Electronic applications such as mobile, embedded computing, and servers often use intermediate level heat pipes to cool systems dissipating 15-150 Watts. Usually, such heat pipes use copper tubes and sintered copper wicks with water as the working fluid.
System designers incorporate heat pipes when the thermal design has limitations of space and/or weight and other materials such as solid aluminum or copper heat sinks cannot achieve the desired cooling. However, heat pipes for electronic applications require several considerations.
Manufacturers often publish thermal conductivity of heat pipes as ranging from 10,000 to 100,000 W/mK, which is nearly 500 times that of solid aluminum, and 250 times that of solid copper. However, unlike solid metal, the effective thermal conductivity of copper heat pipes varies extensively with their length and other factors affect this figure as well, although to a lesser extent.
For instance, a sample heat pipe could reach its published thermal conductivity of 100,000 W/mK when its length was 100 mm, when transporting heat from a 75 W power source. However, on increasing the length of the same heat pipe to 200 mm, its thermal conductivity fell to less than one-third the published figure.
Moreover, customization for a specific electronic application can severely limit the operational and performance characteristics of a heat pipe. This is much like the type of transformation a vehicle requires when fitting it out for a specific activity such as track racing or off-roading.
An engineer often has to customize the heat pipe when navigating the crowded route from the heat source to the evaporator or condenser. They do this by flattening or bending the heat pipe. For a lower thermal resistance, the designer may have to machine a heat pipe assembly to create a flat surface capable of direct contact with the heat source. This may require elimination of the solid metal base plate and the extra TIM layer. To keep the entire fitment small enough, the heat pipe may have to use a diameter that is different at one end from the other.
Drastic changes to customize the heat pipe to the electronic application often results in the heat pipe unable to meet the power handling requirements. To make it suit the application, the engineer may have to make changes to the internal structure of the heat pipe design.
This often requires changing the porosity and thickness of the wick, allowing the engineer to tune the heat pipe to meet specific operating parameters and performance characteristics. For instance, the heat pipe may have to operate at higher power loads or against gravity, requiring a larger pore radius or an increase in the capillary pressure of the wick.
However, a larger pore radius works against gravity, and therefore, in place of increasing the pore radius, increasing the wick thickness may make it more effective. Alternately, the engineer may increase both the wick thickness and porosity along the length of the tube. Some suppliers specialize in custom heat pipes and use unique mandrels and/or custom formulated copper powder to fabricate them. The outward physical characteristics also affect the performance of a heat pipe.
