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High Temp in Small Spaces

Thermal Management for the Small Box

As more embedded users move from large rack-based systems, such as VME and cPCI, into small boxes, designers need to take heat inside and outside the box into consideration.

JOE PRIMEAU, ACROMAG

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Today’s industrial and military customers are demanding computer systems that have the ability to operate in a wider array of applications than traditional industrial and commercial products were designed to survive. This includes not only the need for extended operating temperatures (-40° to +75°C) but also the ability survive in high shock and vibration environments.

At the same time, many of the systems used in these projects have been reduced to a CPU board and specialized I/O. This has led system designers to question the need for the expensive infrastructure of a VME or cPCI-based system. Because of this, they have started to look to small box-type computer packages. The two box-type packages commonly seen in the embedded market today are based on either a PC/104 stack or an Industrial PC design. Each takes a different approach to solve customer design requirements.

PC/104 products address the needs of users by offering a flexible mix of a processor and I/O modules. When PC/104 was first introduced most manufacturers were content to allow the customer to provide a package solution. For simple commercial designs, this was an easy task for the user. As the application became more demanding, many PC/104 manufacturers began to add packaging solutions to their product offerings. These varied from simple, stackable aluminum frames to complete custom enclosures. Several PC/104 manufacturers provide well-designed housings for each of their modules, which incorporate the necessary mechanisms for temperature control as well as cabling and connectors. Others offer nothing more than an aluminum container that the PC/104 boards can be stacked in, with little or no consideration for controlling internal temperatures, cabling or connector requirements. One shortcoming of the stacked housing design is that each manufacturer has their own design, which causes the loss of one of PC/104’s key benefits, that is the ability to mix and match products from different PC/104 vendors.

The Industrial PC offers an assembled box typically based on an Intel processor, with a fixed set of PC-type I/O (Ethernet, USB, audio, video and serial I/O). At first glance, the Industrial PC would seem to offer the easiest path to a fully ruggedized, extended temperature design, since most of the key elements are controlled by the manufacturer. The container can be designed from the outset to control the internal temperature of the components with the use of vents, fans or conduction cooling (Figure 1). Internal cabling can be eliminated or reduced by moving the connectors on to the printed circuit board, thus minimizing shock and vibration issues. Finally, the components can be selected by the vendor to meet the design criteria of the expected application.

What appears to be the strength of the Industrial PC can also turn into its primary weakness—expandability. If the user’s application requires I/O (i.e. analog, digital or specialized serial I/O) beyond the limited set of I/O offered on the Industrial PC, the user is typically forced to add secondary devices to provide this I/O. This means added costs in hardware, cabling, power and, of course, space. Some manufacturers have tried to accommodate I/O need by adding a PC/104 expansion slot inside their package. At first glance, this would appear to solve the problem, but now the work that went into designing a temperature controlled environment is compromised by an uncontrolled element. How will the heat from the expansion boards be dispersed? Can the power supply provide the required extra power needed by the expansion board? And finally, how will the added cabling/connector to the field be handled?

Acromag has introduced an alternative to the standard Industrial PC (Figure 2). Named the I/O Server, this new product is designed to accommodate I/O expansion. The I/O Server’s enclosure, power supply and internal cooling are designed to allow the user to add up to four I/O modules without compromising the ruggedness or operational temperature of the product.

Overcoming the Heat

When designing these smaller, flexible packages, product designers must take into consideration the full thermal management of the design. This means understanding where hotspots are located, how heat can be moved away from the electronics, and where heat will travel before exiting the enclosure. If printed circuit boards are to be stacked, will heat rising from lower boards be captured and dispersed before it can affect boards above? Finally, the designer must consider how customers will use the finished product. Will the product always be oriented in an upright position or will some customers attach the device sideways on a wall or even upside down in a vehicle? All in all, accounting for these aspects can be a daunting task for the product’s designers.

Designing the product’s electronics must start with the selection and placement of components. To ensure proper operation across the full temperature range, components used in the design should be industrial rated (-40° to 85°C). The designer must then carefully consider each component’s location and how much heat it generates. When placing the hottest components, one has to consider how heat generated from that component will affect other components around it. The goal, of course, is to draw heat away from the component to the outside of the enclosure as quickly as possible. While best cooling techniques still cannot prevent some heat from migrating across the board, the goal should be to minimize its effect on other components.

Cooling Techniques

A wide range of techniques have been developed over the years to transfer heat out of the computer enclosure. These techniques include vents, fans and conduction-cooling techniques. All work with varying levels of success.

