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60 The PCB Design Magazine • January 2016 surely be burned, because the amount of heat per unit times that the paper allows to flow now is 20x larger. This evidence proves that when a dielectric thickness is thin relative to the surface area that the heat will readily flow through the material and, in fact, the thinner the dielectric, the more effective the heat will transfer through the PCB thickness and spread across ground planes. An optimal PCB design will have the top layer at nearly a uniform temperature across its entire surface and the bottom layer will be as close as possible to the top temperature. This is the point at which the transfer of heat from the circuitry on the PCB to the atmosphere (or oth- er media in which the PCB would be immersed) is optimal (minimum temperature drop), with both sides of the PCB operating as heat-trans- ferring surfaces. Furthermore, when wide thick copper planes are used, the heat will travel horizon- tally and heat copper planes evenly since the copper plane has high thermal conductivity (raising the temperature of the entire area), and this will facilitate evenly distributed heat across each plane. Once heat is evenly spread, reductions in dielectric thickness will allow the surface temperatures to be closer to each other by improving the layer-to-layer trans - fer of heat. Therefore, PCBs with thinner di- electric thickness will outperform a PCB with thicker dielectrics regarding overall operating temperatures of the CSP ICs, all other variables held constant. ENHANCINg THERMAL PERFoRMANCE oF CSP INTEgRATED CIRCuITS article Figure 3: CSP device mounted on a four-layer PCB, third PCB layer shown (mid-layer 2).