Minimizing Thermal Resistance with Direct Attach Heat Spreaders

Vapor chambers for electronics cooling have typically been manufactured using copper for the envelope and water as the working fluid. As discussed previously, copper has a relatively high CTE and therefore substrate and compliant interfaces are required to attach to low CTE electronics chips. This results in unfavorable additional thermal resistances, lowering the allowable power and heat flux of the electronics.

To address this historical shortcoming, ACT developed a low CTE vapor chamber using aluminum nitride (AlN) ceramic plates with thin layers of direct bond copper (DBC). The aluminum nitride ceramic with DBC has a CTE of approximately 6 ppm/°C, which is similar to many electronic chip packages. The copper DBC on the inside of the vapor chamber provides material compatibility with the water working fluid. The copper DBC on the exterior allows for direct solder attach and electrical circuitry directly etched into the surface of the vapor chamber. Figure 3 shows a multiple evaporator, low CTE, high heat flux vapor chamber, etched and prepped for direct die attach of four (4) chips.

In addition to eliminating the need for an intermediate substrate and the associated interface resistances, the vapor chamber was also designed to handle high heat fluxes. This is made possible through advanced wick designs that enable effective separation of liquid and vapor phases. In this design, a thick wick structure, which can readily absorb the condensate, is co-located with a very thin wick that has very low evaporation thermal resistances. Resulting vapor chamber performances with these wick structures have exceeded 500 W/cm² in heat flux. Heat flux limits of 30 to 50 W/cm² are typical in commercially available off-the-shelf copper/water vapor chambers. Evaporator thermal resistance less than 0.05°C-cm²/W has been demonstrated. Test results in Figure 4 show the performance relative to the heat flux for a 7.6 cm × 12.7 cm, 3mm thick low CTE vapor chamber.

The wick structure design is scalable with heat source sizes from less than 0.6 cm² to 10 cm². This type of performance is favorable for high heat flux chips such as IGBTs and MOSFETs as well as high power laser diode arrays and phased arrays.

Conclusion

Both AlSiC Hi-K plates and high heat flux AlN/DBC Vapor Chambers have demonstrated the ability to be directly attached to electronic devices. An AlSiC HiK plate can provide heat spreading for multiple devices via a custom heat pipe layout that can be designed for manufacturability and thermal performance. The rugged, lightweight, AlSiC HiK plate can also be used structurally in a system. The low CTE, high heat flux vapor chamber is primarily for high, concentrated heat loads and creates a nearly isothermal base. The nearly isothermal vapor chamber can be used for spreading heat in air-cooled applications and for heat transport in edge cooled liquid systems.

This article was written by Bryan Muzyka, Sales Engineer, and Pete Ritt, Vice President, Advanced Cooling Technologies, Inc. (Lancaster, PA). For more information, visit http://info.hotims.com/40430-501.