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34 The PCB Design Magazine • December 2017 supplier. Sometimes they are provided under a non-disclosure agreement (NDA). Add the Board Effects Now that you have component footprints and heat-source estimates and modeled the components as 3D conducting blocks, the sen- sitivity of the results to the board thermal con- ductivity is also something that can be investi- gated. If you can, do this in parallel with refine- ments you make to the component models. No single value exists for PCB thermal con- ductivity. PCBs are made of copper and dielec- tric material. The copper is about 1,000 times more thermally conductive so the dielectric material thermally insulates the layers from one another and insulates individual traces. Before the board has been tracked, you can use a sim- ple isotropic conductivity value varied between 5 to 15 thermal conductivity to see how much of an effect the PCB's thermal data has on the simulation results. As your team gets into more detailed design, this thermal representation of the board will need to be refined. Once the placement has been broadly de- fined, the next step for the PCB design team is schematic capture and electrical simulation (timing). The most useful thermally relevant in- formation that can be obtained after the sche- matic capture, but before the board is routed, is the layer stackup of the board. Estimate how many signal, and power or ground layers, the board may have. Local electrical traces on the surface of the PCB spread heat away from the package interconnect (leads or solder balls), whereas buried power and ground planes in- crease the in-plane thermal conductivity at the macroscopic scale. From a thermal perspective, the contribu- tion of these copper-containing layers on the performance of the PCB is influenced by their thickness. The most common thicknesses are 0.5 or 1.0 ounces of copper, spread evenly over a one square foot area [7] (1 oz. is equal to 1.37 mils (thousands of an inch) or 0.0347 mm). Once you have an estimate of the number of each type of layer in the PCB, you can upgrade the model of the PCB to include each of these layers individually. Before routing, an estimate needs to be made for the thickness and percent- age copper coverage of each non-dielectric lay- er. One-ounce copper should be used for power and ground planes and 0.5 oz. copper for trace layers, with an assumed copper coverage of 80 and 20 percent, respectively. The dielectric contributes little to the ar- ea-averaged conductivity, both in-plane and through-plane. The minimum thickness of the Figure 2: Hierarchy of package thermal information for design. STREAMLINING THERMAL DESIGN OF PCBS