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

Much of the buzz on the show floor at the 2017 IMS in Honolulu was about millimeter-wave devices and circuits. At one time, frequencies above 30 GHz were considered “exotic” and only for military or scientific applications. But times have changed, and available spectrum is scarce. Millimeter-wave frequencies are now used in commercial vehicular radars, and big plans are being made for these small wavelengths in **Fifth Generation (5G) wireless communications networks**, in support of moving massive amounts of data quickly. More and more design engineers are faced with developing practical millimeter-wave circuits to 77 GHz and beyond. But first, they must decide upon the best** transmission-line technology** for those high frequencies as well as the circuit material that can support those circuits with quality, low-loss signal propagation. Drivers and cell-phone users everywhere will be counting on them!

At microwave frequencies, **microstrip** is by far the most popular transmission-line technology, compared to stripline and **coplanar waveguide **(CPW). It has a signal plane on the top copper layer and bottom ground plane. It is relatively simple and cost-effective, and allows surface mounting of components for ease of construction.

Unfortunately, as signal frequencies move into the millimeter-wave range, microstrip circuits can behave like antennas, radiating electromagnetic (EM) energy away from a desired signal propagation path and resulting in much higher radiation losses than at lower frequencies. Microstrip radiation losses are also dependent upon the thickness and **dielectric constant **(Dk) of the circuit substrate material. Thinner substrates suffer less radiation loss than thicker substrates. Also, circuit materials with higher Dk values have less radiation loss than circuit materials with lower Dk values.

In microstrip, the effective Dk is a combination of the Dk of the substrate material and air, since EM waves in a microstrip transmission line propagate in part through the dielectric and in part through the air above it. In contrast to microstrip, stripline is like a flattened coaxial cable. It consists of a conductor surrounded by top and bottom dielectric layers which in turn are covered by ground planes. The Dk of stripline is the same as that of the dielectric material, since air is not involved in the propagation process.

CPW circuits are fabricated with a number of variations, including as standard, **grounded coplanar waveguide **(GCPW), and conductor-backed coplanar waveguide. Standard CPW metallizes parallel conductors (in the form of a flat waveguide) on the top of a dielectric layer, with ground metal areas just beyond the conductors. GCPW adds a bottom ground-plane layer but requires plated-through-hole (PTH) viaholes through the dielectric substrate material to connect the top and bottom ground planes. The extra ground planes on the top copper layer helps GCPW achieve high isolation between signal lines and can be designed to minimize spurious wave propagating modes. Placement of the PTH viaholes is critical, and can impact transmission-line impedance and loss.

Like microstrip, GCPW has an effective **dielectric constant** as the result of EM waves propagating through the dielectric material as well as through the air around the conductors. GCPW, like microstrip, also allows surface-mounting of components for ease of fabrication, in contrast to stripline where PTH vias need to connect the components on the outer circuit layers to the inner signal layer. In terms of millimeter-wave frequencies, GCPW has lower dispersion than microstrip, with less radiation loss, and is capable of supporting higher-frequency propagation than microstrip circuits. GCPW also achieves more effective suppression of spurious propagation than microstrip, and is more amenable to practical signal-launch configurations (such as from waveguide, cables, and connectors) at millimeter-wave frequencies than microstrip.

**Finding the Right Circuit Material**

If GCPW is the optimum transmission line for millimeter-wave circuits, it should then be fabricated on a circuit material with optimum characteristics for millimeter-wave frequencies. Since signal power tends to decrease with increasing frequency, an optimum circuit material for millimeter-wave circuits should have low loss at those high frequencies. The **insertion loss** of millimeter-wave transmission lines is due mainly to the aforementioned radiation losses, conductor losses, and dielectric losses. Radiation losses tend to be design-specific, whereas conductor and dielectric losses will depend upon the choice of circuit material.

Dielectric losses are a function of the type of dielectric material, and usually well defined by a material’s dissipation factor (Df), with lower values indicating lower dielectric losses. A circuit material capable of consistent performance at millimeter-wave frequencies will also exhibit minimal variations in Dk, so that dielectric losses do not change dramatically with frequency.

In considering circuit materials for millimeter-wave circuit applications, the **thermal coefficient of dielectric constant** (TCDk) parameter provides reliable insight into the stability of a material’s Dk with temperature. The TCDk parameter provides an understanding of what to expect of a particular circuit material’s performance at millimeter-wave frequencies, with lower TCDk values indicating less change of Dk with temperature and less variations in frequency phase response resulting from variations in Dk with temperature.

