Lightning Speed Laminates; Some ‘Exotic’ PCB Processes Could Become Commonplace

Technology is advancing quickly and over the years the PCB industry has advanced its processing and capabilities extensively as well. However, due to technology demands, it is likely that some of the processes for PCB fabrication that were previously considered exotic may soon be considered commonplace. Of course, that prediction depends on many issues. Due to the nature of new and evolving applications, the PCB industry will likely need to make more advances in some of its processing.

Those PCB fabricators who support 77 GHz automotive radar applications are probably very aware of the stringent requirements for these very high frequency applications. Generally, applications at higher frequencies will have more sensitivity to several PCB processes and their normal variations. One simple example is circuit etching quality, and for many mmWave applications, the conductor widths are commonly required to be held to tighter tolerances. The trapezoidal shape of a signal conductor is often specified to be within a certain range and even the roughness of conductor sidewalls have shown to cause RF differences at 77 GHz. Copper plating thickness variation can also be very problematic; having tight control of this process is important for very high frequency applications. 

There are several new applications using PCB technology above 77 GHz which require processing capabilities that exceed the current PCB technology. A lot of research work is being done, for example, around 140 GHz for a particular application. The conductor width tolerance needs to be ±1 micron (0.04 mil), conductor thickness needs to be very thin (2 micron), and the thickness variation needs to be at ±0.5 micron. There are some other specifications which are also extremely difficult, or maybe impossible for current PCB technology, such as very tight location tolerances between circuit features and microvias and layer-to-layer alignment. 

One of the 140 GHz applications I read about in a white paper on IEEE.org used a combination of PCB technology and semiconductor technology. The main board was made with PCB technology and that circuit board had 35 GHz for the highest operating frequency. That circuit board is no problem for our current PCB fabricators to meet the requirements. However, mounted on the main board was a small glass circuit made with semiconductor technology and that circuit had castellated vias at the edges, which were used to solder-connect to the main board. The 35 GHz signal on the PCB would transition through the castellated vias onto the glass circuit and it would go through a 4x frequency multiplier to get the signal up to ~140 GHz.

After that, the 140 GHz signal would be processed as necessary on the glass circuit. That is a very interesting way to deal with a 140 GHz application, but I would question the capability of high-volume manufacturing as well as cost concerns. If PCB technology can be used to generate circuit features with extreme precision, the high frequency circuit materials that we currently offer can support this technology and have benefits over the glass semiconductor technology.

One PCB technology that has been around for many years is laser-defined conductors. Some of the recent laser technologies can generate high precision circuit geometry. There are limits to these processes currently, but over time, and if the PCB industry develops this technology for high-volume, this could be a process used for the future millimeter-wave PCB circuitry.

Another PCB fabrication process, which may be beneficial for the industry to exert some effort in advancing, is fusion bonding technology. Fusion bonding has been around for many years, but typically not done in high volume manufacturing. There are many things to overcome to make this technology robust for high volume manufacturing while still maintaining the exceptional electrical properties it can offer.

Fusion bonding uses very controlled lamination equipment and processes to basically melt a thermoplastic material, which acts as a bonding material for a multilayer circuit. It will not surprise those engineers who have investigated different high-frequency materials that there is one type of material that stands out for the best electrical performance, especially for very high frequency: PTFE-based material. To laminate a multilayer that uses a PTFE-based bonding material, a fusion bonding process must be used.

A few years ago, Rogers Corporation brought to market a set of PTFE-based materials specifically formulated to meet the electrical needs of very high-speed digital (56 Gbps, 112 Gbps, etc.) applications. The material is Rogers XtremeSpeed RO1200 family of products, and several independent studies have shown this material set can achieve the best electrical performance for very high-speed digital applications.

However, to get the optimum benefit of this material set, the lamination of the multilayer circuit using this material will need to use the XtremeSpeed RO1200 bondply and a fusion bonding process. (As a quick side note, we have achieved very good high-speed digital results when using the XtremeSpeed RO1200 core with SpeedWave 300P prepreg. SpeedWave 300P prepreg does not need fusion bonding and it will have lamination process parameters more common for high-volume PCB fabrication.) A pure package PCB using XtremeSpeed RO1200 core with XtremeSpeed RO1200 Bondply has the best electrical performance for the most demanding high-speed digital applications today. These materials can support the upcoming advanced high-speed digital requirements of the future.

This column originally appeared in the June 2021 issue of Design007 Magazine.

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2021

Lightning Speed Laminates; Some ‘Exotic’ PCB Processes Could Become Commonplace

06-17-2021

Technology is advancing quickly, however, due to technology demands, it is likely that some of the processes for PCB fabrication that have been previously considered exotic, may soon be considered commonplace. Columnist John Coonrod explains.

