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 does 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 higherwattage components, and the upward spiral in switching frequencies have become the primary drivers in the search for more efficient and cost-effective thermal management. This article will address some of the industry issues and solutions related to 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 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.
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 utilized. An excellent example of the importance of good thermal management is clearly depicted in the Arrhenius Chart (Figure 1), which basically states that for each 10°C rise in component junction temperature, component junction life expectancies will be halved.
Typical methods utilized in transferring heat away from component junctions have included metal back planes, thermal vias, thermal coins, heat spreaders, heat risers, thermally conductive adhesives, air over (forced air), and water cooling (Figure 2). Though widely applied within the electronics industry, the aforementioned cooling methods are often accompanied by negative design factors that typically include increased cost, weight, and size.
Thermal Management for 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 example of such a thermally conductive material would be Arlon 91ML, which 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 heatsinking systems. The advantages include reductions in cost, size, and weight over conventional heatsink requirements.
Thermal imaging (Figure 3) illustrates that materials having "in plane" thermal conductivities of 2 to 4 W/mk will increase a board's ability to remove componentgenerated 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 utilization of thinner materials and higher-strength dielectrics, in conjunction with lower thermal impedance, provides a perfect solution for all highpower and high-density applications.
Thermal Management for 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 heatsinking in high-power RF circuits creates a host of design issues, and therefore it becomes increasingly necessary to incorporate unique or novel heatsinking methods within the active RF realm, typically incurring additional costs for engineering, prototyping, and manufacture.
As RF frequencies and amplifier power demands increase, the need for more efficient heat dissipation becomes the main priority. In addition to the heatsinking dilemma, the thermal stability of a board material also plays a crucial roll in maintaining consistent and reliable operation over a wide temperature range (Figure 4).
A thermally stable material such as PTFE 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.
Benefits for Military Computing
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, multilayer, and rigid-flex technologies with specialty materials can lower overall production costs by reducing or eliminating the need for conventional heatsinking systems. Additionally, the use of such materials and processes can provide improved longevity, reliability, and weight reduction
In most military-related work, we deal not only with commercial off-the-shelf (COTS) orders, but also with very specialized boards and unique computer applications. Commonly, the military is ahead of everyone, and when replacement parts are needed, they can be manufactured for many years. However, recently it has been increasingly harder to find parts.
This presents special problems that now can be solved through revisions. Newer parts are now smaller, faster, and hotter but can be incorporated into the replacement parts via a few alterations. The boards are used in specialty computers in high-speed, short-term use or in very long, intense usage in stressful environments. For this reason, the military is investigating thermal management more closely, particularly parts that can be made easier, faster, and more cost effectively.
This article was written by Brigitte Lawrence, owner of Brigitflex, a printed circuit board manufacturer in Elgin, IL. For more information, click here .