As engineers search for ways to effectively cool the complete system, developments are being made in thermal management techniques.

Figure 1. Bonded and folded heat sink designs employ base plates to hold thin aluminum plates that are used to dissipate heat from electronic devices.
Cooling the latest electronic devices such as video game chips, power transistors and central processing units (CPUs) is not an easy task. As the devices become faster and smaller in size, they require heat sinks that are more efficient, lighter and smaller. In the past, engineers could use an extruded heat sink to tackle heat problems. Simple extruded heat sinks are inexpensive, and the older designs can be effective. However, a number of unique and challenging features are involved when cooling state-of-the-art electronics:

  • Limited available space.
  • High heat flux.
  • Weight and cost limitations.

For example, today's CPUs generate approximately 30 W of heat from a tiny surface area (approximately 0.5 x 0.5"), and the space available for cooling is approximately 2 x 2 x 1.5". In comparison, older CPUs provided approximately 5 W from a 1.5 x 1.5" surface. Because total available space is so small, a heat sink with a large surface area is needed. Yet the only way to have more surface area within a given space is to have narrower gaps between thinner fins. Extruded heat sinks have been pushed to nearly their technical limits, but they are not good enough for some newer applications.

Figure 2. Heat sink designs such as the folded fin have dense fin configurations that require forced-air cooling. In many cases, fan lifetime is longer than the lifetime of the device being cooled.

Some Other Choices

Developments in thermal management techniques include bonded and folded fin heat sink designs, heat pipes and thermoelectric coolers.

Bonded and Folded Fin Designs. Bonded fin designs consist of a base plate with small grooves and thin aluminum plates bonded into the grooves by thermal glue (figure 1). This design achieves a high aspect ratio and has been applied in cooling arrays of power transistors with heat loads of more than a few hundred watts.

Folded fin designs consist of a thin aluminum sheet folded into contiguous fins and bonded onto a flat base plate. The main advantages of this design are higher fin density and lower manufacturing cost compared to plate fins. This design has been used to cool desktop CPUs in large quantity.

Forced-air cooling is a must for heat sinks with dense fins such as the folded fin (figure 2). Improvements in fan technology have made them quieter and more reliable. In most cases, fan lifetime is longer than the electronic device's lifetime.

As effective as bonded and folded fan designs may be, having a large surface area solves only half the problem. As fins get thinner (less than 0.02"), the amount of heat transferred to the far end of the fin is limited. The thinner the fin, the less heat can move through - in other words, fin efficiency is reduced on larger surface areas. Thus, a portion of the fin is not fully utilized for cooling and space is wasted. The challenge becomes how to effectively transfer heat from a small surface area to all of the fins.

The base plate and fin could be made thicker, but this would reduce the space for fins and defeat the goal of having more surface area for cooling. Therefore, careful planning, early design and testing are required for most of today's electronic components.

Heat Pipes. The operating principle of heat pipes is based on the phase change of a working fluid inside a pipe. Fluid is vaporized at the hot end, flows to the cold end and is condensed. Thus, it moves heat away from hot end to cold end. The fluid then returns to the hot end by capillary effect through a wick structure inside the pipe.

This concept can be employed to create thin and flexible pipes that channel heat away from a hot spot to locations with a larger heat-dissipating surface. For heat pipes with diameters ranging from 0.118 to 0.315" (3 to 8 mm), the heat transfer capability ranges from 10 to 50 W. It can maintain a temperature difference of 50°F (10°C) or less between the temperatures of the hot spot and the cold end.

Specially designed for cooling CPUs of notebook computers, heat pipe assemblies consist of a heat pipe, heat sink, heat spreader, thermal tape and mounting plate (figure 3). Due to design constraints, little space is available directly above the CPU, so in general, the heat sink and fan are located about 2 to 4" away from the CPU.

Heat pipe assemblies have two key features:

  • The thermal tape and heat spreader help transfer heat from CPU to the heat pipe and then to the heat sink.
  • The mounting plate holds the heat spreader tightly against the CPU and functions as both a mechanical support for the flexible heat pipe and a heat dissipating surface.

A typical heat pipe used to cool a notebook computer's CPU is about 0.118 to 0.157" (3 to 4 mm) dia. and can handle approximately a 15 to 25 W heat load. Larger size heat pipes ranging from 0.236 to 0.315" (6 to 8 mm) are used for heat loads of approximately 40 W.

Heat pipes can conduct heat approximately 1,000 times more effectively than a similar size copper rod. They are used to channel heat away from a tight place and to spread heat across a large base plate. Thus, the base plate's thickness can be reduced to allow more fin surface area.

Thermoelectric Coolers. Also called TE coolers, Peltier coolers or electronic heat pumps, thermoelectric coolers are semiconductor devices that function like a heat pump. Electronic devices such as laser diodes or infrared sensors must be kept cool to function properly. If cooling below ambient temperature is required, a thermoelectric cooler is an effective device. Thermoelectric cooler design involves:

  • Bismuth Telluride semiconductor construction.
  • Coupled pairs of N and P bars connected in series.
  • Bar areas ranging from 0.0003 to 0.038 in2 (0.2 to 25 mm2).
  • Approximate pumping capacity of 26 W/in2 (40 mW/mm2).
  • Roughly 645 A/in2 (1 A/mm2) produces 86°F (30°C) temperature difference.
  • Technology up to 254 couples.

Based on the Peltier effect, a thermoelectric cooler works by applying a DC current to the device. If voltage is applied to the semiconductor junction, an electrical current will flow in the circuit. As a result of the current flow, a cooling effect will occur at one junction and a heating effect will occur at the other. In effect, heat is moved from one side of the module to the other, and one module face will be cooled while the other side is heated.

Cooling or heating is proportional to the magnitude of current, and this effect is reversible. If the current's direction is changed, the original cooling side will become the heating side and vice versa. A much lower temperature can be achieved by stacking a few layers of thermoelectric coolers to form a multistage unit (figure 4).

Thermoelectric coolers can be used to minimize the effect of Joule heating in semiconductors. In Joule heating (I2R), R is the electrical resistance that occurs in semiconductors as a result of current flow (I). This overhead heat along with the heat from electronic devices must be removed. Typical thermoelectric cooler sizes range from 0.5 to 2 in2 with overhead heat ranging from a fraction of a watt to 100 W. Proper cooling devices such as heat sinks and fan packages are required to prevent thermoelectric cooler overheating.

Because thermoelectric coolers are solid-state devices, they are reliable when used properly. Failure return rate is less than 0.1%, and most are due to improper use such as too much mechanical force or overheating.

Today's electronic devices present many challenging problems and opportunities for the cooling industry. As more devices become hotter and smaller, in-dustry has at-tempted to move toward a system- cooling approach rather than a device-cooling approach. Dis-sipating heat from one device may seriously af-fect the ambient temperature of near by devices. As cooling engineers are challenged to cool an entire system and not just individual devices, special heat sinks, heat pipes and thermoelectric coolers will find more use in cooling applications.