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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A message from Bruce Hoechner, CEO, Rogers Corporation:

Read the corporate financials news release: Rogers Corporation Reports Results for the Fourth Quarter and Full Year 2015.

In Q4 2015 Rogers delivered record fourth-quarter net sales of $152.9 million, a 3.5% increase over Q4 2014. In addition, we achieved non-GAAP earnings of $0.69 per diluted share, exceeding our previously announced guidance.

Screen shot 2013-11-01 at 10.53.55 AMDuring the quarter, we again experienced strong contributions from our synergistic acquisition that were partially offset by a decrease in organic net sales. We believe that the organic sales decline was due to a number of factors, including global macroeconomic conditions that impacted our business segments. I will discuss the segments and details in a few moments.

Our outlook remains cautious in the near term based on the uncertain timing of the global economic recovery. However, our growth strategy remains sound, we are focused on execution, and we remain confident in our longer-term growth prospects in our key megatrend markets.

Roadmap for Growth

I would like to review our growth strategy. This roadmap has enabled us to deliver sound results, and we believe that it positions us well to capitalize on the opportunities that lie ahead, particularly when the global economy recovers.

Q4_2015_roadmap

As a market-driven organization, we leverage our strong understanding of the link between our markets and technology to develop solutions to solve unmet needs in the marketplace. A good example of this is the strong position we have established in Advanced Driver Assistance Systems.

We experienced increased demand for these applications in 2015 and expect that trend to continue based upon our customer relationships and the growth predictions for this market.

In the area of innovation leadership, we continue to cultivate a robust pipeline from the work we’re doing in our innovation centers, as well as in the operating units where our R&D teams are focused on next-generation solutions. One of the innovation centers early developments is a pioneering platform technology that enables substantially smaller, higher performance, higher frequency antennas. This technology is now being scaled up in our Advanced Connectivity Solutions business initially targeted for defense applications, and we expect broader adaption of this in commercial applications.

The Arlon acquisition has demonstrated the strength of our approach to synergistic M&A. The acquisition was completed within our 12-month goal, and it outperformed our expectations during the year. A textbook example of a successful integration, it will serve as a blueprint for future acquisitions.

We continue to see progress from our investments in operational excellence. These initiatives are helping us reduce manufacturing costs, improve inventory management, and achieve greater on-time delivery to request for our customers.

In addition, our ERP upgrade is helping our back office teams work more efficiently as we improve and standardize our systems and processes. This work also prepares us to quickly integrate future acquisitions.

Financial Goals

Our interim three-year financial goals serve as a checkpoint in our long-term plan. While market conditions impacted our 2015 results, we remain confident in our longer-term ability to achieve 15% revenue growth through a combination of organic and acquired growth.

Our commitment to this strategy is delivering sustained progress in revenue and profit performance over the past three years. We saw a slight margin dip in 2013 as we dealt with the macroeconomic conditions, but we were pleased with the contributions from our operational excellence initiatives.

I would like to take a brief look at our profitability from a slightly different perspective, EBITDA. We believe this is a key measure of our performance in assessing our internal core operating results, which improved 260 basis points.

I would like to highlight some achievements from 2015. First, our acquisition and integration of the Arlon business was highly successful. This acquisition led us to record revenues in 2015, and we are excited by the opportunities for organic growth in the future. I will address our megatrends in a bit, but here I would like to highlight that we are confident that our megatrend categories are focused on the right markets for future growth. In 2015, we selected safety and protection as a new Rogers megatrend category based upon the opportunities we see for applications in growing markets like automotive safety and consumer impact and protection.

We believe that our core capabilities align well with this megatrend, and we are executing on those opportunities.

We continue to see strong results from our investments in improving our overall operating capabilities. These initiatives are helping us lower costs and drive efficiencies in both our manufacturing and back office environments. Our focus on delivering shareholder value is reflected in our capital allocation strategy.

In 2015, we initiated a $100 million share repurchase program and bought back $40 million in shares. Building on the success of our North American Innovation Center in Burlington, Massachusetts, we opened the Asia Innovation Center in September 2015 to leverage the talent and opportunities we see in that region.

