Heat pipes have been successfully used in various types of military, computer and medical electronic systems for decades. For example, heat pipe-based thermal solutions are used in “mission critical” military radar electronics, military power conversion, avionics, and satellite thermal control applications. Heat pipes are bendable, flexible, routable, and have proven reliable under demanding conditions such as high-g (i.e., gravity up to 10-g), shock/vibration, and freeze/thaw (1,600 temperature cycles from -40°C to +90°C) requirements typical of military and aerospace applications. To increase customization, working fluid and casing/envelope materials can also be bent and formed to fit custom applications.

Figure 3. Encapsulated APG used in a PWB.
LED electronic components may be directly attached to heat pipe thermal ground planes (TGPs). TGPs are thin, flat or planar heat pipes, typically 1mm to 3mm thick. The planer geometry facilitates attachment of flat electronic devices such as LEDs by epoxy or solder bonding. Direct attachment of the TGP to the LED has the added benefit of minimizing thermal interfaces. Figure 2 is a typical heat pipe TGP. A TGP, such as the unit shown, requires less than 1 gram of fluid and spreads heat laterally with a very high effective thermal conductivity.

Benefits of the Heat Pipe TGP

The heat pipe TGP uses all the same reliable components used in the construction of a cylindrical heat pipe. It also employs a capillary wick structure, typically sintered powder metal, which allows the heat pipe to provide the highest heat flux capability along with the greatest degree of freeze/thaw tolerance and insensitivity to gravitational orientation. The thin, flat structure makes an ideal substrate for mounting electronic devices in packages where vertical space is highly constrained.

The heat pipe TGP represents a proven two-phase cooling approach, where the benefits include very high effective thermal conductivity (500 W/m·K to 2,000 W/m·K or more, depending on the size of the TGP), extreme reliability, and no moving parts, electrodes, or need for external power, allowing the attached LED device to run cooler as a result of improved heat spreading. A reduction in junction temperature of 20- 30°C through the introduction of a TGP heat spreader versus conduction in low-CTE materials is not uncommon. As with a traditional cylindrical heat pipe, the TGP transfers and moves heat through the flow of evaporated working fluid so heat can be transported longer distances with less temperature difference than is possible in a solid conductor.

In addition, the TGP spreads heat laterally due to its geometry which makes the TGP a superior heat transport/heat spreading technology. Generally, the larger the TGP, i.e., the longer the heat transport distance and wider area of spreading, the greater the effective thermal conductivity. For a solid material, the thermal conductivity is a fixed, intrinsic property, so transporting more thermal energy or transporting the same energy over a longer distance requires greater temperature difference, which is undesirable for electronic devices and systems.

Depending on the application, designing a heat pipe TGP with a matching CTE can improve cooling while increasing reliability through reduced junction temperature and reduced thermal stress in the bonding material between the TGP and semiconductor. In addition, the TGP substrate can be tailored to more closely match various semiconductor materials, including silicon, silicon carbide, gallium arsenide, and gallium nitride. Heat pipe TGP structures can be effective up to 350 W/cm2. New electronic systems utilizing the TGP-based thermal solution can run at a higher power density than previous systems based on thermal conduction in solid materials without increased weight or complexity.