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PCBD-Aug2016

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56 The PCB Design Magazine • August 2016 Minimize the via inductance by • Eliminating or reducing stubs • Reducing the via barrel length Minimize the via capacitance by • Reducing the pad size • Removing non-functional pads • Increasing the antipad size There have been a number of solutions put forward to alleviate this issue including: • Back-drilling the stub • Using blind vias • Removing non-functional pads • Increasing the antipad diameter • Terminating the stub • Lowering the surrounding dielectric constant • Plating the via barrel with a lossy material Next month, in Part 2 of this series, I will look into the possible solutions to alleviate this issue, including tuning methods to make the vias more transparent to faster edge rates are evaluated. Via trade-offs are also examined be- cause of manufacturability concerns of higher oven temperatures associated with the recent switch to restriction of hazardous substances (RoHS) processes. Finally, general guidelines and recommendations will be made to show how to design an optimized the vias for better high edge-rate signal transmission will be discussed. Points to Remember • Dangling via stubs distort signals passing through an interconnect and also decrease the usable bandwidth of the signal. • A via stub acts as a transmission line anten- na, and has a resonant frequency determined by the quarter wavelength of the structure. • It is fine to have a plated through-hole (PTH) via providing the signal goes in at one end and out at the other using the entire length of the barrel. • The Nyquist frequency of a discrete signal is defined as one-half of the sampling rate of the signal. The impedance of the via stub drops at this frequency. • The resonant frequency of the via stub is inversely proportional to the dielectric con- stant of the material surrounding the via with a wavelength of four times the length of the un- used portion of the via. PCBDESIGN References 1. "PCB Vias, An Overview," by Bert Simon- vich. 2. Dell products, by Stuart Allen Berke. 3. IBM Corporation, by Bhyrav Mutnury. 4. Altera, Via Optimization Techniques for High-speed Channel Designs. 5. High-Speed Signal Propagation — Advanced Black Magic, by Howard Johnson, Martin Graham Barry Olney is managing director of In-Circuit Design Pty Ltd (ICD) Australia. The company is a PCB design service bureau that special- izes in board-level simulation. ICD has developed the ICD Stackup Planner and ICD PDN Planner software, which is available here. HOW TO HANDLE THE DREADED DANGLERS, PART 1 Lasers operating at the wavelength of 1,550 nanometers power high-speed fiber-optic Inter- net communications. MIT Microphotonics Center Principal Research Scientist Anuradha Agarwal is developing chemical sensors based on this wave- length using a new materials system built of silicon carbide on silicon dioxide on silicon. Ashley Del Valle Morales, a junior at the Univer- sity of Puerto Rico at Mayaguez, will test the silicon carbide-based sensor before and after it is exposed to gamma rays. Del Valle says, "I know it's really important to select a project you like and you're interested in. I also liked the enthusiasm and the interest that the grad students and the principal research scientist showed." Chemical Sensing at Telecom Wavelengths

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