Heat-Sink Attachment: The Key to Optimum Thermal Transfer

Submitted to EEPN by Southco (www.southco.com(www.southco.com)

Incorporating ever-increasing processing power into ever-shrinking electronic components and tighter PC board layouts is creating new thermal management concerns for designers of computing equipment and other electronic devices. Mechanical heat-sink devices, featuring a fin design with a high surface-area-to-footprint ratio, are one method used to transfer heat away from electronic components. But there's more to effective thermal mitigation than simply adding heat sink devices.

According to The Uptime Institute, the trend in the heat load per product footprint has grown tenfold for server and communications applications over the past decade—to more than 6,000 watts/sq. ft. in some applications. And it is projected to go even higher, making thermal transfer a more crucial aspect of electronics designs.

The heat sink attachment option you choose could have a significant impact on the efficiency of your thermal management design, since each available approach comes with its own advantages and disadvantages in terms of size, cost, complexity, and thermal effectiveness. How well each one translates into performance in the intended application can be affected by other considerations, including operating conditions such as shock or vibration. Changes in mounting hardware, tolerance differences among various components, or lack of attention during the assembly process, could all affect the efficiency of thermal transfer performance in the ultimate real-world application.

Some of the simpler designs offer obvious advantages in terms of low cost, but can incur problems in more demanding applications. The use of double-sided tape or adhesive compounds can be difficult to implement in the manufacturing environment and can compromise the thermal transfer efficiency of the heat sink. And as the size of the heat sink increases, withstanding shock or vibration can become more challenging.

Plastic clips are another low-cost alternative whose reliability can be challenged by shock, vibration, or heat sink weight. A different alternative, retaining clips, provides for easier heat-sink replacement. But the reliability of those clips has been shown to vary during shock testing. Also, they can require removal of center fin material on the heat sink, where most of the heat is typically absorbed.

Other mechanical fastening options that provide better tolerance to shock, vibration and alignment include a variety of screw, pin, and spring arrangements. These include loose-hardware as well as captive-hardware designs. The captive designs (see figure and photo) help to avoid problems associated with loose hardware, which pose potential short-circuit problems on printed circuit boards. Captive designs can also simplify screw alignment for easier assembly, even in crowded, compact PCB layouts.

Finally, when specifying heat-sink mounting hardware, remember that maintaining proper heat-sink alignment and contact across the full surface of the chip or other heat-generating electronic device is the key to good, consistent thermal transfer. Compressible heat-transfer pads can help to maintain thermal transfer performance while compensating for uneven surfaces or for inconsistencies in mounting pressures. Spring-loaded, self-leveling, heat-sink fastening hardware that maintains consistent pressure across the surface of the heat-generating electronic component—and automatically adjusts to compensate for thermal expansion—also helps to improve thermal transfer performance, whether used with or without the compressible pads.

If you have experienced problems with heat-sink attachment in past applications, or are in the process of designing new applications with demanding thermal transfer implications, discuss your needs with a hardware supplier who offers fastener options designed specifically to accommodate a broader range of demands in heat-sink applications.