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Lightning Speed Laminates: Using Simulation Software to Assist With PCB Design

04-21-2021

There are many different types of simulation software on the market, which can be very helpful for the PCB designer. Each software has its own set of capabilities and limits. Understanding the basic attributes of the software can help a designer choose the appropriate software for their design task.

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Lightning Speed Laminates: Things to Consider When Creating a Circuit Material Library

02-18-2021

A circuit material library for the fabricator can be advantageous for multiple reasons. Sometimes these libraries are intended to be used for electrical predictions, such as impedance, insertion loss or other issues. Other times the information found in the circuit material libraries are used to assist with thermal issues, potential reliability concerns, circuit construction stackups and some processing issues.

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2020

Lightning Speed Laminates: Understanding Material Interactions With PCB Fab Processes

12-14-2020

Having a good understanding of the circuit material that a designer is working with, along with the potential PCB fabrication influences, is essential for having a successful first-time evaluation of a new circuit design. John Coonrod explains how if a designer only considers the basic material properties of Dk and Df, that may not be enough information to have a successful first round of evaluations.

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Lightning Speed Laminates: Why High-Frequency Materials Have Different Dk Values

10-20-2020

When an engineer researches high-frequency circuit materials, they will notice there are many offerings of what appears to be the same material type but with different Dk values. John Coonrod gives a quick overview of the need for these materials with different Dk values, as related to different high-frequency applications.

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Lightning Speed Laminates: Utilizing mmWave Technology to Optimize High-Speed Designs

08-31-2020

For the past several years, John Coonrod has been working with many different millimeter-wave applications. In this column, he shares areas where understanding the tricks of mmWave technology could help improve design and performance for very high-speed digital technology.

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Lightning Speed Laminates: High-Frequency Circuit Material App Notes

07-22-2020

High-frequency circuit materials are used in a variety of diverse applications. Due to this diversity, it is difficult to write an application (app) note for a specific high-frequency circuit material. John Coonrod explains while exploring Rogers Corporation's app notes.

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Lightning Speed Laminates: Understanding Dk Data Key to Cost-aware Design

04-30-2020

In the development stages of a circuit for a new PCB application, there are usually several iterations to the circuit. These many changes can be costly, and it is not uncommon for a project to have 4–8 changes before it can be released to the market. According to John Coonrod, one item that can substantially reduce the number of changes and the associated costs is the use of a good circuit simulation software.

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Lightning Speed Laminates: PCB Materials for High-power RF Applications

02-15-2020

Most commercial PCB-based applications that use high power are typically associated with cellular base station technology; however, there are other applications. There are also several things to consider when working with high-power RF applications. John Coonrod focuses on PCB-based power amplifiers used in base station applications, but the basic concepts discussed here will apply to other high-power applications.

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2019

Lightning Speed Laminates: Test Vehicles for Materials Evaluation

12-26-2019

There are many different types of PCB designs and constructions that can be used as a test vehicle to evaluate electrical properties. John Coonrod explains how a proper test vehicle to compare different circuit materials would be a design and construction that takes into consideration the different material properties and have the least amount of PCB fabrication variables that can impact the results.

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Lightning Speed Laminates: Material Choices for 5G PCB Applications

10-18-2019

The new 5G cellular infrastructure has many technological differences from previous infrastructures, which certainly impacts the PCBs and materials used to build these circuits. 5G applications are generally split up into two frequency bands; sub-6 GHz and millimeter-wave (mmWave). Most of the initial deployments for 5G technology will be based on the sub-6 GHz band of frequencies; however, there are already mmWave 5G systems, and in the future, there will be more.

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Lightning Speed Laminates: Practical Evaluations of Glass Weave Effect

06-27-2019

There are a few things to consider about glass weave effect. One point of interest is when a laminate is using more than one layer of glass, the glass weave effect is somewhat dampened due to an averaging effect of the glass weave layers with their random alignment to each other. Another point of interest would be when a conductor is much larger than the geometry of the glass bundles, knuckles, and open area. Then, the conductor will experience an averaging of these different Dk values from the glass weave effect. This is typically not a problem for electrical performance.

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Lightning Speed Laminates: How Copper Properties Impact PCB RF and High-speed Digital Performance

05-10-2019

The circuit is a microstrip with a signal conductor on the top copper plane and a ground plane on the bottom of the circuit. Additionally, the concept of skin depth and wave propagation should be considered. Skin depth is the depth within the cross-sectional area of the copper where the majority of the RF current resides and is dependent on frequency. At lower frequencies, the RF current will have a thicker skin depth and use more of the conductor. At higher frequencies, the skin depth is thinner, and less of the conductor is used by the RF current.