Natural convection relies on the natural movement of heat from the hottest location to the coolest. The most common natural convection method is the use of vent holes. Vents holes are usually configured as a set of holes on at least two sides of the enclosure, thus allowing air to move through the enclosure, cooling the electronics. The technique relies on heat rising off the electronics and moving out one vent, causing cool air to be drawn in through the other (referred to as the Stack Effect). Due to a vent’s limited ability to provide sufficient air movement, it is generally restricted to cooling low-power devices in benign environments. Natural convection also has a number of other drawbacks, particularly in applications where contaminants such as moisture, dust or corrosive gasses are an issue. For vents to work properly they must draw air from outside the enclosures, which allows contaminants to enter the enclosure and potentially damage the electronics.

While natural convection relies on the heat source to create air movement, forced convection uses a mechanical movement to force a coolant across the components to draw heat away. In box-based designs, this typically means a fan blowing outside air through the box and out a vent. Although forced convection can cool more effectively than vents alone, they share similar problems. Both methods allow moisture, dust and contaminants to enter the enclosure, threatening the electronics. Concern also has to be given to what happens if/when the fan fails or becomes blocked; this could quickly lead to overheating and possibly failure of the electronics.

Conduction-cooling techniques used in PC/104 and Industrial PC-type box designs typically fall into two categories—heat sinks and heat pipes. Both provide a mechanism to quickly move the heat to the body of the enclosure allowing the outside ambient air to draw the heat away. It is not uncommon to see both techniques used within the same enclosure depending on the placement of the electronics.

In box-based designs, one of the most efficient methods for cooling hot electronics is to use the body of an aluminum enclosure as a heat sink. This is done by bringing the electronics into contact with the body of the aluminum enclosure. This technique works both by absorbing the heat from a individual hot spot and dissipating heat over a large area (the enclosure) then transferring the heat to the cooler air on the outside of the enclosure. Cooling fins added to the outside of the enclosure further improve the efficiency. Fins provide both more surface area and a means of disrupting the air flow across the box, thereby improving thermal transfer. Figure 3 shows a typical heat sink example. A crucial heat sink design component is the thermal gap pad. This thermally conductive material efficiently transfers heat from electronic components to the heat sink by filling any gaps that may form. The thermal gap pad also prevents components from shorting to the metal enclosure when the product vibrates. It is crucial that the pad is of appropriate thickness to prevent compression or separation in high shock and vibration environments.

When electronics are stacked, one row above the other, a variation on the previous technique can be used. A heat spreader is added across the middle of the enclosure and connected to the enclosure’s outside wall. The electronics on lower circuit boards then contact this heat spreader to move the heat to the enclosure body (heat sink). This method has the secondary benefit of capturing rising heat before it can affect electronics placed above. In designs that require customer access to the electronics, a friction plate allows the electronics to slide in and out without damaging the thermal gap pad (Figure 4).

Heat pipes rely on the evaporative cooling effect caused by a temperature differential between two ends of the pipe. A fluid at the hot end of the pipe turns to a vapor; this vapor then flows naturally to the cool end of the pipe and condenses. When particularly hot components are located away from the heat sink created by the enclosure, a heat pipe can be added to transfer that heat to the enclosure’s wall. Figure 5 shows an example of a heat pipe employed to cool a CPU in a PC/104 stack. Using heat pipes in a high shock or vibration environment requires care to prevent flexing that could create stress fractures. A vacuum is required for the heat pipe to operate efficiently. If the heat pipe cracks or a hole forms, the vacuum can be lost, substantially lowering the effectiveness of the heat pipes. To help prevent potential damage, designers often encapsulate heat pipes in an aluminum block.

Well-designed box-based computers, be they Industrial PC or PC/104-based, offer a reliable solution to a wide array of applications. They can be used wherever the environment will allow. In industrial application, the boxes’ rugged design allows placement on or near machinery with less concern for the effect of heat, shock or vibration caused by the machinery. In addition to the typical industrial applications, box-based designs are often well suited for mobile applications such as heavy moving equipment, trains and ships. The extended temperature specification of these products means that they are also well suited for remote outdoor application where it would be cost-prohibitive to build a climate controlled structure to protect sensitive electronics. Properly designed for wide temperature operation, box-based solutions can offer a rugged, reliable solution for a wide range of military, industrial and scientific applications.

Acromag
Wixom, MI.
(248) 295-0310.
[www.acromag.com].