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20 SMT Magazine • March 2015 temperature will be higher than that predicted in the simulation. Refine the model before us- ing the board temperature to estimate compo- nent temperatures. If the board temperature at any point is close to the maximum component case temperature, this limit will be exceeded once the component heat sources are represented discretely. Guesstimate component Power At this stage, make a best- guess estimate of the individu- al power budgets for the main heat dissipating components that will be used in the design and the approximate size of those packages. This will en- able describing them as foot- print heat sources in the simu- lation, smearing the remain- der of heat uniformly over the board surface. Before Selecting the Package, Use 3D component models Include some form of 3D component model in the sim- ulation before the component selection is finalized. By feed- ing the thermal results back be- fore this milestone is reached, the thermal performance will more likely be considered in the package selection criteria. Some ICs are available in more than one pack- age style, and not all package styles perform equally well from a thermal point of view. The need for a heatsink later may be eliminated by appropriate package selection. Component temperature, either in the form of a case temperature or junction temperature depending on how the manufacturer has speci- fied the component, is the key measure used to indicate whether the design is thermally ac- ceptable. In the absence of any other information, the simplest 3D component model can be used is a conducting block. A computational fluid dy- hOW TO STrEamLInE PcB ThErmaL DESIGn continues Feature namics (CFD) thermal simulation software such as FloTHERM includes material properties that are tailored to give a case temperature prediction for different package styles. For plastic compo- nents, a thermal conductivity of 5 W/mK to 10 W/mK is recommended [2] and 15 W/mK for any ceramic components. 5 W/mK will clearly give a worst-case figure for case temperature. The effect of the compo- nent on local air flow and, correspondingly, any down- stream components is taken into account when represent- ing the package body in 3D. Large components can shield smaller, lower profile compo- nents from cooling air. The wake formed behind a com- ponent is a region in which the same air gets recirculated, so any components in that re- gion are likely to be hot. Try to align any rectangular com- ponents so that their long side is parallel to the primary flow direction. This reduces the overall pressure drop be- cause the flow "sees" less of an obstruction and produces a smaller wake, minimizing the effect on downstream components. Provide the Thermal results Back to Design At this stage, information about the PCB's performance can be fed back to the PCB design team. Although the simulation is rela- tively coarse at this stage, the principle simu- lation results—the airflow distribution over the board and the resulting board temperature map—are powerful tools that show what is available for cooling air and how it may affect component temperature. These nominal component case temperature values are subject to change because they are based on an assumed layout, rough power esti- mates, uncertainty about package selection, un- known layer stack-up and copper distribution Include some form of 3D component model in the simulation before the component selection is finalized. By feeding the thermal results back before this milestone is reached, the thermal performance will more likely be considered in the package selection criteria. some ICs are available in more than one package style, and not all package styles perform equally well from a thermal point of view. " "