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Lightning Speed Laminates: Higher Frequencies Pave Way for Flexible Circuit Materials

04-04-2019

The smaller wavelengths of mmWave frequencies tend to highlight circuit material anomalies at those higher frequencies—anomalies that can also influence the radio frequency (RF) performance of the circuit. Such material anomalies include variations in dielectric thickness, dielectric constant (Dk), copper conductor width and spacing, and copper conductor plated thickness. For mmWave circuits, even small variations in these key material properties can impact electrical performance. Fortunately, flexible circuit technology is typically well controlled for thickness compared to thicker, more rigid circuit board materials.

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2018

Lightning Speed Laminates: Skin Depth and Its Interaction with Final Plated Finishes

12-27-2018

Skin depth is the depth within a conductor where the majority of the radio-frequency (RF) current resides. Imagine looking at a cross-sectional view of a circular wire and being able to see how much current is within that cross-section. If the current is supplied by a battery and is direct current (DC), the amount of current is distributed evenly across the cross-sectional area of the wire. The current density is the same everywhere in the area of that wire.

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Lightning Speed Laminates: Different Aspects of Impedance for PCBs

11-29-2018

Many PCBs are specified to have a controlled impedance value. There are several circuit and material properties that impact the impedance performance of a circuit. Some of these properties are obvious to engineers who have worked with controlled impedance boards over the years. However, even experienced engineers are sometimes surprised to find the level of impact these properties have when looking at all of the things that influence PCB impedance performance. Additionally, there are several issues to consider when making impedance measurements to ensure the values are accurately captured.

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Transitioning from FR-4 to High-Frequency Materials, Revisited

08-29-2018

FR-4 materials have been around a long time, and they work well within the range they are formulated to work. The same can be said for high-frequency, or low-loss materials. However, there is a definite gray area that can muddy the waters when a designer is weighing a possible decision to switch from FR-4 to low-loss materials. High-frequency materials also have inherent properties that can benefit some applications that are not related to high frequencies or low loss.

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Lightning Speed Laminates: Considerations for Comparing Material Data Sheets

07-16-2018

The same piece of material can be tested in two different tests and achieve two different Dk values, and both values may be correct. One reason why that statement can be true is since most materials used in the PCB industry are anisotropic, which means that the Dk is not the same on different axes. Some test methods will evaluate the material in the z-axis (thickness axis) only and other test methods will evaluate the x-y plane of the material for the Dk property. When a material is anisotropic, there should be a different Dk in the z-axis than in the x-y plane.

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Lightning Speed Laminates: Not all PTFE Laminates are the Same

05-11-2018

The first attempt to make PTFE friendlier to the PCB fabrication process was to have it reinforced with layers of woven-glass fabric. Over time, there have been other improvements. For instance, adding ceramic filler was a really good idea. However, it can’t be just any ceramic filler; if the right ceramic is not selected, some properties will benefit, and other properties will degrade. But when the right ceramic filler and woven-glass are combined in a PTFE-based laminate, the circuit fabrication and RF performance issues can be overcome.

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2017

Lightning Speed Laminates: Transitioning From FR-4 to High-Frequency Materials

08-30-2017

The move from FR-4 to a high-frequency material is often necessitated by the loss performance of a circuit. The acceptable amount of insertion loss for a particular circuit can vary greatly from one application to another. The material’s loss is categorized by dissipation factor (Df) and it is rather subjective for what is considered low-loss, mid-loss and high-loss material.

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When Comparing Data Sheets, the Devil is in the Details

07-10-2017

One major issue with understanding the information on data sheets and making appropriate comparisons is the test method category. It is possible to test the same piece of material for dielectric constant (Dk) using two different test methods, and arrive at two different—but correct—answers. Most PCB materials are anisotropic, which means the Dk is not the same on the x-, y- and z-axis, where the z-axis is the thickness of the material. Some test methods will test only the z-axis and other test methods will test the x-y plane.

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Lightning Speed Laminates: Woven Glass Weave Effect: Electrical Concerns and Remedies

05-17-2017

The idea of glass weave effect has been around for many years, and it is a topic that is somewhat controversial. In theory, the glass weave effect is a concern that the structure of the glass fabric can have a negative influence on high-frequency or high-speed digital circuit performance. One concern for the glass weave effect is that if a critical circuit conductor is perfectly aligned to the pattern of bundles and open areas, the conductor will experience different Dk in small isolated areas. It is possible at very high frequencies or extremely high speed digital rates that these isolated Dk differences could have an influence on the circuit performance.

