HiK™ Plates

High conductivity (HiK™) Plates are plates with embedded heat pipes to increase effective thermal conductivity.  HiK™ plates and Vapor Chambers are used to collect heat from electronics, 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 primarily when high conductivity is required in one direction.

A typical HiK™ plate has copper/water heat pipes embedded in a standard aluminum plate, using epoxy or solder; see Figure 1.  The heat pipes are strategically placed to get good thermal results while not affecting current geometry or mounting features.  The heat pipes and solder weigh slightly more than an aluminum baseplate (1.1 to 1.2 times more), but have effective thermal conductivities of 600 to 1200 W/m K, which is 3 to 6 times higher than aluminum.  HiK™ plates can also be used as structural components within systems.  Magnesium HiK™ plates can be used to reduce the density, while AlSiC HiK™ plates allow for direct die attach; see Figure 2.

Figure 1. Typical HiK™ Plates with embedded heat pipes to improve the effective thermal conductivity.

Figure 2. AlSiC HiK™ Plate for Direct Die Attach

Spot-cooling heat pipes transfer heat in a single dimension, while vapor chambers transfer heat in two dimensions.  HiK™ plates transfer heat in 1.5 dimensions:  They have a very high effective thermal conductivity in the direction of the heat pipes but also spread heat perpendicular to the heat pipes by conduction in the plate material.

HIK™ PLATE BENEFITS AND LIMITATIONS

Table 1 gives the primary benefits and limitations of HiK™ plates.  The benefits include passive operation, high effective thermal conductivity, low cost, and the ability to place mounting holes as desired.  HiK™ plate limitations are the same as for other passive two-phase heat transfer devices.

Figure 3. HiK™ Card Guide reduces the ΔT between cards and the chassis.

HiK™ plates are often used for conduction-cooled cards, where cold rails at the top and bottom of the card are cooled by a flowing liquid.  The maximum height is around 18 to 20 inches (46 to 51 cm), assuming edge cooling on the top and bottom of the HiK™ plate.  Another application is to spread the heat over forced-air heat sinks.  The embedded heat pipes spread the heat over the entire heat sink, increasing the efficiency and lowering the overall ΔT.  In addition to plates, HiK™ card guides provide improved thermal performance over traditional metal chassis assemblies; see Figure 3. HiK™ dual condensers can reduce the ΔT through wedge-lock joints.

TABLE 1.  BENEFITS AND LIMITATIONS OF HIK™ PLATES.

Figure 4. HiK™ plates showed an experimental reduction of 22°C in peak temperature compared to a baseline aluminum plate. (a) Aluminum plate thermal analysis. (b) HiK™ plate thermal analysis. (c) HiK™ plate. Note that the heat pipes are tailored based on the electronics locations to give the maximum effective thermal conductivity.

Figure 4 shows a thermal performance comparison of an edge-cooled aluminum baseplate versus a HiK™ plate.  Figure 4(b) shows the modeling results of the baseline solid aluminum plate without any heat pipes. You can clearly see the 3 hot spots, two at the top and 1 at the bottom.  Figure 4(b) shows the analysis results for a HiK™ plate.  The maximum temperature has dropped by roughly 22°C (confirmed by experimental measurements), while the temperature uniformity has greatly improved.  The actual HiK™ plate is pictured in Figure 4(c).  The heat pipes can be seen as silver lines and were routed to maximize the effective thermal conductivity from the high power electronics components to the water-cooled edge rails.

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 5.  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.

Figure 5. 3-Dimensional HiK™ plate, with the condenser, oriented 90° from the evaporator

Figure 6. A HiK™ natural convection heat sink reduces the mass by over 34% when compared with an all-aluminum heat sink with the same thermal performance.

Embedded heat pipes can improve performance and reduce the mass of forced and natural convection heat sinks.  ACT fabricated a HiK™ heat sink and an all-aluminum heat sink with the same performance; see Figure 5.  The total heat dissipation is 150W in both cases.   The conventional aluminum heat sink is 12 inches (30.5 cm) long, weighs 9.6 lbs. (4.4 kg) and has a base thickness of 0.6 inch (1.5 cm).  Introduction of 5 heat pipes, 3 in close proximity to the heat source and another two a little further out for improved spreading reduced the length to 10 inches (25.4 cm), reduced the thickness to 0.28 in (0.7 cm), and reduced the mass to 6.3 lbs. (2.9 kg) for an overall material reduction of over 34%.

Thermal images that demonstrate the improvement are shown in Figure 6.  The Hi-K heat sink seen on the right maintains the same source temperature, even though the heat sink is shorter, lighter, and thinner. The improvement is directly attributable to the addition of heat pipes which can be seen as red lines in the picture on the right.

Figure 7. Thermal images of the two natural-convection heat sinks show that the HiK™ has similar performance to the standard heat sink, with a reduction in mass of over 34

HIK™ PLATE SELECTION PARAMETERS

HiK™ plates are best suited for:

• Enhanced thermal conduction to cold rails
• More efficient/smaller forced and natural convection heat sinks
• Card guides and metal chassis assemblies

Selection criteria are given in Table 2.  The important things to remember about HiK™ plates are that they have similar benefits and limitations to heat pipes:  high thermal conductivity, maximum heat fluxes of 60-70 W/cm², passive operation, and can be used for direct die attach with AlSiC baseplates.   HiK™ plates are often used for edge-cooled cards, moving heat from the center of the conduction plate to the cold rails.  They also can be used with forced and natural convection heat sinks, spreading the heat so that the heat sink is more efficient.  In addition to plates, HiK™ card guides provide improved thermal performance over traditional metal chassis assemblies; see Figure 3.  HiK™ dual condensers can reduce the ΔT through wedge-lock joints.

TABLE 2.  HIK™ PLATE SELECTION CRITERIA

More information on When to Use Heat Pipes, HiK™ Plates, Vapor Chambers, and Conduction Cooling: