Taking the Heat

Heat is the limiting factor in getting high-end desktop performance out of a portable computer. The more heat a notebook design can dissipate, the faster you can reliably run the CPU or the more memory you can hold. Fan-induced convection cools most desktop systems, but fans are unacceptable in notebooks -- they're noisy, power-consuming, and a potential failure point. Convection also needs space for air to move, something compact notebook designs lack.

Good notebook designs rely instead on passive conduction. Heat is conducted away from the CPU, hard drive, and the graphics subsystem to exterior surfaces capable of radiating heat. The keyboard, the I/O port area, and (if it's not a docking unit) the bottom surface of the notebook (that isn't blocked by the battery) are all good surfaces for dissipating h eat.

An aluminum heat sink will quickly draw heat away from the CPU, but the heat must then be conducted to the outside. Typically, metal sheets or frame elements, often made of a lightweight, conductive magnesium alloy, serve this purpose. Compaq's Armada 4100 series, for example, has a magnesium chassis that provides structural stiffness to the notebook, a path for outward heat conduction, and external radiation surfaces. Tadpole's notebooks can handle the latest desktop processors because they have a magnesium-alloy case that is thermally connected to the CPU.

Some designs use heat pipes to beef up thermal conduction by metal plates. These small-diameter sealed metal tubes contain a liquid that wicks from one end of the tube to the other, driven by a thermally induced fluid-to-vapor cycle. The pipe serves as a passive but effective heat pump.