The Circuit Designer's Companion - diagramas.diagram...

324 The Circuit Designer’s Companionfasteners with controlled torqueclampingbarheatsinkFigure 9.16 Package clamping barpressure for each package. With suitably ingenious mechanical design, it may alsocontribute to the total thermal performance of the whole assembly.9.5.4 Placement and layoutIf you are only concerned with designing circuits that run at slow speeds with CMOSlogic and draw no more than a few milliamps, then thermal layout considerations willnot interest you. As soon as dissipation raises the temperature of your components morethan a few tens of degrees above ambient, it pays to look at your equipment and pcblayout in terms of heat transfer. As was shown in section 9.4.3, this will ultimatelyreflect in the reliability of the equipment.Some practices that will improve thermal performance are• mount pcbs vertically rather than horizontally. This is standard in card cagesand similar equipment practice, and it allows a much freer convectiveairflow over the components. If you are going to do this, do not then blockoff the airflow by putting solid metal screens above or below the boards; usepunched, louvred or mesh screens.• put hot components near the edge of the board, to encourage a good airflowaround them and their heatsinks. If the board will be vertically mounted, putthem at the top of the board.• keep hot components as far away as possible from sensitive devices such asprecision op-amps or high failure rate parts such as electrolytic capacitors.Put them above such components if the board is vertical.• heatsinks perform best in low ambient temperatures. If you are using aheatsink within an enclosure without forced air cooling, remember to allowfor the steady-state temperature rise inside the enclosure. However, don’tposition a heatsink near to the air inlet, as it will heat the air that iscirculating through the rest of the enclosure; put it near the outlet. Don’tobstruct the airflow over a heatsink.• if you have a high heat density, for example a board full of high speed logicdevices, consider using a thermally conductive ladder fixed on the boardand in contact with the IC packages, brought out to the edge of the board andbonded to an external heatsink. PCB laminates themselves have a lowthermal conductivity.• if you have to use a case with no ventilation, for environmental or safetyreasons, remember that cooling of the internal components will be by three