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Lightning Speed Laminates: PCB-Based Antennas and PIM Concerns

01-11-2017

A large variety of PCB-based antenna structures are used at microwave frequencies, and some are used at higher frequencies. A common PCB antenna structure is a microstrip patch antenna. A microstrip structure is a two-layer copper circuit with a signal plane and a ground plane, but it is more common for this type of circuit to be the outer layers of a multilayer circuit.

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2016

The Blending of High-Speed Digital and High-Frequency RF

11-02-2016

When the terms high-speed and high-frequency are mentioned, people think they describe the same issue. But in reality, they can be two very different matters. The term high-speed generally refers to digital technology which transfers data at very high rates. But the term high-frequency is typically related to radio frequency (RF), which involves analog signals moving energy at high frequencies.

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Smaller Circuits--Material Properties and Thermal Issues

08-29-2016

Coefficient of thermal expansion (CTE) is typically considered for PCB reliability, but it can also have an impact on circuit performance for applications exposed to varying temperatures. Due to CTE, a circuit will change physical dimensions when the temperature changes. If the circuit has small features or tightly coupled features, the physical change of the circuit dimensions can cause a shift in electrical performance.

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The Dilemma--Solder Mask for High-Frequency PCBs

07-14-2016

Typically, PCBs with RF traces on the outer layers have minimal or no solder mask in the RF circuitry areas. Many times the solder mask is applied in areas where components are soldered to the PCB but the solder mask is developed away in the areas where conductors have critical RF performance. There are many reasons to avoid solder mask coverage on RF conductors, due to inherent solder mask properties.

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Lightning Speed Laminates: What is Signal Launch and Why Should You Care?

06-01-2016

Signal launch is a term often used to describe how the signal is introduced to the PCB. Many times the signal may be coming to the PCB by a cable and the transition from the cable to the PCB is done through a connector. In the case of RF applications, the cable is typically coaxial and the connector has the same coaxial configuration. The orientation of the electric fields in the connector are different than the orientation of the electric fields in the PCB.

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Why Do Different Test Methods Yield Different Electrical Values?

01-20-2016

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.

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2015

Lightning Speed Laminates: Impact of Final Plated Finish on PCB Loss

11-05-2015

A variety of plated finishes are used in the PCB industry. Depending on the circuit construction and other variables, the plated finish can cause an increase in PCB insertion loss. The plated finish used on the outer ground planes of a stripline circuit have minimal or no impact on insertion loss. However, microstrip or grounded coplanar waveguide circuits, which are common on the outer layers of multilayer high-frequency PCBs, can be impacted by the plated finish for increasing the insertion loss.

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A Brief Overview of High-Frequency Laminates

09-02-2015

The PCB laminates used for high-frequency applications possess several unique attributes that are distinctly different than that of FR-4 materials. Obviously, the electrical properties are better for high-frequency laminates than FR-4; however there are many other beneficial characteristics which may be less obvious for those unfamiliar with these specialty laminates. These other properties lead technologists to sometimes use high-frequency laminates for applications that are not high-frequency.

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The Art of Bending and Forming PCBs

07-15-2015

Flexible circuits are designed to be bendable, but bending rigid PCBs is a little unusual. However, many applications that do not use flex circuit technology will also require bending and forming the circuit. Some of these applications use high-frequency circuit materials to create a circuit in a form that enables improved antenna functionality. Another application involves wrapping a circuit around a structure, which sometimes functions as an antenna as well. John Coonrod explains.

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RF Power Capabilities of High-Frequency PCBs

05-06-2015

I often hear this question: “How much RF power can be applied to a high-frequency PCB?” My answer sometimes surprises engineers. I tell them that they can put as much RF power into the PCB as they want, with the assumption that the PCB does not exceed its maximum operating temperature (MOT). MOT refers to the maximum temperature to which a circuit can be exposed without degradation of critical properties.

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Insertion Loss: A Bigger Concern in High-Speed Digital

03-04-2015

In the past, one of OEM customers’ main concerns when dealing with their PCB fabricators was characteristic impedance. Many times, a PCB design is considered controlled impedance and the PCB fabricator is held to a specification for impedance control.

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Lightning Speed Laminates: Making Connection with Conductor Discontinuities

01-07-2015

Columnist John Coonrod writes, "The title may be confusing for many technologists accustomed to dealing with electrical issues in traditional PCBs, but if you design PCBs that operate at microwave frequencies, it makes perfect sense. With microwave PCB design, it is not uncommon to have a conductor run come to a stop, followed by a space, followed by another conductor run, with the RF energy propagating through the discontinuity without the slightest problem."

