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March 2015 • SMT Magazine 21 hOW TO STrEamLInE PcB ThErmaL DESIGn continues Feature within the PCB, and a preliminary heatsink size and design (if already known to be necessary). However, the model is useful for investigating the effect of placement on the temperature of a component and its neighbors because adjust- ments can be made easily and the model re-run in a matter of minutes not hours. The results provide some indication as to which components, if any, may need some form of heatsink, which can be investigated next. When more information is known about package selection, this helps prioritize where to invest effort in developing and redefining the components' thermal model. Size the heatsinks Early in Design For any components that may be too hot, investigate how effectively a heatsink brings down the component's temperature. If the flow is mainly normal to one side of the package, a plate (or extruded) fin heatsink is likely suit- able. If not, then a pin fin heatsink should be considered. The heatsink geometry can be defined para- metrically with CFD thermal simulation soft- ware. Start by making the base of the heatsink the same size as the package and investigate dif- ferent numbers of fins, fin height, and fin thick- ness. The aim is to see if the heatsink can simply be mounted on top of the package or if a larger heatsink might be needed, which will require board real estate for the mechanical attachment (Figure 1). If so, it is essential to select an ex- isting heatsink that provides adequate cooling performance or to design a custom heatsink be- fore the board can be routed because the me- chanical attachment for the heatsink may affect component placement. Including the thermal resistance of the ther- mal interface material between the package and figure 1: example of a heatsink that extends beyond the package body with retaining pins.