For many circuit designers, plated through holes (PTHs) form pathways, from one circuit plane to another. PTHs, also known as viaholes, 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. But 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 viaholes that pass from top to bottom through the many layers of a multilayer circuit and buried viaholes, 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 viahole, 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 viaholes for connecting conductive layers will have less capacitance than longer viaholes. Also, viaholes 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 viahole 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.
Viaholes 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 viahole’s electrical effects. Signal loss through a viahole, 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 viahole impedance for microstrip transmission-line circuits. And modern finite-element electromagnetic (EM) simulation software programs include models for viaholes and can simulate changes brought about by different viahole 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 viaholes through commercial PCB materials.
To characterize various viaholes, 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. (www.rogerscorp.com). The viahole structures were kept simple to better understand what physical changes in the viaholes would mean in terms of electrical performance at microwave frequencies. The test circuit included signal launches to standard through-circuit viaholes and to back-drilled through-circuit viaholes, as well as three buried viaholes connected to the signal paths of the through-circuit viaholes. This simple test circuit, which included 2-in.-long stripline transmission lines with no transition viaholes, made it possible to evaluate the performance of the different viahole types and determine what effects that back drilling would have on the performance of a through-circuit viahole compared to a standard through-circuit viahole.
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 viaholes, even the buried viaholes. Different versions of the test circuit were constructed, all with input and output 2.4-mm coaxial connectors at the launch through-circuit viaholes. 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 viahole characteristics, such as diameter size and length, to see if measurements could reveal what those changes might mean in terms of high-frequency performance.
Without delving too deeply into the data, the test results revealed superior impedance ripple behavior for the back-drilled through-circuit viaholes compared to the conventional through-circuit viaholes, for better impedance match, return loss, and signal integrity for circuits with these viaholes. 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 viaholes compared to the conventional through-circuit viaholes.
Measurements performed with and without gating were used to decipher the impedances of the three buried viaholes, 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 viaholes and how changes made to the viaholes or the circuit pads around them could fine-tune the electrical performance of both types of viaholes 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 viaholes. But once the correlations between physical characteristics and electrical performance are known, viaholes 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 viaholes, along with comprehensive test data, can be found in a report to be presented by the author at the IPC APEX EXPO 2016, scheduled for March 13-17, 2016 at the Las Vegas Convention Center, Las Vegas, NV. Contact www.ipcapexexpo.org for more information.)
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