Once the theory of two-phase thermal devices (Heat Pipes and Vapour Chambers) is understood, that they are devices that can transport heat utilizing the phase change characteristics of a working fluid inside a tube or container and that they have extremely high effective thermal conductivities, typically 10 to 10,000 times more conductive than solid materials. Then it is clear that this technology can be applied to solve thermal issues by providing a brand new solution concept or improving an existing solution.
Columbia-Staver can provide a design service to work together developing an optimised system or simply build to print a customer’s design. The possible solutions are endless but can fall into the following main categories.
Heat Pipe Fin Stacks
This type of assembly usually consist of three components:
This is the part of the assembly that interfaces with the hot component, it can be machined from solid, extruded, die-cast or cold forged. It can be manufactured from many different materials but most common would be Aluminium or Copper.
The evaporator commonly sits on top of the hot component with a Thermal Interface Material between. It can be designed to include fixing hardware.
The evaporator must of course also interface with the Heat pipe or Heat pipes. This interface can be into a dry press fit slot, glued in place with a thermal epoxy, soldered or even inserted into a drilled hole. Each of these methods has a cost & performance characteristic.
The Heat Pipe/Pipes:
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 constraints) that is required in order to fit into the customers system. The size and number of heat pipes required is determined during the design phase and is a function of the system requirements.
The Fin Stack:
At the other end of the heat pipe from the evaporator is the condenser section, this can be a simple spreader plate or the wall of an enclosure but is more commonly a set of fins designed to increase the surface area for dissipation. The number and size of the fins depends on the system requirements and the available airflow if any. These fins can be extruded, stamped, skived, machined and can be attached in similar ways to the evaporator i.e. Interference fit, epoxy glued, soldered or even expanded.
By changing the scale of the design i.e. size and number of Heat pipes, the size and number of fins & the design of the evaporator, Heat pipe fin stacks can be designed to carry a few watts or kilo watts of power.
Heat Pipe Tower Systems
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 by forming standard heat pipes into an ‘L’ or ‘U’ shape.
In the case of the ‘L’ shaped heat pipes one leg is embedded into an evaporator block that sits on the hot component and the other leg has a fin stack fitted. Multiple ‘L’ shaped heat pipes can be used in order to cope with the power of the component.
In the case of the ‘U’ shaped heat pipes the bottom section of the ‘U’ is embedded into the evaporator with the two legs sitting vertically onto which a fin-stack can be fitted.
An alternative method of making a Tower system is to use a special heat pipe. If the heat pipe is of large enough diameter then one end of the pipe is finished square and the sinter wick structure is contiguous sown the inner radius of the pipe and across the circular end section. This end section is then on contact with the heat source and it is from this section that the working fluid is evaporated. The section of pipe away from the flat end is fitted with Fins. As the heat pipe has no bent section the fins can be positioned very close to the evaporator section.
Embedded Heat Pipes
The performance of aluminium heat sinks can be improved by embedding heat pipes into the base plate of the heat sink. The high thermal conductivity of the heat pipe effectively changes the thermal conductivity of the base. High Heat flux in concentrated areas can be spread across a heat sink by placing the hot spot over one end of the heat pipe which becomes the ‘evaporator’ and the heat is transferred to the cooler part of the heat sink where it condenses, releasing the heat to the heat sink.
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 Nickle 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.
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 one in the hole or they can be glued in place with thermal epoxy.
Embedded VC Heat Sink
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 the heat pipes, vapour chambers are capable of transporting heat in two directions and completely isothermalizing 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 directly attached to one side of the Vapour chamber.