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NOVEMBER 2022 I DESIGN007 MAGAZINE 25 frequencies below a kilohertz, the series induc- tive reactance between two points is lower than the resistance between these points. erefore, at low frequencies current patterns are controlled by conductor resistance. e field required to move current in a solid plane at DC is very small. Why Does a Thin Trace Overheat and Behave Like a Fuse? e way I see it: electromagnetic energy moving through the dielectric induces cur- rent to flow in the conductors. e current is a result of the energy in the dielectric. It is an effect, not the cause. e trace and the plane bound the energy and thus it is the energy den- sity between the conductors that determines the current density in the trace geometry. e wider and thicker the conductor (Figure 2), the more energy can be contained in the field. us, a larger cross-sectional area of copper is required to deliver higher energy at DC. How- ever, the plane has sufficient capacity to sup- port the energy flow. If the trace is too narrow, then the contained energy will dissipate as heat until the trace disconnects. A DC power integrity simulator highlights areas of high current density, which will lead to higher temperatures. A low thermal resistance ensures that the heat is transferred through the material much faster. is resistance is directly proportional to the length of the thermal path and inversely proportional to the cross-sec- tional area and thermal conductivity of the thermal path. For instance, if the load is shorted, then the EM field will try to maintain the output voltage by providing more energy. As the energy level of the field increases, it looks for the weakest link, which is the point of lowest resistance on the trace segment. e excess energy will then transform into thermal energy, vaporizing the trace at the weakest point. e trace also cools by conduction through the substrate. But as the trace temperature increases, the dielectric material beneath degrades, passing through the stages of the glass transition to thermal decomposition and finally delamination. Deterioration of the underlying dielectric material will reduce the effective cooling capacity of the material and thus increase the temperature of the trace. If there is current flow in a conductor, an electric field must exist. At DC, this field will be the same at any depth. However, a changing magnetic field associated with AC flow forces the current to bind to the perimeter of the con- ductors. is is known as the skin effect and applies to digital circuits, where currents flow very near the surface of the conductors. For a sine wave current at 1 MHz, the skin depth in copper is only 66 µm. is means that the current at this depth is the surface current reduced by 8.68 dB. At 1 GHz, the skin depth is 0.066 µm and decreases with higher frequency. As frequencies and rise times continue to increase, the PCB designer needs to consider the path of electromagnetic fields through the substrate. To fully understand the opera- tion of digital circuits, we must focus on the Figure 2: Small traces (left) confine less energy than larger traces (right).