Conductor losses can be traced to a number of variables at millimeter-wave frequencies, including the **surface roughness **of the copper conductors and the choice of plated finish for the conductors. Copper is an excellent conductor, but increasing surface roughness results in increasing conductor loss and greater propagation phase delays. The main area of concern for copper surface roughness is at the copper-substrate interface, with conductor loss due to the copper surface roughness increasing as a function of increasing frequency. The small wavelengths of millimeter-wave signals result in less skin depth in the circuit material as part of EM propagation, and circuit materials with greater copper surface roughness will more severely impact the insertion loss and phase response at millimeter-wave frequencies. The effect of copper surface roughness on insertion loss is also dependent upon the thickness of the circuit material, with thinner circuits affected more by copper surface roughness than thicker circuits.

At millimeter-wave frequencies, circuit materials with excessive copper surface roughness will have more impact on the conductor loss of microstrip circuits than on the conductor loss of GCPW circuits. Switching to a circuit material with smoother copper finish will bring less of an improvement in conductor-loss performance for a GCPW circuit than for a microstrip circuit, especially at millimeter-wave frequencies. In particular, tightly coupled GCPW circuits, which feature closely spaced conductors and ground areas, are less subject to the effects of copper surface roughness than loosely coupled GCPW circuits (with greater spacing between conductors and ground).

An optimum circuit material for millimeter-wave circuits should cause minimal phase angle variations, since such behavior can be critical to many millimeter-wave applications, such as 77-GHz vehicular radar systems. By minimizing variations in certain material-based attributes, such as copper thickness, Dk, conductor width, and substrate thickness, variations in phase angles can be minimized at millimeter-wave frequencies.

Additional details on finding the right combination of circuit material and transmission-line technology are available as part of the *Microwave Journal* webinar, “**Design Considerations and Tradeoffs for Microstrip, Coplanar and Stripline Structures at Millimeter-wave Frequencies,**” presented by John Coonrod.** **

Download the **ROG Mobile app** to 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.

Power electronics is changing rapidly. New packaging technologies are facing a rise in chip temperatures as seen in such applications as EVs / HEVs. Electronics increasingly need longer lifetimes to function in harsh conditions, such as wind turbines. Power electronics based on SiC components need to handle faster switching in the face of higher temperatures. While beneficial, the challenge is that these changes produce considerable stress on power systems and, consequently, result in reliability problems.

Traditional power module designs were primarily based on Al_{2}O_{3} or AlN ceramics. But the need for higher performance is leading designers to choose more advanced substrates. substrates with Si_{3}N_{4 }ceramics use the excellent bending strength, high fracture toughness, and good thermal conductivity of Si_{3}N_{4 }to build substrates that can handle a variety of today’s challenges.

In the following video, the **Power Electronics Solutions** team at Rogers takes a look at how to make the right substrate choices to support demanding applications.

**VIDEO: 3 Steps to Select the Right DBC / AMB Ceramic Substrate for Your Application**

In this video, Olivier Mathieu, Product Innovation Manager, discusses how to use the latest substrates in power electronics designs. He walks us through: (a) How to select the right ceramic thickness and its influence on basic isolation voltage, (b) How to select the right ceramic grade and its influence on heat removal, and (c) How to select the right copper thickness and its influence on functional isolation, ampacity, and heat removal.

Watch the video, **Selecting the Right DBC / AMB Ceramic Substrate****.**

The end of summer is right around the corner in the Northern Hemisphere with the Autumnal Equinox on September 22^{nd}. Happy Spring Equinox to all of you in the Southern Hemisphere 🙂

Did you know that equinoxes are not daylong events? They actually occur at the exact moment the Sun crosses the celestial equator, the imaginary line in the sky above the Earth’s Equator. At that instant, the **Earth’s rotational axis**** is not t**ilted with respect to the Sun. In 2017, the Sun crosses the celestial equator from north to south on **September 22**^{nd} at 20:02 UTC. Parts of Australia, New Zealand, and Russian will experience it on September 23^{rd }due to time differences.

In the meantime, Rogers Corp. has had a busy summer. The Rogers Germany team won the Bosch Preferred Supplier of the Year Award. Congrats!

