This type of assembly usually consists of three components:

The Evaporator

This is the part of the assembly that connects to the hot component and the heat pipe. It can be machined from solid material, extruded, die cast or cold forged. It can be manufactured from many different materials but the two most common are copper and aluminium.

The evaporator commonly sits on top of the hot component with a thermal interface material between them. The evaporator can also be designed to incorporate fixing hardware.

In order for the assembly to operate, the evaporator must of course interface with the heat pipe or heat pipes. This interface can be a dry press fit slot, glued in place with a suitable thermal interface epoxy, soldered or even inserted into a drilled hole. Each of these methods has a different cost/performance characteristic.

The Heat pipe or Heat Pipes

Depending on system power and performance requirements, Columbia-Staver can provide solutions with single or multiple heat pipes, with water or other working fluids. The heat pipes can be flattened and shaped into complex geometry (within certain design rules). The size and number of pipes is determined during the design phase. The function of the heat pipe is to efficiently move the heat away from the hot component to a cooler area, typically a fin stack.

The Fin Stack

At the other end of the heat pipe from the evaporator is the condenser section, this can be attached to a simple spreader plate, the wall of a metallic enclosure or more commonly a set of fins designed to increase the surface area to help dissipate the heat into the ambient. The number and size of these fins will depend on the system requirement, the ambient temperature, and the available airflow.

These fins can be extruded, stamped, skived or machined from solid and can be attached to the heat pipes as an interference fit, epoxy bonded or soldered.

Increasing the size and number of the heat pipes and the associated fins, the design can carry from a few watts to kilo watts of power.

Using the high thermal conductivity of heat pipes, heat can be removed from inside a sealed box or cabinet and be passed to the ambient outside the box or cabinet, provided the outside temperature is lower than that inside the box or cabinet.

The heat pipes are fitted to a flat plate that will act as a bulkhead when fitted to the box or cabinet. Half of the heat pipes are inside the box or cabinet and half outside; the heat pipes will transport the heat through the bulkhead and dissipate into the ambient. Fins can be added to the heat pipes inside and outside to increase the surface area in contact with the air. Fans can be added inside and outside if deemed necessary to achieve the required thermal performance.

Heat pipe tower systems are intended to move heat vertically from a PCB into a fin stack that is positioned in a suitable air flow.

Tower systems can be made using standard heat pipes that are formed into an “L” shape or a “U” shape.

In the case of “L” shaped heat pipes, one leg is embedded into an evaporator block that sits onto the hot component and the other leg has a fin stack fitted. Multiple “L” shaped heat pipes can be used to cope with the power of the component.

In the case of “U” shaped heat pipes, the bottom section of the “U” shape is embedded into the evaporator block with the two legs sitting vertically above the heat source, fins can be fitted.

An alternative method of producing a tower system is to use a very special heat pipe. If the heat pipe is made from a large diameter tube and the ends are flat, the sinter wick normally on the inside walls of the tube can be continued across the flat base of the pipe. This end is positioned above the heat source and the fluid contained in this section will evaporate, moving the heat vertically and passing into a fin stack. As this pipe has no bent section, the fins can be positioned very close to the evaporator section.

The performance of aluminium heat sinks can be improved by embedding heat pipes into the base of the heat sink. The very high thermal conductivity of the heat pipe effectively changes the thermal conductivity of the aluminium base. High heat flux in concentrated areas can be spread across a heat sink, by placing the hot spot over the heat pipe which becomes an evaporator transferring the heat to the cooler part of the heat sink base where the working fluid condenses, releasing the heat into the heat sink.

Surface Embedding

The heat pipes can be embedded into the surface of the heat sink and attached by simple dry press fitting, by gluing with thermal epoxy or even by soldering. In surface embedding, the heat pipes will be pressed into a half round groove and flattened to the surface of the heat sink.

Dry press fitting is the most cost-effective way of embedding a heat pipe into the surface of a heat sink.

Care must be taken to get a very good fit between the heat pipe and the slot that has been cut into the heat sink base as any gaps will adversely affect the performance.

Mechanical strength and the elimination of possible voids between the heat pipe and the heat sink can be achieved by using a thin layer of thermal epoxy between the heat pipe and the heat sink.

Finally for the best thermal performance the heat pipe can be soldered into the heat sink base. Due to incompatibility, however this process requires that the heat sink is first nickel plated to enable the soldering process to take place.

The heat pipes can of course be formed into complex shapes prior to embedding, this means that they can avoid holes and other features in the heat sink base.

Total Embedding

For total embedding, the heat pipe channels can be closed over by friction stir welding (FSW) an aluminium cover plate over the heat pipes.

If the heat sink base is of sufficient thickness, then holes can be gun drilled into the heat sink base and the heat pipes pushed into place, the heat pipes can be expanded once in the hole, or they can be glued in place with thermal epoxy.

For maximum heat sink improvement, a vapour chamber can be embedded into the base of the heat sink. Vapour chamber solutions are based on the same technology as heat pipes.

In contrast to the linear heat transport in one direction inside heat pipes, vapour chambers are capable of transporting heat in multiple directions and are completely isothermal using the base of the heat sink thereby making sure that each fin can dissipate an equal amount of heat.

The vapour chamber can be inserted into a recess that has been machined into the base of the heat sink and held in place by thermal epoxy. In some cases, it is even possible for the vapour chamber itself to form the heat sink base and for fins to be directly attached to one side of the vapour chamber.

Vapour Chambers offer the following benefits:

• Reduced thermal resistance and spreading resistance
• Provides significant weight reduction
• Optimised form factor for space-constrained applications
• Compatibility with various form factors
• Reduced device temperatures
• Enhanced heat transfer efficiency
• Minimised thermal throttling