Advanced Connectivity Solutions (ACS) delivered record fourth-quarter net sales of $63.8 million, driven by $16.4 million from the acquisition, which was an increase of 11% over Q4 2014. ACS organic sales for the quarter declined 16.4% on a currency neutral basis. Strong demand for Advanced Driver Assistance Systems and aerospace and defense applications were offset by weaker demand and inventory rebalancing in the 4G LTE base station market, primarily in China.

Looking ahead in ACS, we anticipate that the base station recovery in China and growth in small cells will help drive demand for high-frequency circuit materials. We are very pleased with the consistently strong demand for applications for Advanced Driver Assistance Systems as these features continue to expand into mass-market automobile models.

In addition, we expect the adoption of new technologies linked to the Internet of Things and e-mobility to help drive growth in ACS.

Elastomeric Material Solutions (EMS) achieved net sales of $42.5 million, including $6.1 million from the acquisition, which is roughly flat year over year. Organic net sales were down 13.5% from 2014 on a currency neutral basis. Solid results in consumer impact and protection and automotive applications were offset by weaker demand for certain portable electronics applications. The EMS organization is addressing the headwinds in display gasket applications for portable electronics by refocusing the business on other solutions. For example, we are seeing strong interest in the new smartphone back pad solutions that we introduced in 2014, which are partially offsetting the weaker demand for display gaskets.

We also have had several design wins for sealing applications in the automotive market. In addition, we see opportunity through European and Asian geographic expansion in general, industrial and mass transit, as well as greater market penetration in consumer impact and protection applications.

Power Electronics Solutions (PES) net sales were $36.7 million, a 12% decrease compared to Q4 2014. On a currency neutral basis, PES net sales declined 3.4% from Q4 2014. Foreign exchange rates and a global slowing in industrial and infrastructure investments due to challenging macroeconomic conditions impacted PES more than our other two business segments.

During the quarter, increased demand in EV, HEV, as well as certain renewable energy applications were offset by weaker demand in mass transit.

We believe the outlook for the mid- to long-term is positive. Government mandates and climate change agreements are strengthening demand for energy-efficient motor drives and renewable energy applications. In addition, fuel efficiency regulations continue to drive demand for EV, HEV, and vehicle electrification applications.

Megatrends

We continue to be encouraged by the positive growth expectations for our key markets. Demand in areas like mobile data traffic, EV, HEV, and automotive safety are expected to drive substantial growth in the mid- to long-term.

For our 2016 plan, we are not anticipating significant movements in commodity pricing or foreign exchange rates at this time. While lower global GDP growth is muting revenue expansion, wage stability across our operating geographies appears to be in check, helping us manage operating costs.

In the face of weaker global economic conditions, we are addressing issues such as selective pricing pressures by continuing to reduce manufacturing costs. The industrial slowdown that we believe started during Q2 2015 affected sales into certain applications we serve within the industrial sector. We do expect to see an improvement in demand for these industrial applications when the global markets recovery.

Our market-driven approach is helping us form deeper partnerships with our customers. These relationships are contributing to a strong sales pipeline and positioning us to be designed into new applications and technologies.

I am very encouraged by our product development opportunities. Our investments in next-generation and new products closely align to our megatrend markets, which accounted for 66% of our sales in Q4 and full-year 2015.

Synergistic M&A opportunities continued to be actively pursued by Rogers, and we will use the successful integration of Arlon as a blueprint for future acquisitions. We are capitalizing on the investments we have made in our processes and systems to reduce costs and gain efficiency, which will also serve to accelerate the integration of future acquisitions.

In order to increase productivity and reduce costs, we are in the process of shifting some manufacturing to lower-cost locations, as well as rationalizing capacity. In addition, we are leveraging our shared service organizational model to better utilize resources.

And finally, we will continue to practice disciplined capital deployment, making strategic investments to move our growth strategies forward and deliver value to our shareholders.

Q4 and Full Year 2015 Earnings Call Full Transcript.

Q4 and Full Year 2015 Financials Press Release.

Q4 and Year End 2015 Earnings Call Slides.

 

Technology and Invention is a series by Rogers Corporation about the innovation, pioneering spirit, and transformative technologies that are creating a cleaner, safer, more connected world. Part 1: Historical Milestones, Part 2: The Plastics Revolution.