Rogers Elastomeric Material Solutions/Asia held their Preferred Partners Conference in July in Phuket, Thailand. About 140 people from 52 companies were in attendance. The conference theme of “Winning Partnerships” aptly describes the collaborative relationship between Rogers and its customers — partnering and working together to secure business.

Of course, there were also a few BBQs along the way, with happy, smiling Rogers employees around the world.

**Evergem, Belgium **

**Rogers, Connecticut, USA**

**Eschenbach, Germany, Interns**

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

Printed circuits for high-speed and high-frequency applications rely on fine-featured transmission lines for signal transmission. Three of the most commonly used transmission-line technologies for these applications are **microstrip, stripline, and grounded coplanar-waveguide** (GCPW) transmission lines. Ideally, the loss through these transmission lines is minimal, and this requires an electrical impedance that is consistent and without interruptions, and with a value most appropriate for the types of signals to be transferred through the circuit. Variations from the nominal impedance of a circuit can result in increased insertion loss, increased return loss, higher radiated energy, degraded signal integrity (SI), and degraded rise time. A number of factors can affect the impedance of a PCB, including the physical and electrical characteristics of the circuit and circuit material, but by reviewing and better understanding these variables, their effects can be minimized.

Several different types of **impedance **are associated with high-speed, high-frequency PCB transmission lines, including wave impedance, input impedance, characteristic impedance, and frequency-dependent impedance. For RF/microwave circuits, for example, a characteristic impedance of 50 Ω is typically used for low-loss performance. The characteristic impedance, Z_{0}, of a circuit can be defined as the ratio of voltage to current for a wave propagating in one direction without interference from any other wave in the circuit. In mathematical terms, it is simply:

Z_{0} = V(x)/I(x)

where V(x) is voltage and I(x) is current. To include the effects of inductance (L) and capacitance (C) as required for high-frequency/high-speed circuits, the characteristic impedance for microstrip can be determined by the square root of inductance divided by capacitance:

Z_{0} = (L/C)^{0.5}

where C is a function of the product of the circuit substrate dielectric constant (Dk) and the area between the signal plane and the ground plane divided by the substrate thickness, or C = (Dk area)/thickness. For thicker substrates, the capacitance decreases and the impedance increases. The lower capacitance is less supportive of the flow of electrons needed for current flow. For thinner substrates or substrates with higher Dk, the capacitance increases and the impedance decreases in support of greater current flow. A wider conductor will increase the area and achieve the same effect.

This relationship for impedance, relating inductance and capacitance, is lossless, with no frequency dependence. To account for the numerous inductances and capacitances of complex circuits, such as differential circuits, a more comprehensive relationship can be used. This relationship is frequency dependent and does include conductor loss and dielectric loss:

Z_{0} = [(R + jωL)/(G + jωC)]^{0.5}

where

ω = 2πf is the angular frequency;

G is related to dielectric loss; and

R is related to conductor loss.

Inductance and capacitance have opposite effects on a circuit’s **impedance**. An increase in inductance can cause an increase in impedance, while an increase in capacitance can result in a decrease in impedance. For example, in a 50-Ω system in which impedance variations may reach 45 Ω at one time and 55 Ω at another, the decrease in impedance may be due to an increase in capacitance while the rise in impedance may be the result of an increase in inductance.

The physical characteristics of transmission lines can affect capacitance and impedance, which in turn impact circuit impedance. As conductors are made narrower, the inductance increases and the impedance increases. As conductors are made wider, the capacitance increases and the impedance decreases.

Sudden changes in impedance or differences in impedance are problematic for maintaining high performance in high-speed/high-frequency circuits. Such changes can occur at any transmission-line junction, such as between a coaxial connector and the feed point of a PCB. The change in impedance can cause reflections of RF/microwave signals or high-speed digital signals back to the signal source. This results in less energy delivered to the load as well as with the reflected energy interfering with the propagation of forward energy from the source to the load.

**Tracking Transmission Lines**

For high-speed/high-frequency circuits, an ideal signal path maintains the same impedance throughout, such as a 50-Ω characteristic impedance, with minimal losses in energy along the path. Most of these signal paths use **microstrip, stripline, or GCPW**, which may be integrated into complex multilayer circuits. Closed-form equations are available to determine the impedances of these circuit structures, although field-solving techniques typically provide more accurate results.

