older CPUs, parts that do not dissipate much power (such as the chipset), computers with low-power processors, and equipment where silent operation is critical and fan noise unacceptable. Usually a heatsink is clamped to the integrated heat spreader (IHS), a flat metal plate the size of the CPU package which is part of the CPU assembly and spreads the heat locally. A thin layer of thermal compound is placed between them to compensate for surface imperfections. The spreader's primary purpose is to redistribute heat. The heatsink fins improve its efficiency. Several brands of DDR2, DDR3, DDR4 and DDR5 DRAM memory modules are fitted with a finned heatsink clipped onto the top edge of the module. The same technique is used for video cards that use a finned passive heatsink on the GPU. Dust tends to build up in the crevices of finned heatsinks, particularly with the high airflow produced by fans. This keeps the air away from the hot component, reducing cooling effectiveness; however, removing the dust restores effectiveness. Peltier (thermoelectric) cooling Peltier junctions are generally only around 10-15% as efficient as the ideal refrigerator (Carnot cycle), compared with 40–60% achieved by conventional compression cycle systems (reverse Rankine systems using compression/expansion). Due to this lower efficiency, thermoelectric cooling is generally only used in environments where the solid state nature (no moving parts, low maintenance, compact size, and orientation insensitivity) outweighs pure efficiency. Modern TECs use several stacked units each composed of dozens or hundreds of thermocouples laid out next to each other, which allows for a substantial amount of heat transfer. A combination of bismuth and tellurium is most commonly used for the thermocouples. As active heat pumps which consume power, TECs can produce temperatures below ambient, impossible with passive heatsinks, radiator-cooled liquid cooling, and heatpipe HSFs. However, while pumping heat, a Peltier module will typically consume more electric power than the heat amount being pumped. It is also possible to use a Peltier element together with a high pressure refrigerant (two phase cooling) to cool the CPU. Liquid cooling Liquid cooling is a highly effective method of removing excess heat, with the most common heat transfer fluid in desktop PCs being (distilled) water. The advantages of water cooling over air cooling include water's higher specific heat capacity and thermal conductivity. The principle used in a typical (active) liquid cooling system for computers is identical to that used in an