Choosing a heat-sink-attachment system
The right attachment method can maximize heat-sink operation in critical power designs
BY BAHMAN TAVASSOLI
Advanced Thermal Solutions
Norwood, MA
http://www.qats.com
When choosing an effective heat sink, engineers usually consider its price, size, weight, performance and other features. But equal care should be given to deciding how the heat sink will be attached to the hot component so that the sink works most effectively.
To maximize heat transfer, intimate contact is essential between a sink and its assigned component. Insufficient contact can leave air gaps at the heat-sink-to-component interface that will reduce the amount of heat flow. For this reason, most heat-sink systems include a thermally conductive interface material, such as a layer of thermal grease or a thin pad. On the other hand, a heat sink attached too tightly can damage a component or cause the pc board to warp or camber.
Fig. 1. A frame-clip attachment system includes a plastic frame that fits tightly around the component. The ends of a metal clip that runs through the sink's fin field hook securely into integral tabs in the frame.
Several heat-sink attachment methods are now available; many are designed for certain kinds of package types while others are unique to a specific type of heat sink. The most common attachment systems are metal clips, pins, screws and adhesive tapes.
Clips
Metal spring clips and wire clips provide a convenient way to securely fasten a heat sink with the required contact pressure. They are fast to apply, snapping onto slots around the package or into holes in the pc board, and they are easily removed. Clips provide a uniform, consistent pressure to keep the sink's mating surface against the component (usually with an interface material between them). Because the clips provide consistent pressure, no torque-gauging tools are needed.
Despite their tight hold, clips allow enough motion within the sink-component assembly to prevent damage to a package when the coefficient of thermal expansion for the component differs widely from the interface material and the heat sink.
However, wider spring clips can take up enough space in a heat sink's fin field to reduce its fin count. A wide clip may also inhibit air flow through the field. As a result there can be a reduction in the sink's heat-spreading ability.
Thin wire clips fit within a full fin field, and unlike some high-arched spring clips, the wire clips don't increase the profile height of the attached sink. The wire can also be custom shaped more readily than a wider spring clip.
Until recently, wire clips had to be fastened to the pc board on each end, typically by using metal anchors soldered onto the board. This design can provide a useful grounding path from the metal sink to the pc board's ground plane, but the need for anchors takes up precious board space.
A new frame-clip design includes a strong, resin-based frame that is flexed, and then tightly fit around the component's perimeter. This frame collar includes integral tabs into which the ends of a spring clip or wire clip can be securely hooked with just hand pressure (see Fig. 1 ). The frames are omni-directional, which allows the heat sink to be mounted in an optimum position within the system's airflow. Frames are made for specific package sizes, say for a 20-mm flipchip or a 23-mm square BGA.
Screws and spring fasteners
Another heat-sink mounting option is to use screws and related hardware. A typical example is attaching a TO-220 package, having an integrated mounting tab, to a heat sink or heat spreader bar using screws and nuts. Other designs feature sinks with integral studs that pass through the device's mounting hole and are soldered onto a pc board.
While screw and stud attachment methods are long-proven, some care is need with their application. With screw-and-nut based mounting there can be a potential for over- or under-torquing. Over-torquing can create a camber situation in a pc board while under-torquing can compromise thermal transfer (or lead to loose-hardware problems). Soldering heat-sink studs is a labor step that is impractical in some manufacturing processes.
Other heat sinks are designed to attach directly to a pc board with spring-loaded fasteners inserted at each corner of the sink's base. These push-pin fasteners apply force to the heat-sink base to maintain a desired pressure on the thermal interface material. They also hold the heat sink in place during dynamic loading. Mounting fasteners are available for use with pc boards of different thicknesses.
Adhesive tapes
A hardware-free and very practical method for heat-sink attachment is to use thermally conductive double-sided adhesive tapes. The tapes provide both the heat-sink-to-component attachment system and the thermal interface material, thereby replacing clips or fasteners (see Fig. 2 ).
Fig. 2. Thermally conductive double-sided adhesive tapes provide both attachment and a thermal interface, eliminating the need for attachment hardware.
There are many choices of thermally conductive adhesive tapes, including versions reinforced with fiberglass or aluminum mesh. Their thermally conductive fillers are similar to those of dry thermal pads, that is, boron nitride, aluminum oxide, titanium diboride and others. Some tapes are specifically made to bond sinks to plastic packages, and others have a dielectric film layer. And many thermal tapes are embossed with a tight pattern to maximize their surface conformability and minimize any air pockets.
Some thermal tapes have one side optimized to adhere to the metal mating surface of a heat sink and an opposite side formulated to work with plastic packages. It's critical, of course, to match the tape sides to the correct surfaces. It's also important that all attachment surfaces be clean and as flat as possible.
Thermal tapes can possess high strength bonds, but they're challenged by the physical nature of some applications. Larger and heavier heat sinks mounted to components on vertically aligned boards may be more safely secured with a clip system. The same is true for parts exposed to strong shock and vibration. The thermal performance of thermal adhesive tapes is also surpassed by grease and phase- change interface pads, which don't provide attachment properties.
Tapes, clips, pins, and other attachment choices can be explained and compared by a competent thermal technology engineer. For designers looking to maximize the rate of heat transfer from hot components to heat sinks, it is well worth investigating the different choices in heat-sink-to-component attachment systems.
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