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22 SMT Magazine • March 2015 the heat sink is essential. A standard thermal pad with a thickness of around 0.2 mm and a thermal conductivity of around 1.0 W/mK is conservative for early design use. Guidance can also be given to be the de- sign team on the most relevant thermal met- rics to use to compare the thermal perfor- mance of candidate components. For compo- nents without a heatsink, the most relevant thermal metric to compare is the junction-to- board resistance [3] . For components that are expected to have a heatsink the junction-to- case resistance is the most relevant because the resistance is usually defined for the face that is in contact with the heatsink [4] . For TO- type packages, this face is normally soldered to the PCB. If both of these metrics are avail- able, a JEDEC standard 2-Resistor model (Fig- ure 2) can be created and the thermal model re-run to get a first estimate of junction tem- perature [5] . The next level up for predictive accuracy is a DELPHI model [6] . DELPHI models are bet- ter for heatsink selection than 2-Resistor mod- els because the top surface is subdivided into inner and outer regions that have different temperatures. They can be used to initially in- vestigate the effect of heatsink base thickness. However, for thermally critical packages that require a heatsink, a detailed model should be used. Incorporate PcB Design Details and Import Back to EDa Once the placement has been broadly de- fined, the most useful information that can be obtained after the schematic capture, but be- fore the board is routed, is the layer stack-up of the board. Obtain an estimate of the number of each type (signal, or power/ground) of layer in the PCB, then upgrade the model of the PCB to include each of these layers individually. Be- fore routing an estimate needs to be made for the thickness and percentage copper coverage of each non-dielectric layer. The final step is to import component place- ment data from the design system to ensure that placement within the thermal tool is cor- rect and should be re-imported whenever the layout is changed. hOW TO STrEamLInE PcB ThErmaL DESIGn continues Feature figure 2: The package thermal information needed for good design.

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