Generally, a passive heat sink in combination with a system fan is the most efficient, most cost-effective cooling solution. However, active fan heat sinks solve many real-world problems and get products to market on time. These compact units, consisting of a fan mounted directly on top of a heat sink, cool a single IC. In some cases, they are an interim measure to get a product out the door. In other cases, fan heat sinks are the only viable solution. An example of this scenario is cooling of embedded PCs in hazardous (to the PC) industrial locations. Where particles or corrosive fumes occupy the same environment as the PC, sealing the chassis is the best way to ensure long life. Fan heat sinks can cool sensitive chips in this sealed environment, offering a solution where traditional methods fail.
A video card manufacturer developed an efficient passive solution that effectively cooled a 4-watt video chip. The next generation IC jumped to 8 watts, overpowering the passive solution. The low profile (6 mm high) fan heat sink shown in Figure 1 allowed the manufacturer to offer the more powerful video chip without redesigning the card. Figure 2 shows a thermal model of the video chip with the fan heat sink. Note that the simulation has captured the enhanced cooling at the tips of the heat sink fins due to the flow entrainment.
Socket 7 devices use the motherboard connector for the Pentium, AMD K6 & K5, and the Cyrix/IBM 6x86 family of processors. The Socket 7 is a zero-insertion force (ZIF) design that makes CPU installation quick and easy. Socket 7 devices are prime candidates for fan heat sinks. These devices perform similar functions, but dissipate vastly different amounts of power. A fan heat sink ensures any device inserted in the socket will receive sufficient cooling.
Some low-end PCs use fan heat sinks in place of a system fan. This solution lowers the overall cost and provides a quieter system. Since office noise level requirements are stricter in Europe, many PCs made for the European market use fan heat sinks specifically to lower the noise level. As functionality and processor speed increase, fan heat sinks may no longer be able to keep up with the total cooling needs of the system, possibly requiring alternative cooling strategies.
Custom-built PCs usually have less than optimum air flow and heat dissipation. An off-the-shelf chassis may include any one of a number of motherboards with customized options for video, communications, etc. Board manufacturers selling into this market often include fan heat sinks in order to guarantee the reliability of critical components.
An integrated fan improves the performance of any heat sink by about 50 to 100%. The directional airflow provided by the fan ensures more efficient heat transfer from the fins to the ambient. When fan heat sinks are engineered into the system, they can provide many years of uninterrupted service. To maximize the effectiveness of this kind of solution, the designer must take into account the properties of the heat sink itself. The first property to consider is reliability. Any moving part has a finite lifetime, and fan heat sinks are no exception. Sleeve-bearing fans are the least reliable, typically rated for 10,000 hours. Because of this short lifetime, designers should shy away from their use for most applications. Single-ball single-sleeve bearing fans carry a typical rating of 20,000 hours, twice as reliable as the sleeve-bearing fans. While some PC manufacturers use these fans, the 2.2-year constant-use expected lifetime is probably not acceptable to most end-users. Dual-ball bearing fans are the most reliable, rated for 50,000 hours. These fans are recommended for all industrial applications, especially embedded systems.
The cost differential between these different fans may influence the decision to use one fan over another. Single-ball, single-sleeve bearing fans cost 15-20% more than single-sleeve bearing fans. The jump to dual-ball bearing fans accounts for another 15-20% increase in cost. In most applications, the reliability improvement is well worth the additional cost.
Some fan heat sinks go one step further, incorporating a failure detection method into the design. These heat sinks include a signal interface that connects directly to the processor. The fan emits two pulses per rotation and sends these pulses through the signal interface to the processor. The processor can then clock the pulses and compare with the speed when the fan is new. When the fan begins to slow down, indicating imminent failure, the processor can begin an automatic shut down procedure, allowing the user to save all work-in-progress and repair the problem before damage occurs to the IC or the data.
Another innovative, proactive fault-detection technique is the incorporation of thermisters into the heat sink design. These temperature probes sense chip temperature, allowing a controlled shut down when the temperature approaches the maximum the chip can handle.
A second property of the heat sink to consider is its ability to function in a particular environment. If the heat sink does not have sufficient clearance above it, it may not only be less reliable, it may also not perform to specification. The temperature of the enclosure also has an effect on fan functionality. Air temperatures greater than 45 °F put strain on the fan, shortening its life.
Fan heat sinks are providing cooling in many short-term and long-term applications. Their small size, ability to provide spot cooling, and relatively efficient nature make them an ideal solution when there is no time for a system redesign. They can also provide the necessary cooling inside closed environments. For applications requiring maximum uptime, proper fan selection and the use of fan speed or IC temperature to predict failures can enhance system reliability. Fan heat sinks give designers the speed and flexibility they need to meet rapid time-to-market requirements.
Aavid Thermal Technologies, Inc., One Eagle Square, Suite 509, Concord, NH 03301. Tel: (603) 224-1117; Fax: (603) 224-6673.
Icepak is a trademark of Fluent Corp.