The Birth of Electronics

The word “electron” was first proposed by Johnstone Stoney in 1891 when he realized that an electric charge had a natural unit which could not be subdivided any further. Shortly thereafter, a series of experiments by J.J. Thomson led to the discovery of a light particle that carried a charge; the name “electron” was applied to it. The many applications of electrons moving in a near-vacuum or inside semiconductors were later dubbed “electronics.”

magnet-electron-shellThe electron itself has turned out to be a bit different than what J.J. Thomson originally described. Albert Einstein and others showed that electrons aren’t either particles or waves, but in some conditions act like particles and in others like waves. In fact, J.J. Thomson’s grandson, G.P. Thomson, received a Nobel Prize for his work on the wave character of electrons. We now know that electrons are part of a whole family of related particles — all of them infinitesimal points carrying charge, mass, and spin.

Printed Circuit Boards

PCBs are a combination of mechanical and electrical connections etched from copper sheets and laminated onto a non-conductive substrate. The first printed circuit boards (PCBs) can be traced back to the need to eliminate complex wiring and provided consistent results. In 1903, a German inventor, Albert Hanson, described flat foil conductors laminated to an insulating board, in multiple layers. In 1904, Thomas Edison experimented with chemical methods of plating conductors onto linen paper. In 1925, Charles Ducas submitted a patent that involved creating an electrical path directly on an insulated surface.

The commercial development of PCBs began in the 1940’s, post-WWII. After years of discussions with lithograph companies, Dr. Paul Eisler began making the first real operational printed circuit boards in Austria. Eisler found no demand for his product until the Americans started work on the proximity fuse to bring down V1 rockets; printed circuits were a critical component. Following the end of the war, the USA released the secret of printed circuits, then all electronics in airborne instruments were printed.

DrPaulEislerIn 1966, Rogers purchased technology from Westinghouse Electric Corporation that, although never used commercially, led to the development of flexible circuits for computers, telecommunications, and other electronics applications.

Also in 1966, Rogers established its first plant outside of Connecticut, in Chandler, Arizona, closer to where the electronics industry was growing rapidly. The new Circuit Systems Division began operations in a 40,000 square foot plant manufacturing flexible circuits, busbars, and the Mini/Bus, designed to distribute voltage to integrated circuits on printed circuit boards. The Microwave Materials Division spun off from the Circuit Systems Division and is now known as Advanced Connectivity Solutions.

Today, as circuit speeds continue to escalate and wireless communications proliferate, the need for high frequency circuit boards becomes paramount. Rogers’ high performance dielectrics, laminates and prepregs offer high frequency performance and low cost circuit fabrication for a wide range of electronic products, from adaptive cruise control to airborne antenna systems to network gear.

The Critical Role of Power Electronics

Power electronics technology converts and controls electrical power using high-efficiency switching mode electronic devices. The technology is embedded in AC and DC power supplies, electrochemical processes, heating and lighting control, electronic welding, photovoltaic and fuel cell power conversion, and motor drives. Power electronics play a central role in industrial automation, high-efficiency energy systems, energy conservation, renewable energy systems, and electric and hybrid vehicles.

Bardeen_Shockley_Brattain_1948Early power electronics technologies included mercury-arc rectifiers for converting AC to DC power, hot-cathode ray tube rectifiers first invented by GE, and virtually indestructible magnetic amplifiers. The modern era of solid-state electronics launched in 1948 with the invention of the transistor at Bell Labs. From the Computer History Museum:

Using improved semiconductor materials developed for radar detectors during the war, in early 1945 [William] Shockley experimented with a field-effect amplifier, similar in concept to those patented by Heil and Lilienfeld, but it failed to work as he intended. Physicist John Bardeen suggested that electrons on the semiconductor surface might be blocking penetration of electric fields into the material. Under Shockley’s direction, together with physicist Walter Brattain, Bardeen began researching the behavior of these “surface states.” On December 16, 1947, their research culminated in a successful semiconductor amplifier. Bardeen and Brattain applied two closely-spaced gold contacts held in place by a plastic wedge to the surface of a small slab of high-purity germanium. On December 23 they demonstrated their device to lab officials and in June 1948, Bell Labs publicly announced the revolutionary solid-state device they called a “transistor.”

