Vias: Those Holes are Part of the Circuit

On March 8, 2016, in Uncategorized, by sharilee

This post, authored by John Coonrod, Technical Marketing Manager, and team, originally appeared on the ROG Blog hosted by Microwave Journal.

For many circuit designers, plated through holes (PTHs) form pathways, from one circuit plane to another. PTHs, also known as via holes, can provide a path from a conductive layer to a ground plane, from one signal plane to another, and from high-current or power planes to signal planes. But they are not simply PTHs through a printed circuit board (PCB). To some designers, they are necessary evils, required to make those transitions from plane to plane. Some designers view them as design elements; not only do they provide signal pathways through a PCB, but they contribute electrically to the PCB, having an impact on the final performance of the PCB.

The key to making PTHs work for the benefit of a circuit design is to understand their effects on electrical performance, especially at higher frequencies. They should be considered as circuit elements, and they can have a great deal to do with a number of analog circuit transmission-line performance parameters, including insertion loss and return loss, and they can also affect high-speed digital circuit performance by degrading signal integrity (SI) and bit-error-rate (BER) performance.

Forming PTHs in PCBs calls for precision mechanical processes, such as drilling and plating, but also requires consideration of the electrical effects of those PTHs on circuit performance. Just as the thickness and dielectric constant of a PCB material can influence performance at microwave frequencies, the number and sizes of a circuit’s PTHs can affect high-frequency performance. To better understand the impact of PTHs on RF/microwave circuit performance, they were put to the test in multilayer evaluation circuits, using two different types of PTHs: through-circuit via holes that pass from top to bottom through the many layers of a multilayer circuit and buried via holes, which may connect just a few conductive layers or conductor and ground layers within a multilayer circuit.

The electrical contributions of a PTH vary according to its physical properties, such as the length of a via hole, its diameter, the amount and type of conductive metal (such as copper or gold) used for plating the hole, and the thickness and dielectric constant of the substrate material through which a PTH is drilled. In microstrip circuits, for example, shorter via holes for connecting conductive layers will have less capacitance than longer via holes. Also, via holes with larger hole diameters will exhibit more capacitance (and lower impedance) than PTHs with smaller hole diameters. These many variables combine to determine the ultimate effects of PTHs on circuit performance, with those effects highly dependent on the frequency/wavelength of analog circuits and the data rates of digital circuits. For low loss in a high-frequency transmission line, the electrical characteristics of a via hole would ideally be well matched to those of the connected transmission line, so that no impedance discontinuities (or reflections or loss) result. Of course, some circuit designers may choose to incorporate the parasitic capacitance, resistance, inductance, and transconductance characteristics of a PTH into their circuit designs, such as to fine-tune the response of a passive filter. Knowing those PTH characteristics in advance can certainly make it easier to work with PTHs in high-frequency analog and high-speed digital circuits.

Via holes at microwave frequencies are often modeled as two-port networks, with an input port and an output port and changes occurring to the input signal as a result of the via hole’s electrical effects. Signal loss through a via hole, for example, typically increases with increasing frequency. A number of mathematical models have been developed to predict the electrical effects of PTHs on microwave circuit performance, including the use of closed-form equations to calculate via hole impedance for microstrip transmission-line circuits. And modern finite-element electromagnetic (EM) simulation software programs include models for via holes and can simulate changes brought about by different via hole diameters and circuit board thicknesses. Unfortunately, such software tools can be expensive and complex to use, especially for modeling fine circuit features such as PTHs. There is no substitute for laboratory measurements performed on actual via holes through commercial PCB materials.

Back-Drilled vs Conventional Through-Circuits

To characterize various via holes, a test stripline-based PCB with four conductive layers was designed and fabricated from commercial laminate and prepreg materials: 7.3-mil-thick RO4350B™ LoPro® laminate and several plies of RO4450F™ prepreg material, both from Rogers Corp. The via hole structures were kept simple to better understand what physical changes in the via holes would mean in terms of electrical performance at microwave frequencies. The test circuit included signal launches to standard through-circuit via holes and to back-drilled through-circuit via holes, as well as three buried via holes connected to the signal paths of the through-circuit via holes. This simple test circuit, which included 2-in.-long stripline transmission lines with no transition via holes, made it possible to evaluate the performance of the different via hole types and determine what effects that back drilling would have on the performance of a through-circuit via hole compared to a standard through-circuit via hole.

With the aid of a commercial vector network analyzer (VNA) with frequency-and time-domain analysis capabilities, it was possible to not only measure scattering (S) parameters through 40 GHz for the test circuit, but to determine any impedance variations occurring at the various via holes, even the buried via holes. Different versions of the test circuit were constructed, all with input and output 2.4-mm coaxial connectors at the launch through-circuit via holes. The connectors attach by pressure contact and do not require solder, which would have added its own electrical contributions (variations) to the test circuits. The connectors were not matched to the circuits, but were oriented in a similar manner to ensure consistency in measurements. These different test circuits were consistent except for changes in via hole characteristics, such as diameter size and length, to see if measurements could reveal what those changes might mean in terms of high-frequency performance.

Test Results

Without delving too deeply into the data, the test results revealed superior impedance ripple behavior for the back-drilled through-circuit via holes compared to the conventional through-circuit via holes, for better impedance match, return loss, and signal integrity for circuits with these via holes. Loss measurements showed consistent performance at higher frequencies, with smoother, more consistent insertion-loss performance over a wider usable bandwidth for the back-drilled through-circuit via holes compared to the conventional through-circuit via holes.

Measurements performed with and without gating were used to decipher the impedances of the three buried via holes, since one impedance junction followed by another can mask the true value of the second and third impedance junctions. The results revealed how different modifications affected the impedance values of these buried via holes and how changes made to the via holes or the circuit pads around them could fine-tune the electrical performance of both types of via holes for improved overall circuit performance. Attention to detail is critical in designing circuits with PTHs since even the copper plating thickness can affect the impedance of the via holes. But once the correlations between physical characteristics and electrical performance are known, via holes can be added to a design as with any other circuit element, and can even help improve the electrical performance.

(Note: Additional details on the construction of these test circuits and the design variables used with the different via holes, along with comprehensive test data, can be found in a report to be presented by the author at the IPC APEX EXPO 2016, March 13-17, 2016 at the Las Vegas Convention Center, Las Vegas, NV. Contact www.ipcapexexpo.org for more information.) 

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