: 0.5W heat source
One good example of a thermally-stable material is Arlon's PTFE, which combines excellent thermal conductivity with a high dielectric constant, remaining stable over an unusually wide temperature range. Maintaining a good thermal dielectric constant can dramatically reduce changes in circuit impedance, RF reflection, and dead band shift. (Figure 7).
Figure 7: Temperature Sensitivity to Dielectric Constant
Advancements
The unique combination of specialty materials, appropriate product development methodology, and state-of-the-art manufacturing processes will pave the way for more rapid advancements in digital and RF circuits where high heat and high energy are an inherent part of the design.
The integration of single-sided, multi-layer and rigid-flex technologies with specialty materials can lower overall production costs by reducing or eliminating the need for conventional heat-sinking systems. Additionally, the use of such materials and processes can provide improved longevity, reliability, and weight reduction.
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