Today, power MOSFETs, which first appeared on the market in the 1970s, have become popular for low-voltage, high frequency applications. The insulated-gate bipolar transistor (IGBT or IGT), invented in 1983, is the most popular semiconductor device for medium-to-high power applications.

Improving Transmission Efficiency with Busbars

Rogers-new-RO-LINX-Power-Circuit-BusbarsAs the need for power increased after WWII, it became important to improve power transmission efficiency and reduce switching losses within electrical devices.

Rogers developed the busbar product line – strips or bars of copper, brass, or aluminum that collect and distribute electricity within electrical devices –in 1959 as an engineering solution to the problem of power distribution in the first transistorized computer, the IBM 1401. These busbars quickly captured a main share of the growing computer market.

By the mid-1960s, virtually all manufacturers of mainframe computers were Rogers’ customers. This was the company’s first significant step toward vertical integration from materials into components, using Rogers’ materials as the base. This was to become one of the key aspects of growth in the next twenty years.

Significant research has taken power distribution technology to the next level. Today that takes three forms at Rogers: ROLINX® busbars, curamik® ceramic substrates, and curamik® cooling solutions.

ROLINX PowerCircuit busbars help attain higher power efficiencies by limiting switching losses. They serve as power distribution “highways” that connect power sources with capacitors, IGBTs, or complete modules. These busbars combine the traits of traditional laminated busbars with the processing capabilities of PCB assemblies. The result is a high-performance busbar that supports 3D design, helps achieve optimum thermal management, and is well suited for medium-to-high-volume assembly processes.

Power Substrates for High Heat Conductivity

curamikIn 2011, Rogers acquired curamik Electronics GmbH, a manufacturer of power electronic substrate products headquartered in Eschenbach, Germany. Founded in 1983, curamik develops and produces direct copper bonded (DCB) ceramic substrate products used in the design of intelligent power management devices, such as IGBT modules.

These ceramic substrates are found in a growing array of clean technologies that are powering the world. The basis of the substrate is a ceramic isolator to which pure copper is applied. The result is ceramic substrates with high heat conductivity and great heat capacity, and heat spreading provided by the thick copper layer.

curamik’s DBC (Direct Bond Copper) substrates are manufactured by bonding copper foils directly to electrically insulating industrial ceramics. curamik also uses AMB technology (Active Metal Brazing), another form of joining metal to ceramic. The result is customized ceramic substrates for highly efficient thermal management, providing high-performance electrical power management.

Rogers Corporation provides innovative solutions for power electronics, advanced foams for cushioning and protective sealing, and high-frequency printed circuit materials. For over 180 years, we have empowered breakthroughs in reliability, efficiency, and performance, to help our customers build a cleaner, safer, and more connected world.

 

 

 

By John Coonrod, Technical Marketing Manager, Rogers Corp., Advanced Connectivity Solutions. As seen in The PCB Design Magazine (Dec 2015)

A variety of different test methods may be used for any one electrical concern. This article will discuss the issues related to determining the dielectric constant (Dk) and dissipation factor (Df or Tan-Delta). On a data sheet, a designer may see a Dk value for a material to be 3.5, as an example. Once the designer buys the material and performs necessary evaluations, it may be found that the Dk of the material is 3.8. In some applications this difference in Dk is probably not meaningful; however, for many RF and high-speed digital applications, this difference could be very significant. What is really interesting about this example is that the two Dk values may both be correct, depending on the test methods used.

Screen Shot 2016-01-29 at 11.06.44 AMMost laminates used in the PCB industry are anisotropic and this means that the electrical properties are not the same on all three axes of the material. Typically the thickness (z-axis) of the material will have a different Dk value than the x or y axes of the material. The reasons for this depend on what type of material is being considered.

The laminates used in the PCB industry are typically woven-glass reinforced, however there are notable exceptions. The glass reinforcement layer typically has a different Dk and Df than does the raw substrate of the laminate. The standard E-glass most often used in PCB laminates has a typical Dk value of about 6 and a dissipation factor of around 0.004. The common FR-4 laminates use relatively simple resin systems and the resin itself has a Dk that is around 3 and a Df of about 0.03. Different ratios of resin to glass will cause the laminate to have a Dk that is somewhere between the value of the resin and that of the glass. However, the glass-resin ratio impact on Dk is usually considered when evaluating the material through the thickness axis and if the x- or y-axis is evaluated, the Dk value may be very different than the z-axis result.

