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, Z0, 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:

Z0 = 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:

Z0 = (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:

Z0 = [(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:

Z0 = [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:

Z0 = [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.

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As we celebrate our 185th 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.”       

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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.

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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 185th year.

 

 

You think the pace of technology innovation is fast now, wait til you see what’s going to happen with 5G wireless. 5G will drive an Internet of Things (IoT) ecosystem of intelligent, fully connected sensors and devices, capable of improving economies small and large, and further blurring geographical borders.

According to the 2017 Cisco Mobile Visual Networking Index (VNI), 5G networks will be able to support advanced applications such as remote surgery, immersive experiences through virtual and augmented reality, autonomous cars, and so on.” All are game changers in their industries.

By 2021, it is predicted there may be as many as 25 billion 5G-capable devices and connections globally. The first few years of IoT deployment will run mostly on LTE networks using NarrowBand IoT (NB-IoT) and LTE M. But 5G growth is expected to be fast.

The Cisco VNI report states, “While 4G was the network that made smartphones prolific as a personal infotainment device, 5G is going to be network of the IoT. 5G will be capable of offering a new high bandwidth benchmark of 1 Gbps or higher and sub 1 ms latency. Operationally, 5G’s support for dynamic resource allocation and application prioritization will accommodate a variety of M2M devices, including those that require very low bandwidth.”

Today’s cellular networks operate in the 700 MHz to 2.6 GHz bands. Mobile service providers have introduced 4.5G and 4.9G technology to improve speeds immediately. 5G fixed wireless access will follow shortly; Verizon has been running trials in 11 cities. 5G mobile technology will follow shortly; AT&T estimates as soon as late 2018.

5G, when it arrives, is expected to handle far more traffic at much higher speeds than current cell network base stations. This requires new technologies, including millimeter waves (mmWaves), massive MIMO (multiple-input multiple-output), beamforming, and full duplex. Let’s take a look at a few of these key developments.

Millimeter Wave Spectrum

5G networks will be based on millimeter wave technology that can run in the underutilized portions of the 10-300 GHz band. Some cellular providers have begun to use mmWaves to send data between stationary points, such as base stations. Providers are considering the next step, using millimeter waves to connect mobile users to nearby base stations.

They are called mmWaves because the wavelength varies in length from 1 to 10 mm vs. the radio waves that serve today’s wireless devices, which measure tens of centimeters in length. Because these signals are shorter, there are hurdles to overcome. The higher frequencies carry more data, but are easily blocked by buildings and foliage, and sometimes even rain.

Sub-6 GHz Spectrum

Certain frequencies under Sub-6 GHz will be defined as 5G frequencies by 3GPP. That means these defined frequencies will be included in 5G standards and will apply to all countries.

A wide range of 5G trials are underway across the globe. China Mobile recently demonstrated the first 5G remote driving technology using a consumer car. They also completed a commercial Massive MIMO deployment that attained speeds of 2 Gbps.

In Massive MIMO, a high number of antennas, potentially hundreds, are incorporated into advanced chips that are smaller, deliver more processing power, and use less battery. The large number of antennas helps minimize signal loss and energy consumption, and can mitigate obstruction issues by steering signals in specific directions.

The Challenge of High Frequency Materials

Designers of these high frequency devices need to balance cost, weight, size, and radiation characteristics (such as gain, beamwidth, side-lobe levels, polarization). High frequency circuit materials deliver the performance needed by wireless base stations, satellite antennas, and network servers and storage.

PCB materials with dielectric constant (Dk) values of about 2.8 to 3.5 are preferred for sub-6 GHz and millimeter wave circuit applications. The consistency of the Dk across a circuit board can also be an important concern at these frequencies since variations in the Dk can introduce variations in the signal phase. Performance can also be affected by the composition of the PCB material.

Given this challenging set of requirements, what types of real-world materials are suitable for sub-6 GHz and millimeter-wave circuits?

The majority of initial 5G applications will be under 6 GHz, frequencies that are similar to those used in 3G and 4G materials. For such applications:

  • Rogers’ RO4350B laminates provide tight control of Dk and low loss. They are rigid thermoset materials that do not use PTFE, but can achieve excellent RF/microwave performance over time and even at elevated temperatures.
  • The RO4835 laminates are a high-performance material for high-frequency applications, but because it is not based on PTFE, it does not require special preparation (such as a sodium etch) to enable the formation of reliable plated through holes.

For microwave and millimeter wave applications:

  • The RO4730G3™ UL 94 V-0 antenna-grade laminates are designed to meet present and future performance requirements in active antenna arrays and small cells, in 4G base transceiver stations (BTS) and Internet of Things (IoT) applications, as well as emerging 5G wireless systems. These flame-retardant (per UL 94V-0), thermoset laminate materials are an extension of Rogers’ dependable RO4700™ circuit materials, which are a popular choice for base station antennas. RO4730G3 laminates provide the low dielectric constant (Dk) of 3.0 favored by antenna designers, held to a tolerance of ±0.05 when measured at 10 GHz.

Other new technologies are being created to enable the IoT networks that will deliver valuable insight obtained from massive amounts of data:

  • High temperature silicone materials serve as gaskets and seals, cushions, and thermal and acoustic insulation in demanding and remote environments.
  • Laminated multilayer busbars provide efficient and compact connections for propulsion, auxiliary, and other IGBT based converters in connected car and connected rail systems.

5G and sub-6 GHz networks will impact every aspect of our lives. Finding the right balance of material performance and cost is a challenge, especially technologies that is are still being defined.