A closed-form equation to determine the impedance for microstrip, for example, is:

Z_{0} = [87/(Dk +1.41)^{0.5} ]{ln[5.98H/(0.8W + T)]}

where

H is the thickness of the dielectric substrate;

W is the width of the transmission line; and

T is the thickness of the transmission-line copper layer.

To ease the calculations, the MWI-2017 impedance-modeling software is available to download for free from the **Rogers Corp. Technology Support Hub**.

The calculations for finding the impedance of stripline transmission lines are similar to those for microstrip:

Z_{0} = [60/(Dk)^{0.5}]{ln [1.9B/(0.8W + T)]}

where

B is the thickness of the dielectric substrate from ground plane to ground plane;

T is the thickness of the stripline conductor; and

W is the width of the stripline conductor.

GCPW is a somewhat more complex than microstrip or stripline, with correspondingly more complex closed-form equations for predicting impedance. But again, the **free MWI-2017 software** provides a quick and straightforward way to perform the calculations based on proven closed-form equations.

**Delving into Differences**

The physical characteristics of high-speed/high-frequency circuits play significant roles in determining PCB impedance, since differences in substrate thickness, copper conductor thickness, and conductor width lead to differences in impedance in such circuits. To explore the effects of differences in these parameters, various microstrip test circuits were fabricated using a 20-mil-thick substrate with 2-mil-thick copper and 43-mil-wide conductor (50.07 Ω characteristic impedance) as a baseline. One of the other substrates had a slightly lower **dielectric constant** (Dk), one had a 1-mil-thick copper conductor, one had an 18-mil-thick substrate, and one had a 42-mil-wide conductor, to explore what the changes in physical parameters would do to the impedance in each case. The thinner substrate exhibited significantly lower impedance, the use of a circuit material with lower Dk resulted in a minimal difference in impedance, the thinner copper conductor provided only slightly lower impedance, and the narrower conductor also led to only slightly lower impedance.

**Copper conductor** surface roughness is yet another PCB parameter, although it is not always considered when analyzing different circuit variables for impedance. Smoother copper conductor surfaces exhibit lower conductor losses than copper conductors with rougher surfaces, but how do they affect impedance? For one thing, rougher copper surfaces will slow the velocity of a wave propagating through the circuit. The slower wave is perceived by the circuit as a higher effective Dk, even if the Dk of the circuit material itself has not changed.

As is apparent, a number of variables can affect the impedance of high-speed/high-frequency circuits, including substrate thickness, copper thickness, conductor width, and Dk. A number of additional factors can influence PCB impedance. For example, absorbed moisture can decrease the impedance of microstrip lines since water has a high Dk. Circuit substrate materials with high moisture absorption will suffer these effects on impedance, especially in conditions of high relative humidity (RH). This post is based on a presentation that will be given by the author, John Coonrod, at the **PCB West Conference & Exhibition** on September 13, 2017. The presentation will provide additional information on the variables that affect PCB impedance.

**ROG Mobile App**

Download the **ROG Mobile app** to 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.

As we celebrate our 185^{th} year, we honor our history and look ahead to a promising future. We are proud to be listed on the prestigious New York Stock Exchange, where we rang the closing bell to commemorate the anniversary of our founding.

*“The NYSE congratulates Rogers Corporation on its 185th Anniversary of Founding. We’re honored that* *Rogers, which is among our oldest listed companies, celebrated this milestone by ringing the NYSE Closing Bell.” *

Photo Credit: NYSE

Since 1832, Rogers has anticipated and adapted to the changing world around us. From our earliest days, we have been on the forefront of groundbreaking technology. Our passion to be ahead of vital market trends still drives us today.

As a market-driven innovator, Rogers’ leverages our technology expertise, deep understanding of our markets and extraordinary customer relationships to envision and enable the future. For leading technology and manufacturing companies, we are the engineered materials solutions partner trusted for reliability, innovation and collaborative support that enable their products to power, protect and connect our world.

Our global team of 3,100 employees propels us forward with their talent, dedication and commitment to an ethical culture. At Rogers, we are proud of our history, committed to the present and planning for an exciting future. It was a thrill to ring the closing bell at the New York Stock Exchange for the fifth time in our history in recognition of our 185^{th} year.