RTC Magazine

Getting the maximum performance out of embedded PCs while keeping their size, weight and cost within reasonable limits requires careful thermal design. Excessive internal temperatures can wreak havoc on embedded PCs and their electronic components. Exceeding thermal limits results in loss in performance first, and ultimately, component failures and downtime.

Thermal management has always been important. As processor operating frequencies go up and IC processes continue to shrink, the issue has turned from important to critical. Hot spots occur easily and are quite a challenge in today’s embedded PCs. An executive from a well-known processor vendor at a recent technology forum remarked, “Thermal issues are the number one problem we face today.”

Embedded systems cover a wide range of mechanical variations, from rackmount VME systems to small portable designs. The computer architecture in embedded systems also varies tremendously, from low power, special-function RISC processors to general-purpose processors common in desktop or laptop PCs. PC processors such as Intel’s Pentium and Motorola’s PowerPC are very common in embedded systems because their use in consumer systems drives the cost down.

That said, these processors and their support circuitry lack the power and thermal management capabilities suited to the rigors of industrial embedded systems. For example, the narrow slot spacing in a VME chassis forces a designer to use alternatives to the large heatsinks and fans found in desktop motherboards. Kontron’s ASM3-VME chassis shown in Figure 1 illustrates the relatively small inter-board space common to VME.

System designs using embedded PCs require heat dissipation from a few watts for a 386-based system to over 100 W for a dual Pentium 4 system. Compounding the thermal management task are the special environmental, high-availability and longevity requirements common in embedded systems. Dissipating even 10 W can be difficult in a system that can’t use fans, must be completely sealed, and must operate in an environment where the ambient air temperature is 50°C and the maximum operating temperature is 85°C. Such restrictions are common in many industrial and military applications. Obtaining the proper thermal design requires both thermal modeling and verification by actual measurements.

Rings of Thermal Management

It’s helpful to model thermal management in an embedded PC-based system as a set of concentric rings. Each ring represents a thermal interface that transports heat away from the ring inside of it. Within the innermost ring is the silicon of the high-performance ICs used in the system that are the primary sources of heat. The first thermal interface layer is the packaging provided by the IC vendor. The outermost ring is almost always thermally stable ambient air, but this is determined or controlled by the user.

The system designer must implement application-appropriate thermal interfaces to transfer the heat of the system, starting at the ICs to this outer, stable heat reservoir of air. Table 1 shows the layers and simple description of their function. The number and type of layers in the thermal solution varies from system to system according to the environmental and application-specific requirements. In any given system, not all layers may be present, or allowed, and the layers might vary in order from system to system.

The packaging and features of the components, the source of system heat, represent the first layer of thermal management. Packaging material is commonly plastic for economic reasons, but in high-power ICs a metal heat slug or “flip chip” style packaging allows closer thermal coupling to the silicon die for more efficient heat transport. Many ICs, particularly processors, offer programmable power reduction features such as dynamic clock speed control. Low-power processors may be required in situations where active cooling isn’t allowed and space limitations exist.

Board Selection Key

For most system designers, the use of embedded PCs starts by purchasing a board-level product. Board-level embedded PCs typically provide only layers 0 and 1 of the thermal solution. To implement a complete system based on this type of product typically requires a heat sink and/or fan. Following the thermal model, in that case layer 2 is absent, layer 3 represents the processor to heatsink interface, and layers 4 and 5 are implemented by the heatsink and fan. That scenario so far only transfers the heat into the air inside the enclosure, so there must be some airflow through the enclosure or some other means to transfer the heat to the environment—layer 6.