 

Selected quotes from our recent earnings call. Read the corporate financials news release: Rogers Corporation Reports Second Quarter 2017 Results

In Q2 2017, Rogers achieved another quarter of exceptional net sales and earnings. Net sales were $201 million, an increase of 28% over Q2 2016. This robust performance was driven by double-digit organic growth in each of our three business units. In addition, our recently acquired businesses, DeWal and Diversified Silicone Products (DSP), continue to perform very well. Topline performance combined with our continued focus on operational improvements resulted in an outstanding profit increase.

Bruce Hoechner, CEO, on Innovation Leadership

During the past several years, Rogers has intentionally transformed into a more diverse company by expanding our portfolio through new product innovation, driving geographic penetration and executing on prudent acquisitions. This diversification has helped us perform consistently and deliver steady growth.

A good example of this is in the Advanced Connectivity Solutions (ACS) business. As the telecom market moves towards 4.5G and 5G technologies, this has moderated the 4G/LTE build-out. ACS has been able to maintain growth by taking advantage of other market opportunities, such as automotive safety and aerospace and defense.

  

Bruce Hoechner, CEO, on Growth Drivers

Our market driven focus is helping us advance our position in the markets we serve. One example is our emphasis on developing next generation technology to meet growing demand for wireless data. In e-Mobility, which is a key growth engine for our Power Electronics Solutions (PES) business, consumer demand and a global push to reduce CO2 emissions are contributing to growth for electric vehicle (EV) and hybrid electric vehicle (HEV) products. We are making additional investments to build upon our innovation leadership by adding a third innovation center in Chandler, Arizona which will focus on antenna systems to enhance 4.5G and 5G performance and other promising technologies.

We hold significant market positions in our two key growth areas of advanced mobility and advanced connectivity, which are aligned with our technology portfolio, marketing initiatives, and new product pipeline.

Bruce Hoechner, CEO, on Rogers’ Business Units

Advanced Connectivity Solutions (ACS) achieved second quarter net sales of $74 million, an 11% increase over Q2 2016. Growth in applications for automotive Advanced Driver Assistance Systems (ADAS) and aerospace and defense were partially offset by lower demand for wireless 4G/LTE applications. As we look ahead, we see strong indicators that 5G is gaining traction with the deployment of fixed wireless access. Several major operators are pursuing plans to put high-speed Internet into the home to compete with existing cable and fiber-to-home applications, often referred to as The Last Mile.

We are also well positioned to meet the needs of advanced automotive safety systems as they expand into mass market models. ACS holds a leading market position with a portfolio that spans the full range of customer requirements for short, mid, and long-range radar sensors.

The Elastomeric Material Solutions (EMS) team achieved all time record quarterly sales of $78 million, a 70% increase over Q2 2016. The EMS focus on geographic expansion and market penetration is paying off with healthy demand for general industrial applications across all regions. In portable electronics, we have benefited from a large increase in back pad wins. Our automotive sector is also growing with greater adoption of our sealing and vibration management solutions across a number of leading EV and HEV OEM’s and further penetration of our unique water resistant PORON® foam material into automotive sealing applications.

Power Electronics Solutions (PES) achieved strong Q2 net sales of $44 million, a 14% increase over to 2016. This growth was driven by broad-based demand across all markets, including EV and HEVs, laser diode coolers, renewable energy, and variable frequency motor drives. We continue to see impressive growth in the e-Mobility market, where consumer demand for high performance electric vehicles and government mandates are driving an increase in EV, HEV sales.

Q2 2017 Earnings Call Full Transcript

Q2 2017 Financials Press Release

Q2 2017 Earnings Call Slides

 

As electronic devices continue to shrink in size and increase in power, demand grows for power electronic circuits with higher power density. Increased operating temperatures are one of the tradeoffs of higher circuit power density, resulting in an increase in thermal stress for the circuit materials that serve as substrates for modern power electronic circuits. New processes and materials are available to address these challenges.

Soldering vs Silver Sintering

Soldering can present a number of complications that reduce a circuit’s performance, such as solder bridges and heel cracking. At higher operating temperatures, solder fatigue becomes an issue. Common die attach technologies are based on soft solders with melting points below 250°C. With the development of new generations of semiconductors (e.g. based on wide band gap materials like SiC and GaN), operating temperatures increase to more than 200°C. This will lead to a significant decrease in the solders‘ strength and reliability.

Sintering is a heat treatment process applied to a powdered material in order to give it higher strength and integrity. Silver sintering has become a promising technology for high temperature power electronics packaging as an alternative to soldering.

In the soldering process, heat is applied until a solid reaches its melting point and is then allowed to cool down and solidify to form a bond. In the silver sintering process, heat is applied to a silver paste, resulting in densification. Several actions occur simultaneously, including grain growth, pore growth, and densification, resulting in a stronger bond.

Find out what you need to know about silver sintering in the tech note: curamik® Substrates for Silver Sintering.

PCB Materials for High Power, High Temp Applications

The requirements for PCB materials capable of supporting high-density power electronics circuits are quite challenging, since they include both mechanical and electrical stability at high temperatures.

To meet these demands, curamik® ADVANTAGE provides a ceramic-materials based solution for smaller, higher power-density PCB circuits in power electronic applications. These ceramic substrates provide low dielectric loss and low-loss copper conductors that support high voltages and currents in power-grid, energy, and industrial power applications.

To improve the performance and usability of the substrate materials, all curamik ADVANTAGE products include a choice of plating materials, addition of solder stop to control solder coverage, and treatment for surface roughness. As an alternative to soldering, a state-of-the-art silver sintering process provides an attachment option to solder for critical high-temperature applications

Download the Power Magazine article for more details: Tailoring Circuit Materials for Power Electronic Applications

Watch the curamik ADVANTAGE video:

 

 
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