Heatsinks always bring up disagreement; but I believe that most of the disagreements are based on availability of materials being traded off against optimum performance.
Let's talk basics. For heat sinking, you need to consider conduction, radiation and convection. The heat must conduct through the heatsink. Metals have excellent thermal conduction properties; so metal is the bulk material of choice. Any coatings or paint on the metal would usually be an insulator (usually either an organic material or an oxide), so they tend to hurt performance. As for radiation, materials with black coloring abosorb and radiate faster. So black is optimum. Increasing surface area with ribs, or roughening also increases radiation. As for convection, anything that increases surface area or turbulance at the surface helps transfer heat to the air. So, rough, ribbed, wavy surfaces are ideal. Forcing an air flow over the surface with a fan also maximizes convection. Of the three heat transfer methods conduction works the fastest. So do all that you can to maximize conduction first. So, use a metal heatsink, thermal grease (or a gap pad) and smooth surfaces to mate the item that you are cooling to your heatsink. Microscopic air gaps prevent conduction and kill performance. Once you have maximized conduction, then you need to design your heatsink for maximum radiation and convection. For these purposes, you would ideally like a solid, black, metal heatsink with a rough, ribbed wavy outer surface with no coatings whatsoever. This type of heatsink is rare, if not impossible to make. For instance, there aren't many black metals; and there aren't many machining, extruding or molding processes which produce a rough surface. So, here is where the tradeoffs (and confusion) begins. Note that I'm using Aluminum as the example; but other metals may work. A cheap heatsink would be an Aluminum block. A little more money gets you some thermal grease to help transfer heat from the item to be cooled to the Aluminum block. For a little more money, you can have ribs put in your Aluminum block. A little more money gets an Aluminum form with ribs and sandblasting to roughen it. A little more money gets an Aluminum form with ribs, sandblasted roughing and black anodize. Note that the black anodize is an oxide coating, which hurts conduction at the surface a little bit; but the black color helps radiation. Black anodize also helps with cosmetics (covers up scratches); and it helps to prevent corrosion. A little more money gets you a fan to blow over your heatsink. This also costs a power penalty because the fan needs power. So, be an engineer, do some calculations, experiments or educated guesswork to figure out how much money you want to spend to get the job done. By the way, light clothes are worn in the summer to reflect the sun's heat. The light clothes are very poor at radiating our own heat away. That doesn't matter anyway. We radiate heat through our skin, not our clothes. Dark skin radiates better, hence the dark coloring of people with African ancestry. I hope that I've helped you. Chris Maxwell | Design Engineer - Optical Division email chris.maxw...@nettest.com | dir +1 315 266 5128 | fax +1 315 797 8024 NetTest | 6 Rhoads Drive, Utica, NY 13502 | USA web www.nettest.com | tel +1 315 797 4449 | This message is from the IEEE EMC Society Product Safety Technical Committee emc-pstc discussion list. Visit our web site at: http://www.ewh.ieee.org/soc/emcs/pstc/ To cancel your subscription, send mail to: majord...@ieee.org with the single line: unsubscribe emc-pstc For help, send mail to the list administrators: Ron Pickard: emc-p...@hypercom.com Dave Heald: davehe...@attbi.com For policy questions, send mail to: Richard Nute: ri...@ieee.org Jim Bacher: j.bac...@ieee.org Archive is being moved, we will announce when it is back on-line. All emc-pstc postings are archived and searchable on the web at: http://www.ieeecommunities.org/emc-pstc
