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OCTOBER 2022 I PCB007 MAGAZINE 87 other, the reactivity changes and influences the performance. If the circuit is more induc- tive, current in the circuit will lead the voltage by a phase angle specific to the amount differ- ent from resonance. Conversely, if the circuit is more capacitive the voltage will lag the cur- rent by a phase angle specific to the difference from resonance. Still with me? What has this all got to do with the PCB? We have a coil on the PCB. Now, if we had capacitance in parallel to the primary winding, we could make some magic. By add- ing capacitance to the inductor, we can create filters. Specifically, filtering can be done to sig- nals that stop certain frequencies from passing or allow only a specific range of frequencies to pass. ese are called "band pass" and "band stop" filters. So, when using a coil (inductor) with a capacitor/resistor, we are doing what is called "tuning the circuit." is allows the pre- determined signals to process optimally, while stopping or eliminating parasitic signals. What does this all mean? e inductance in a passive or active inductor within the PCB is critical to final performance. erefore, the standard electrical test of the coil's primary and secondary windings cannot be satisfied. A short within the primary or secondary wind- ings may not trigger a fault but will affect the performance of the tuned circuit. us, mea- suring the proper inductance of the coil is crit- ical. Although an inductor circuit may pass the standard opens and shorts test, it does not mean that the circuit is stable. at is why inductance should be tested whenever buried coils or inductors are man- ufactured within the PCB with specific end results expected. PCB007 Todd Kolmodin is VP of quality for Gardien Services USA and an expert in electrical test and reliability issues. To read past columns, click here. Chapter4: Copper Foil Copper foil is the standard conductive layer used for metal-clad laminates, although other options are available. There are two main types of copper foil used for PCB boards today: electrodepos- ited (ED) foil and rolled a n n e a l e d ( R A ) f o i l . E D copper foil is pro- duced by a continu- ous process which y i e l d s a w e l l - c o n - t r o l l e d p r o d u c t i n mass volume and low cost as compared to RA copper foil. ED copper foil has a wide range of thicknesses, from 5–400 µm, for PCB appli- cations. IC substrate application requires an ultra-thin foil which is supplied on an 18–72 µm copper carrier and range in thicknesses from 1.5–5 µm. Rolled annealed foil yields a very smooth surface where the process deforms the cop- per crystalline structure to achieve thickness. Unfortunately, the foil is only available in a 25" wide format. Most processes are designed around a 50" wide machine direction of the glass using large hydraulic presses with platen sizes to accommodate the 50" glass width. Use of the RA foil reduces productivity and results in higher cost. With the development of newer smooth ED foils that are as smooth as RA foil, the need for RA foil and the associated cost has been largely mitigated. Continue reading. BOOK EXCERPT The Printed Circuit Designer's Guide to... High Performance Materials

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