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30 PCB007 MAGAZINE I JUNE 2019 Creeden: If you've ever seen lightning strike in the air, you're probably seeing it from the cloud to the ground. Or if you've ever seen static electricity, when the lights are low, you can see the spark fly. That's a good visualiza- tion to understand every time you're routing a trace. Historically, circuitry traces were DC in nature, and their environment didn't matter as much. Now, you are managing an electromag- netic field. The field is capac- itive, and that's best (high capacitive) when a trace is close to its return path. It's also magnetic, which is in- ductive. That is how a sig- nal propagates (low induc- tance) down the line. You're also managing an EM energy field; you're not just con- necting two points with a trace. The energy field is not in the trace. Rather, the trace and it's return path—typi- cally a GND plane—serve as reference points; thus, the energy exists in the dielec- tric material between them. Therefore, the material with all of its parameters are an integral part of the performance of the circuit. The material's electrical properties are mea- sured by the dielectric constant (Dk) denoted as ϵr. This measures how well energy will per- meate through the material at different fre- quencies. Also, they are measured by the dissi- pation factor (Df) also known as loss tangent. This measures how much energy can be dis- sipated or lost into the material. The energy field travels within the dielectric material. The material can resist the flow of energy, and each material has a known measured rate. As a ref- erence, air has a dielectric constant of ϵr = 1. Your average FR-4s have a dielectric constant of approximately ϵr = 4. Faster circuitry re- quires less resistance and less energy loss, so you see high-speed material go down to the range of ϵr = 3. With circuitry achieving increasingly faster speeds, most people equate speed to the cir- cuit's frequency, but it must be understood that the burst of energy delivered in every pulse as measured in the rise time (Tr) as re- lated to voltage/frequency. That is when the signal transitions from zero to its voltage and that burst of energy defines its field. To man- age that, you must understand that the mate- rial selection is an integral part of the function of the circuitry. These are all factors that en- gineers and designers must take into consideration from day one. Material plays an impor- tant factor, but the designer must practice good design skills to ensure that signal integrity issues are not cre- ated by violating a signal's return path, impedance matching, or crosstalk to an- other signal. Another elec- tronic consideration is the consistency of the weave pattern because of the Dk difference between weave and resin, so as a solution, they have what's known as spread weave. They spread the weave out to get a consistent Dk, which is essential for the performance of differential pairs of traces. Mechanical and physical properties affect the structural integrity and manufacturing pro- cess. With the glass transition (Tg), the ma- terial's resin will transition from a hard to a rubbery state as a factor of temperature. That's imperative when you're considering high layer count boards. The dielectric material typically is comprised of a glass weave, which expands in the X-Y axis, and a resin that expands in the Z axis. The Z-axis expansion is measured as the coefficient of thermal expansion (CTE), which threatens the structural integrity of the via plating, and the vias are the most vulner- able entity on a PCB. The other physical prop- erty to be considered is the thermal decompo- sition temperature (Td) when you start doing HDI boards where you have multiple lamina- tion (thermal) cycles to accomplish the con- Mike Creeden

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