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.
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).
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.
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.