A large number of test methods are available to evaluate materials for Dk and Df. The methods that are most often used in the PCB laminate industry for making these measurements are typically tailored to evaluating materials in very large volume. Because of this issue, these test methods need to determine Dk and Df relatively fast, have good repeatability, and be used for quality control. A common test method used is the clamped stripline resonator, where a clamping fixture is used to form a stripline structure; the layer structure of a stripline is ground-signal-ground. This test method determines the Dk and Df of the material in the clamped fixture and more specifically, it is reporting these values related to the thickness axis of the material.

Other tests used in high-volume testing include SPDR (split-post dielectric resonator), rectangular cavity and open cavity resonance methods. All three of these methods have electric fields oriented perpendicular to the material, which means these test methods will evaluate the x-y plane of the material and not the z-axis. In the case of the common FR-4 material which is a resin-glass composite, the Dk number can be very different in the x-y plane than in the z-axis due to the impact of the glass.

Returning to the original example, where a material is tested and found to have a Dk of 3.5 and then another test is done on the same material and the Dk is found to be 3.8, both of these numbers can be correct when using two different test methods. These numbers are actually based on real life experience when testing high frequency laminates that are PTFE based with ceramic filler and have woven-glass reinforcement. With this type of material it is possible to have the same piece of material tested in the clamped stripline test and get a value of 3.5 and then tested in SPDR and obtain a result of 3.8. Since the clamped stripline test is evaluating the z-axis of the material and the SPDR is evaluating the x-y plane of the material, both results are obviously different but still correct. Essentially, these measurements show to some degree how anisotropic the material is, where the z-axis Dk of the material is 3.5 and the x-y plane Dk of the material is 3.8.

Knowing the anisotropic Dk values of a laminate is typically critical for RF applications with edge-coupled features. In the case of high-speed digital circuits, these values can be important for differential pair structures. Having these values can be important, but also having a modeling software which can incorporate these values into predicted circuit performance is an important supplement to the design process.

It is always recommended to contact your material supplier if you have questions about the Dk or Df of a laminate. You should ask which test method is used and which axis or axes of the material is being evaluated. Another good question to ask your material supplier is the frequency at which these values are generated, because the Dk and Df of a material is frequency dependent. Having the most accurate laminate information for the design phase of a project is critical to its success.

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This post authored by John Coonrod originally appeared on the ROG Blog hosted by Microwave Journal.

Microwave circuit dimensions are related to their wavelengths / frequencies and to the dielectric constant (Dk) of their substrates. Quite simply, higher-frequency signals have smaller wavelengths and their electromagnetic (EM) energy of those smaller wavelengths will propagate through circuits with smaller dimensions. Phase velocity is related to wavelength, with slower EM waves having shorter wavelengths which propagate through circuit structures with smaller dimensions than faster waves with their longer wavelengths.

For a given frequency / wavelength, the circuit dimensions can also be reduced in size by fabricating the circuits on laminates with high dielectric constants, such as RO3010™ and RT/duroid® 6010.2LM circuit materials from Rogers Corp. The high Dk values of these circuit materials results in shorter wavelengths and smaller circuit dimensions, but also slows the propagation of the EM waves along the conductors of a printed circuit board (PCB).

Transmission-line conductors on PCBs for RF/microwave circuitry take on various forms, such as microstrip, stripline, coplanar waveguide (CPW), and even substrate-integrated-waveguide (SIW) transmission lines for circuits at millimeter-wave frequencies. For a conventional microstrip transmission line with solid conductors, for example, the phase velocity has relatively high speed. It is related to the current density, with most of the current running along the edges of the solid microstrip conductors in a circuit. If the solid conductors are modified into a conductor pattern that is not solid, such as a ladder pattern, the phase velocity will be slowed and the wavelength will be reduced.

