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32 The PCB Design Magazine • December 2017 can be investigated next. You can also use this information to prioritize which component thermal models need to be refined once more information is known from package selection. Decide on Heatsinks Early For any components that may be too hot, you can now investigate how effectively a heatsink lowers a component's temperature. If the airflow is mainly normal to one side of the package, a plate (or extruded) fin heatsink will likely work best. If not, then consider a pin-fin heatsink. The thermal-simulation software includes pre-made parts that you can use to parametri- cally define heatsink geometry. Start by making the base of the heatsink the same size as the package and investigate different numbers of fins, fin height, and fin thickness. The aim is to see if the heatsink can be mounted on top of the package or if a larger heatsink might be needed, which will require board space for the mechani- cal attachment (Figure 1). The PCB design team needs this information as early as possible. Se- lect an existing heatsink that provides adequate cooling performance or design a custom heat- sink before the board can be routed because the mechanical attachment for the heatsink may affect component placement. When including a heatsink, include the thermal resistance of the thermal interface ma- terial (TIM) between the package and the heat- sink in your analyses. At this stage of model- ing, a standard thermal pad with a thickness of around 0.2 mm and a thermal conductivity of 1.0 is a good conservative choice. Refine the Thermal Models for Greater Accuracy At this point, you can consider the most relevant thermal metrics to compare the ther- mal performance of candidate components. For components without a heatsink, the most rel- evant thermal metric is the junction-to-board resistance [3] . For components that are expected to have a heatsink, the junction-to-case resis- tance is the most relevant because the resistance usually is defined for the face that is in contact with the heatsink [4] . For TO-type packages, this face is soldered to the PCB. If both metrics are available, you can create a JEDEC Standard two- resistor model (Figure 2), and rerun the thermal model 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 better for heatsink selection than two-resistor models be- cause the top surface is subdivided into inner and outer regions that have different tem- peratures. Thus, you can use them to ini- tially investigate the effect of heatsink-base thickness. However, for thermally critical packages that require a heatsink, use a detailed model. You can do an In- ternet search for the components' data- sheets to obtain more specifications. Or, if a thermal-simulation model is not already available, you can re- quest one from your Figure 1: Heatsink with retaining pins that extends beyond the package body. STREAMLINING THERMAL DESIGN OF PCBS