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36 The PCB Design Magazine • September 2016 allow the definition of an oblong antipad on in- ternal layers. Where differential pairs are present, an ob- long antipad can be placed around both pairs (see Figure 3) to reduce the via-to-plane capaci- tance. A ground stitching via should be posi- tioned at either end of the oblong antipad to reduce return path inductance. With connector pin arrays, the oblong clearances also preserve the continuity of the current return path in the reference plane. 5. Terminate the Stub The via stub acts like an unterminated trans- mission line. If a terminating element is placed at the bottom end of the stub, then the reflec- tion of the stub may be minimized. The imped- ance terminating element may include one or more resistors, capacitors, and/or inductors be- tween the via stub and a ground layer. Simu- lation would determine the most appropriate solution. The impedance terminating element may be formed internally to the PCB, or mount- ed to the PCB surface. For instance, a resistor equal to the characteristic impedance of the via (50 ohms) could be placed from a single ended signal to ground or power or a 100 ohm termi- nating resistor across a differential pair. 6. Lower the Surrounding Dielectric Constant Lowering the dielectric constant, of the ma- terial surrounding a via, by positioning non- PTHs in proximity to the interconnect is a so- lution proposed by Bhyrav Mutnury and col- leagues, IBM Corporation. Air in the non-PTH has a dielectric constant of 1, whereas FR-4 is approximately 4. This serves to increase the res- onant frequency of the via stubs. By increasing the resonant frequency of the via stub, beyond the frequency of the signal, the attenuating ef- fects of the via stub are no longer problematic. But, it may also make Swiss cheese of the refer- ence plane, which is not a good approach for high-speed design. 7. Plate the via Barrel with Lossy Material Stuart Allen Berke and colleagues, Dell Prod- ucts, have postulated reducing the Quality (Q) factor of the via stub. The resonance of a via stub can be dampened by plating the via barrel with a material having a low conductivity. For example, a via can be plated with tin, which has a conductivity of approximately 8×10^-6 S/m, while copper has a conductivity of approxi- mately 6×10^-7 S/m. Thus, tin can be referred to as a "lossy" medium as compared to copper. Plating a via barrel with a lower conductivity material, such as tin, reduces the Q factor of the via. Resistance is inversely proportional to the conductivity of the material used to plate the via, thus a lower conductivity material results in a lower Q factor for the via dampening the resonance. In conclusion, dangling via stubs distort a high-frequency signal and also decrease the usable bandwidth of the signal. Since the via capacitance varies in proportion to the overall size of the hole and the plane clearance of the antipads, vias should be kept small with large clearances where possible. Oblong antipads also reduce capacitance and preserve the con - tinuity of the reference plane. Of the seven so- lutions put forward to alleviate this problem, using blind vias and back-drill stubs on back- planes combined with oblong antipads are the best, and most economic solutions for high- speed design. Points to Remember • Back-drilling typically requires specialized equipment and is expensive. HOW TO HANDLE THE DREADED DANGLERS, PART 2 Figure 3: Oblong antipads reduce via capacitance.