RTC Magazine

The three most important aspects in designing successful high-performance embedded systems are efficient PCB design, meeting power requirements and an effective cooling solution. Since heat reduces system life, long-term reliability is a direct result of effective cooling solutions to heat dissipation issues. OEMs that partner with their solution providers early in the design and integration process will find that a variety of cooling techniques are available. Often, innovative solutions in heat dissipation issues create an improved operational envelope and lower total cost of ownership for those systems.

A successful cooling solution meets or exceeds the performance goals of the OEM’s design. High-reliability designs must thrive in high-temperature environments while maintaining sufficient processor throughput to meet the computational demands of the embedded application. Market forces are driving designs to optimize both price and performance, and to continue the trend toward full CPU utilization for a given deployment. Thus, it is important for thermal management solutions to be capable of handling worst-case loads.

Designers of high-performance embedded systems have

also been challenged by increasing power consumption requirements, another driver of thermal management solutions. The incorporation of Intel’s Pentium M CPU into small form-factors such as PC/104 has pushed PC/104 solution power envelopes to new levels. In addition, full utilization of Intel’s Core Duo processor will more than double the 15W maximum power consumed by the previous Pentium III generation.

Traditional convective and conductive cooling solutions may not be adequate to meet the new demands and will end up costing more over the life of the system. Ineffective mounting and planar alignment problems degrade the performance of most thermal management solutions. By bringing aerospace technologies to address this issue, system life can be extended and both time-to-market and the overall cost of ownership can be reduced.

Traditional Thermal Management Solutions

One popular method for traditional conductive applications is the deployment of a flat-surface heat-spreader. Such a spreader is designed to make thermal contact with the CPU and other support chips, providing a highly heat-conductive path from the contacted chips directly to the chassis wall. Unfortunately, these one-piece, rigid solutions have the undesirable result of transferring the shock from an impact on the chassis directly to the CPU die. Additionally, in order to accommodate interior flat chassis wall mounting, heat-conductive solutions must adapt the board topography to a flat planar mounting surface.

For heat-convective applications that allow active cooling, the traditional extruded heat sink can be improved with the addition of an appropriately sized fan. Cooling solution attachment points are usually required on each chip to accomplish this. In the case of a Pentium M system, individual heat sinks for the CPU and the highly integrated 82855GME chipset are required. However, the ownership costs of maintenance—e.g., renewal of worn fans and cleaning of contaminated heat sinks—may only be acceptable to OEMs in certain market segments, or during early design, proof-of-concept creation and prototype phases.

In cases where traditional extrusion offers insufficient performance, pin-fin heat sinks can be considered. Integrated combinations of pin-fin heat sink and fan, or fan-sinks, are available to obviate the problem of bolting fans to a pin-fin heat sink. For example, Advanced Digital Logic’s fan-sink combines a flat motor design with a customized pin-fin heat sink base (Figure 1). The fan-sink solution can dissipate more than 30W when the full base area is in contact with the heat source, in a volume of space 37% smaller then equivalent traditional extruded heat sink solutions.

This fan-sink also protects the bearings, the most common point of failure in mechanical devices. By directing air flow through the central opening in the impeller, and therefore through the motor, the precision double ball bearings of the fan-sink are placed directly in the path of the inlet air flow. This keeps the bearings cooler than conventional brushless-DC motor designs, in which air is drawn around the motor, leaving the bearings shielded in a zone where heat can concentrate.