There are a growing number of high power applications across industries that require highly efficient cooling solutions. Some of these products have very high localized heat fluxes, greater than 300 W/cm2, but must maintain a tight temperature range so as not to disrupt sensors or optical devices. Direct die attachment can lead to mechanical stresses at the interface if the coefficient of thermal expansion (C.T.E.) is mismatched between the die and the substrate. The standard method to overcome this is to add an interface material, such as thermal gap pads or thermal pastes, to accommodate the mismatch. Unfortunately the presence of this thermal interface layer increases the thermal resistance and likewise increases the temperature on the device itself.
Vapor Chambers are an important tool in the thermal management toolbox, since they act as flux transformers, spreading the high input heat flux over the entire surface of the vapor chamber. This allows the heat to be removed from the vapor chamber by conventional cooling methods. Most vapor chambers are limited to an input heat flux of about 75 W/cm2, however, ACT has developed a C.T.E matched vapor chamber that allows for direct bonding of the heat source; see Figure 1. This unique device has been demonstrated to dissipate heat fluxes as high as 700 W/cm2 and 2kW overall. The evaporator thermal resistance of vapor chambers with this wick design is only 0.05 K-cm2/W.
The overall envelope structure is aluminum nitride with a direct bond copper exterior. The copper on the inside of the vapor chamber ensures that the well-known water/copper performance is maintained. In areas where the heat source is to be attached, the copper layer is removed exposing Aluminum nitride. Aluminum nitride has a CTE of ~5.5 ppm/⁰C, which is close to many common semiconductor materials. The devices can be directly attached to the vapor chamber, eliminating the need for a thermal interface layer.
At the printed circuit board (PCB) level, the heat flux is the highest in the system. While it is advantageous to dissipate heat as close to the source as possible, this can be difficult to do while simultaneously satisfying the electrical isolation requirements. Unfortunately, in many cases electrical isolation is only achieved using materials that are thermally insulating, such as with FR4 boards. Recent work at ACT has explored adding heat pipes to the structure of Metal Core Printed Circuit boards to help spread heat right at the source. If the ratio of the heat source to circuit board area is sufficient, this can be an effective way to improve heat spreading at the board level while requiring minimal design impacts to the system.
Figure 1 shows an example of heat pipes embedded into a Metal Core Printed Circuit Board (MCPCB). Heat pipes are seen on the left, while the reverse ‘circuit side’ is seen on the right.
In Figure 2, a 3 LED structure is shown on the right, while a thermal image of that structure is seen on its left. Measured results show that embedded heat pipes can reduce the heat spreading resistance by 45% over the standard aluminum MCPCB and even 15% over a copper MCPCB. This is a valuable improvement, particularly so close to the heat source.
High Conductivity (HiK™) heat sinks can also improve the Size, Weight, and Power (SWAP) compared to standard heat sinks. Placing a discreet heat source on a large metal heat sink will produce large thermal gradients as the heat slowly conducts through the aluminum to the fins. Embedding heat pipes in a HiK™ heat sink can increase the thermal conductivity from around 180 W/m K to 500-1,200 W/m K, providing an opportunity to reduce heat sink plate thickness and fin area. This approach has been proven in a variety of weight/volume sensitive applications including: Ruggedized Electronics, UAVs, Handheld/Portable Devices, LEDs and optical devices.
Embedded heat pipes can improve the performance and reduce that 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 1. 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 2. 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.
Heat pipes have an effective thermal conductivity of 10,000 to 100,000 W/m K, compared with aluminum’s thermal conductivity of about 180 W/m K. Therefore, the performance of large aluminum heat sinks can be improved with embedded heat pipes. The high effective conductivity allows the heat pipes to spread the heat throughout the heat sink. This heat spreading reduces the thermal gradient and likewise reduces the max temperature at the LED source. Benefits include:
- Improved lifetime and reliability by operating with the same power at a lower temperature
- Increased optical output by operating at the same temperature with higher powers
- Reduced heat sink size and weight
Our testing and analysis has confirmed that the longer the extrusion, the more heat pipes improve performance. As seen in Figure 1, the percentage improvement in thermal resistance with heat pipes increases approximately linearly with increasing heat sink length. For example one can expect to see a 5% improvement in thermal resistance for a 5 cm long heat sink, increasing to 30% for a 30 cm long heat sink. Please note that one can expect the benefit will be more noticeable in natural convection heat sinks, as fan operation plays a major role in forced convection performance.
Figure 2 shows a photograph and an infrared (IR) image of a heat sink with embedded heat pipes. In this case heat pipes were embedded in a 200mm long radial heat sink that that was dissipating 100 watts of heat. The heat pipe improves the efficiency of the heat sink, by transferring heat with a very low ΔT from the LED at the bottom to the entire length of the heat sink.
Figure 3 shows calculated temperatures for identical heat sinks, with and without embedded heat pipes. The heat pipes decreased the LED maximum temperature by 10⁰C, which will help achieve long, reliable operation.
In many lighting applications the LED device must fit in a fixed space to accommodate a variety of customer requirements, which often do not account for thermal management considerations. A common example is luminaire design, where the ceiling or wall fixtures are based on pre-existing designs using non-LED technologies. These designs commonly have both restricted space for heat dissipation through conduction, and limited air flow to remove heat via convection. In cases where there is space to remotely dissipate the heat, heat pipes can be used to transport the heat from the device to a heat sink located elsewhere. This is called the remote sink.
The remote sink solution has a heat pipe in direct contact with the LED device at one end, which serves as the evaporator. At the other end the heat pipe is connected to the heat sink, the condenser. A sketch of a conceptual design can be seen in Figure 1. Here two heat pipes are in direct contact with both the LED at the bottom and heat dissipating fins at the top. A wall or other enclosure can be placed in between the LED and heat sink to separate the two.
Aluminum has a thermal conductivity of about 180 W/m K, while the thermal conductivity of copper is only 400 W/m K. In contrast, the effective conductivity of a heat pipe can range from 10,000 to 100,000 W/m K. This high effective thermal conductivity allows the heat sinks to be located remotely from the LED.
Figure 2 shows a photograph and an infrared (IR) image of a heat pipe transporting heat to a remote sink. The heat pipe heat sink is operating at natural convection conditions with 30 Watts of applied heat. The heat pipe clearly demonstrates the transport of heat isothermally from the heat source to the heat sink and the even distribution of heat to the heat sink. A slight increase in temperature is measured across the heat sink (<0.5 °C), due to the sensible heating of air rising through the heat sink.
Heat pipes can efficiently transfer heat approximately 8 inches with minimal thermal gradient, and over even greater distances when the heat pipe is gravity aided. Note that the number, size shape and location of heat pipes would be specific to the design.