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Know Your Choices for Mounting Heat Sinks to Hot Components

Thu, 09/01/2011 - 2:02pm
Dr. Kaveh Azar, Advanced Thermal Solutions, Inc., www.qats.com Edited by Chris Warner
Because of the performance demands on ICs, the use of cooling heat sinks on top of ASICs, CPUs and other processors is a standard industry practice. As the available real estate on PCBs has become sparse, physically attaching heat sinks to such devices has become a challenge for most applications, specifically for high-density boards. Thus, an engineer starting the process of thermal management must first determine the cooling needed and then consider the mechanical aspects of attaching the heat sink. This article provides a general review of the available attachment technologies and their respective salient points.

We can quickly determine the relevant parameters to make the right choices for effective cooling including proper heat sink attachment. The thermal consideration is foremost on our decision tree. Once we have resolved the cooling issue, including the heat sink size and the type of thermal interface material needed, we need to ask the question of how this heat sink will be attached to the device or the PCB.

Why is a heat sink’s attachment system so important? Because of two key factors:

* First is to ensure that the heat sink is securely placed on the device to meet requirements such as the shock and vibration standards that are integral parts Mil-spec and NEBS testing.

* Second, every heat sink that is attached to a device, unless directly bonded, requires some sort of thermal interface material, commonly referred to as a TIM. The thermal performance of TIMs is directly related to pressure. The higher the pressure, more compact the material will becomes, hence, its thermal conductivity will be higher. Of course, if the pressure is too high, it will force the TIM to migrate from the gap between the heat sink and the device.

A search through patent archives will turn up many creative designs that engineers have come up with to address the issue of heat sink attachment. If you normalize this search and focus on broadly practiced and market-available solutions, you will see that there are several traditional as well as newer attachment systems to choose from. Each method provides advantages and drawbacks. Here is a summary of the most common choices:

thermal tapesThermal Tapes

Thermally conductive adhesive tape is the most cost effective heat sink attachment method. It is proven suitable for low-mass heat sinks and for components with low power dissipation. It’s not recommended for attaching heavy heat sinks. These double-sided adhesive tapes must be applied to clean, flat surfaces. The sink must be applied with sufficient pressure so there is full, even contact with mating surfaces. Thicker tapes tend to conform better to uneven component surfaces, but thickness adds to thermal impedance. So, trials with different kinds of thermal tapes are advised.

epoxyThermal Epoxies

Epoxy is more expensive than tape, but provides a greater mechanical bond between the heat sink and component, as well as improved thermal conductivity, (typically 2-3 times better). Most epoxies are two-part liquid formulations that must be thoroughly mixed before being applied to the heat sink, and before the heat sink is placed on the component. The epoxy is then cured for a specified time, which can vary from two hours to 48 hours at room temperature. The epoxy bond between the heat sink and component is basically permanent. Re-work can be very difficult if not impossible. The most typical damage caused by rework is the separation of the component die heat spreader from its package and often damage to the component and the die itself


Z-Clip Attachment to the PCB Z-clip attachment

More expensive than tapes and epoxies, wire form z-clips provide a mechanical attachment that offers other benefits. As the name suggests, the z-clip is a wire in the general shape of a Z. The printed circuit board must have holes or anchors, typically soldered, at opposite corners of the component’s mounting area. To mount a heat sink, one side of the clip hooks onto an anchor. The clip is deflected and passed through the sink fins. Its other end hooks onto the second anchor. The deflection develops a spring load on the component, which maintains very good contact. This consistent spring pressure allows for the use of higher performance phase-change thermal interface materials to improve thermal transfer. However, a poorly designed Z-clip, or anchoring system, along with the warping in the component or possibly the heat sink, could cause point contact rather than surface contact, resulting in poor thermal performance of the assembly.

clip on attachClip-On Attachment to the Component

While priced comparably, clips that attach directly to the component provide an advantage of z-clips in that no soldering or holes are needed in the board. They are often used to attach heat sinks to BGAs. These systems (e.g. superGRIP from ATS) make use of the gap the component’s underside and PCB top surface. A plastic frame is fitted around the component to provide a mounting platform for a spring clip. The ends of the clip attach to tabs on the frame. Once secured, the spring clip provides a precise, constant compression force to hold the heat sink in place and maximize the performance of the thermal interface material. The clip is easily detached with a hand tool to freely allow rework.

Push Pins

For larger heat sinks, push pins with compression springs are an effective mounting choice. The push pins, typically made of brass or push pins 1plastic, have flexible barbs at one end that engage with pre-drilled holes in the PCB. Once installed, the barb retains the pin. The compression spring holds the assembly together and maintains firm, reliable contact between the heat sink and component. Care is needed in selection of push pin size. Too high an insertion force can result in the die cracking and consequent component failure. In addition, Printed Wiring Board (PWB) warping is very likely in this situation that could cause the component to separate from the PWB.

Threaded Standoffs

For very large heat sinks, there is no substitute for the threaded standoff and compression spring attachment method. A threaded standoff is essentially a hollow metal tube with internal threads. One end is secured with a screw through a hole in the PCB. The other end accepts a screw which compresses the spring, completing the assembly. A typical heat sink assembly uses two to four standoffs, which tends to make this the most costly heat sink attachment design. Another disadvantage is the need for holes in the PCB. Furthermore, for very large heat sinks and thinner PCBs, or less metallization where the component resides, there is a requirement for a support plate the secure the standoffs to it rather than the PCB. The support plate will be placed on the backside of the board and prevents the warping of standoffsthe PCB that may cause mechanical as well as operational failures.

Summary

In general, thermal tapes and epoxy adhesives are the most cost effective choices for attaching heat sinks. But, they are only useful on small (low mass) heat sinks and don’t provide the thermal transfer capabilities of phase change interface materials. Z-clips and other clip-on methods have a wide range of applicability and are moderately priced. Push-pins and standoffs are excellent for mounting large heat sinks, but their cost – as well as those of the customized heat sinks – can be significantly higher. 
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