The phase velocity is a function of the separation distance between the storage locations in a conductor for the EM energy’s electric and magnetic field components, resulting from capacitive loading (and storage capabilities) within a transmission line’s conductors. When the magnetic and electric energy storage locations are in close proximity, the phase velocity is higher or faster than when the magnetic and electric energy storage locations within the conductor are further apart. For the slower phase velocity, the wavelength of the transmission-line’s conductors is shorter than for the conductors with the faster phase velocity. Slowing the phase velocity of a microstrip transmission-line’s conductors is a way to miniaturize circuits fabricated on PCB materials with a given Dk value.

By modifying conventional microstrip conductors into unconventional patterns, such as a ladder configuration, distances between stored magnetic and electrical energy field components are increased beyond the separation distances for standard solid microstrip conductors. A microstrip ladder pattern effectively conducts EM energy, but with slower phase velocity than a solid microstrip transmission-line conductor because of its greater distance between magnetic and electric energy storage locations. Just how much slower the phase velocity is than a standard solid microstrip conductor will depend upon the geometry of the microstrip ladder pattern.

Such slow wave structures reduce the group velocity of a transmission line, effectively increasing its group delay compared to a conventional solid microstrip transmission line, and reducing its wavelength compared to the wavelength of a conventional microstrip transmission line. As an example, consider a half-wavelength gap-coupled resonator constructed from microstrip transmission lines. When fabricated with a standard solid conductor pattern on 20-mil-thick RO4835™ laminate from Rogers Corp., with typical Dk of 3.48 at 10 GHz in the z-direction of the material, the circuit structure will resonate at 4 GHz. When using the appropriate microstrip ladder conductor pattern on the same material, the resonant frequency of the gap-coupled resonator circuit can be reduced by 25%, to 3 GHz.

Coupled resonators are often used to realize various high-frequency filter functions, so this gap-coupled-resonator example with its microstrip ladder pattern of conductors can be applied to these types of filters to achieve a reduction in the size of the filter circuitry for a given design frequency. An even greater reduction in filter size can be gained by combining the ladder conductor pattern approach with high-Dk PCB material, such as RO3010 ceramic-filled polytetrafluoroethylene (PTFE) laminate with a Dk of 10.2 measured in the z-direction of the material at 10 GHz and RT/duroid 6010LM ceramic-PTFE composite material with Dk of 10.2 in the z-direction at 10 GHz.

What is the impact on circuit size when using one of these high-Dk PCB materials? If a circuit material such as RO3003™ laminate with a Dk of 3.0 in the z-direction at 10 GHz is used as a starting point for a filter circuit with microstrip edge-coupled structure, including edge-coupled resonators, and the same filter is designed on a PCB material with much higher Dk, such as RT/duroid 6010LM laminate with its Dk of 10.2, a healthy reduction in size is possible. The size of the same microstrip edge-coupled filter function will be about 30% smaller on the higher-Dk material.

By combining slow-wave circuit patterns such as the microstrip ladder pattern with high-Dk circuit laminates such as the 10.2 Dk RO3010 or RT/duroid 6010LM PCB materials, the circuit-shrinking effects of both approaches can be applied to an RF/microwave circuit design for a significant reduction in circuit size. Both of these low-loss circuit materials feature tightly controlled Dk values for excellent high-frequency performance and reduced circuit sizes.

Screen shot 2014-08-08 at 1.33.54 PMLadders are just one type of microstrip conductor pattern that can be used to create slow-wave structures, with many different patterns capable of achieving slow-wave propagation and contributing to a reduction of circuit size for a given design frequency and circuit laminate Dk value. The key to success for forming any slow-wave pattern (and shrinking the dimensions of a high-frequency circuit) lies in separating the electric and magnetic energy storage within the structure, thereby slowing the phase velocity and reducing the wavelength of the transmission line for a given circuit material and Dk value.

ROG Mobile App

Download the ROG Mobile appto access Rogers’ calculators, including the popular Microwave Impedance simulation tool, literature, technical papers, and the ability to order samples of the company’s high performance printed circuit board materials.

Ask an Engineer

Do you have a design or fabrication question? Rogers Corporation’s experts are available to help. Log in to the Rogers Technology Support Hub and “Ask an Engineer” today.

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