Heat pipes are commonly used for cooling electronics by transporting heat from one location to another. They may part of a system that cools a certain very hot component, but they are used, typically in multiples, to bring cooling to electronic assemblies. Here are some common attachment methods used when assembling heat pipe-based cooling applications.
First, we look at a cooling system where several heat pipes are integrated with a series of cooling metal fins. As shown, the fins may be mechanically press fit over the heat pipes resulting in a structure like that in Figure 1.
At this finned end of the assembly the heat transfers from pipe to fins where it dissipates to the air. These fins are typically stamped from sheet metal and the holes stamped through as well. When they’re properly sized, the fins press fit tightly on the raised heat pipes. The heat transfer is normally very good. To optimize thermal transfer, the fins can be soldered to the pipes, but press fitting into tight holes should provide more than sufficient performance.
The other ends of these heat sinks are soldered into grooves in an aluminum plate. (Figure 2) This is an aluminum plate and the heat pipes are copper. In order to solder we need to nickel plate the aluminum. Then we add solder paste into the grooves and then the heat pipes are inserted into the grooves.
The solder paste is usually a low temperature solder paste, typically based on tin bismuth alloys with melt temperature of about 138°C. That’s important because you really can’t bring the heat pipes to more than 250°C or else the water in the heat pipes will boil and the heat pipes will burst. So, during the assembly process you would put the solder paste into these grooves, then insert the heat pipes, and then clamp it with some sort of fixture to maintain the contact.
Then the whole assembly will go through an oven to reflow the solder paste. The reflow oven will precisely control the temperature of the air inside and will also have some kind of circulating fan so that the part heats evenly and quickly. Temperature control in the oven is critical to avoid exceeding the max temperature of the heat pipes. Other reflow methods for heating up an assembly might include a soldering iron, torch or hot air gun. But these methods can be risky and difficult. It is difficult to heat the part evenly and to control the temperature that the heat pipe is being exposed to.
In a prototype environment you might turn to an epoxy for attaching heat pipes to assemblies. There are number of thermally conductive epoxies available. Their thermal conductivity ranges from 1 to 6 W/mK. When a heat pipe is epoxied into an assembly, the bond line is so thin that it really doesn’t make too much of a temperature difference, even when compared to solder. There might be a few degrees difference which is usually acceptable in a prototype when you’re in testing mode and are aware that there could be a temperature difference of a few degrees. That’s easily calculated from the specs on the epoxy.
To begin the epoxying process, first you either mix your epoxy or use a mixing tube. You apply a thin layer into the groove and then insert the heat pipe. The grooves shown here are for heat pipes that are pre-bent and fit very precisely. Once in place, a flat plate that goes on top and is clamped down during the epoxy curing period.
In the example here, the epoxy has room temperature cure. Once the heat pipes are in and clamped down, the assembly can be conveniently left for a time at room temperature for the epoxy to cure. For a shorter time, the assembly can go into an oven at a high temperature – not a soldering temperature, but still hot enough that it will accelerate the cure time.
When embedding heat pipes into a surface a good practice is to machine the grooves slightly deeper than the heat pipes are. Then, you can create a fixture that is like a negative of this plate with raised areas where those heat pipes. Such a fixture will press the heat pipes down into those grooves. After they’re epoxied or soldered in the assembly the heat pipes and base will be at the same height for optimum thermal contact.
In this kind of application, flat heat pipes should be used. They can maximize the surface contact area where there are hot components. And in applications where the components do not come in direct contact with the pipe it’s often easier to use round heat pipes. This is because round heat pipes are easier to bend and have slightly better thermal performance than the flat heat pipes. So whenever possible we use the round heat pipes, but when they are embedded into a surface and they have contact with the components then we use the flat heat pipes.
For More Information
The above article is taken from a descriptive video by Advanced Thermal Solutions, Inc. that you can find on the ATS YouTube page at: https://www.youtube.com/watch?v=I5CQsBWKtOg