Standard heat pipes only transfer heat along the axis of the heat pipe, so they are best suited to cooling discrete heat sources. High Conductivity Plates (HiK™ plates) or Vapor Chambers are used to collect heat from larger area sources, and either spread the heat, or conduct it to a cold rail for cooling. Vapor Chambers are generally used for high heat flux applications, or when genuine two-dimensional spreading is required. The lower cost HiK™ plates are used when only high conductivity in a tailored direction is required.
Aluminum and aluminum alloys have thermal conductivities around 180-200 W/m K. Copper, with a thermal conductivity of around 400 W/m K can be used when higher thermal conductivities are required, however, it is more expensive than aluminum, and weighs more than three times more than copper. Materials with higher thermal conductivity than copper are significantly more expensive.
When high conductivity structures are required in thermal management, heat pipes can be embedded in aluminum to create a HiK™ plate, achieving effective thermal conductivities that can be as high as 1200 W/m K (2400 W/m K for large HiK™ plates), which is higher than any material other than high quality diamond heat sinks.
The first step in fabricating a HiK™ plate is to determine the location of the high power components on the aluminum board, as well as the location of the cooling areas (typically water-cooled cold rails at the sides of the circuit board). Slots are then milled in the board from the high power components to the heat sink, and flattened copper/water heat pipes are inserted into the slots; see Figure 1. The heat pipes are soldered in place, and then the surface is machined to leave a smooth surface, as shown in Figure 2. In this figure, two of the high power locations are one-quarter and three-quarters of the way up near the right side. Note the three sets of heat pipes that spread the heat over the right hand side of the cooling rails on the top and the bottom.
A thermal analysis was conducted on the HiK™ plate in Figure 2 to help determine the heat pipe locations in the HiK™ plate. As shown in the top half of Figure 3, there were three hot spots in the aluminum plate design, one on the left, and two smaller areas on the right. The bottom half of Figure 3 shows the benefits of the embedded heat pipes. The addition of the heat pipes reduced the peak temperature by 22.1°C, as verified by experimental testing.
Analyses such as that shown in Figure 3 are used to calculate the effective thermal conductivity of the HiK™ plate. The thermal conductivity of the plate is increased in the CFD model until the temperature profile measures the experimentally measured temperature profile. The effective conductivity is dependent on distance (it is higher over longer distances, since the internal heat pipe ΔT is very low). Typically, the effective thermal conductivity of a HiK™ plate ranges from 500 to 1200 W/m K, depending on the specific application.
While most HiK™ plates are flat, ACT also has the ability to embed heat pipes so that the condenser is oriented at an angle from the evaporator; see Figure 4. In this case, the heat pipes are bent into an L-shape, so that heat can be removed from the flange in the front of the picture.
- When to Use Heat Pipes, HiK™ Plates, Vapor Chambers, and Conduction Cooling
- HiK™ Plates
- Webinar on Advanced Passive Thermal Management Solutions for LEDs