If you’ve worked with a notebook computer resting on your knees for any length of time, or held a smart phone to your ear for a long chat, you’ll know how hot these portable devices can get. Given that smaller portable devices such as tablets and phones are unable to accommodate water-cooling pumps or even fans, manufacturers have relied on conductive sheets of metal or other materials to carry the heat away from the really hot spots and keep high temperatures from damaging chips. But it’s increasingly clear that if portable devices are to continue to improve, we’ll need something better.
Engineers at Fujitsu Laboratories in Kawasaki, Japan, think they have that something. The lab recently announced development of a loop-shaped heat pipe that could do the job. The device is a super-thin loop of copper filled with water. At one end is the evaporator, which sits over the smartphone or tablet’s hottest chip—the CPU—using the heat to boil the water. At the other end is the condenser, where heat escapes and the water recondenses and flows back to the evaporator.
The real innovation is that the Fujitsu Lab managed to pack a liquid-cooling system into a frame slim enough for a smartphone.The actual pipe and evaporator sections are just 0.6 millimeters thick, the world’s thinnest, according to Fujitsu, while the thermal diffusion plate or condenser they connect to tops out at 1 mm. Overall, the device forms a somewhat rectangular frame measuring 107 mm by 58 mm.
The heat pipe from the condenser to the evaporator and the evaporator segment consist of two outer and four inner stacked copper sheets, each measuring 0.1 mm thick. This gives the pipe and evaporator its overall 0.6-mm thickness—way, way thinner than the 10-mm thick evaporator the lab previously developed.
Etched into the inner copper sheets are patterned holes slightly offset from the adjacent copper layers. These form a structure of miniature pores that create capillary action and cause the fluid to flow from the condenser to the evaporator. The heat from the CPU turns the water to vapor, which then flows through the hollow portion of the heat pipe back to the condenser, where the heat is dissipated and the vapor changed back into water and recirculated again.
Compared to previous thin loop heat pipes, Fujitsu says its new device enables roughly five times greater heat transfer.
What’s more, “the capillary action enables the liquid to easily circulate and readily transfer heat no matter how the device is positioned,” says Takeshi Shioga, a researcher at Fujitsu Laboratory’s Product & Systems Engineering Laboratories.
Another of the loop’s merits is that its layout can be customized. “The position of the evaporator and condenser can be changed according to the shape of the product,” says Shioga. “So there’s no necessity to compromise a product’s functionality, performance, or specifications during the design phase.”
Fujitsu will refine the design to bring down costs. It plans to implement it in mobile devices in 2017 and later in communication infrastructure and medical equipment, as well as in small notebook computers.