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2014

Chilling Out with Conductive Adhesives

10-29-2014

"There are several options to attach heat sinks to PCBs, and TECA materials are being used more often. Even though the other options are usually higher in thermal conductive because they use metal or direct metal-to-metal interface, TECA is generally more consistent for heat flow path, relatively easy to apply, and comparable to the RF electrical performance of other options," writes columnist John Coonrod.

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Environmental Effects on High-frequency Material Properties

09-03-2014

PCBs can be subjected to a variety of environmental conditions, which can cause changes in the material and alter how a PCB operates. For those who are less familiar with circuit material properties, there is often an unrealistic expectation that material shouldn't change electrical performance when subjected to different environments.

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Lightning Speed Laminates: Environmental Effects on High-frequency Material Properties

09-03-2014

PCBs can be subjected to a variety of environmental conditions, which can cause changes in the material and alter how a PCB operates. For those who are less familiar with circuit material properties, there is often an unrealistic expectation that material shouldn't change electrical performance when subjected to different environments.

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When is Controlled Impedance Important?

07-02-2014

The importance of controlled impedance hinges upon many variables, such as the PCB’s characteristics and how it is to be used. A PCB designed for digital applications will often have different impedance requirements than a circuit designed for RF applications. Within both of these categories, however, there are sub-categories of specific types of applications.

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Lightning Speed Laminates: When is Controlled Impedance Important?

07-02-2014

The importance of controlled impedance hinges upon many variables, such as the PCB’s characteristics and how it is to be used. A PCB designed for digital applications will often have different impedance requirements than a circuit designed for RF applications. Within both of these categories, however, there are sub-categories of specific types of applications.

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Hybrid High-frequency Multilayer PCBs

05-14-2014

A hybrid multilayer PCB uses materials with significantly different critical properties than those associated with a traditional multilayer PCB. A hybrid could use a mix of FR-4 materials with high-frequency materials, or a mix of different high-frequency materials with different dielectric constants. As hybrid construction becomes more popular the benefits and challenges should be better understood.

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Lightning Speed Laminates: Hybrid High-frequency Multilayer PCBs

05-14-2014

A hybrid multilayer PCB uses materials with significantly different critical properties than those associated with a traditional multilayer PCB. A hybrid could use a mix of FR-4 materials with high-frequency materials, or a mix of different high-frequency materials with different dielectric constants. As hybrid construction becomes more popular the benefits and challenges should be better understood.

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FAQ - Microwave PCB Materials

03-26-2014

The landscape of specialty materials changes so quickly that it can be hard to keep up. As a result, PCB designers are inundated with data about microwave PCB materials. But very often, it's difficult to find useful information regarding these specialty substrates. This month, Columnist John Coonrod presents some of the most frequently asked questions about microwave materials.

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Lightning Speed Laminates: FAQ - Microwave PCB Materials

03-26-2014

The landscape of specialty materials changes so quickly that it can be hard to keep up. As a result, PCB designers are inundated with data about microwave PCB materials. But very often, it's difficult to find useful information regarding these specialty substrates. This month, Columnist John Coonrod presents some of the most frequently asked questions about microwave materials.

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Thermal Management of High-Frequency PCBs

01-29-2014

Thermodynamics can be a difficult enough subject to understand. But when combined with high-frequency PCB design and fabrication, it can really get complicated. Thermal management of PCBs has received much attention over the past few years and it will most likely continue as new technology pushes the limits of this issue.

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2013

Lightning Speed Laminates: High-frequency Materials for Lead-free Soldering

12-11-2013

Some OEMs' qualification procedures dictate that PCBs be subjected to multiple passes through a lead-free solder reflow cycle. The materials that make up the PCB can have a major impact on the ability of the PCB to survive the lead-free solder evaluations, and some materials perform better than others. Columnist John Coonrod explains.

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Lightning Speed Laminates: High-Frequency Laminates for Hybrid Multilayer PCBs

11-06-2013

A hybrid multilayer is a PCB construction that uses dissimilar materials. Reasons for using dissimilar materials include: Improving reliability, reducing cost, optimizing electrical performance, and improving manufacturability. For the past several years, hybrid multilayer PCB construction has flourished in the arena of high-frequency RF applications.

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Lightning Speed Laminates: PCB Materials that Empower Signal Integrity

10-09-2013

Impedance control for PCBs is typically a concern when considering signal integrity. The factors impacting impedance control are circuit thickness, conductor width and spacing, copper thickness and the dielectric constant (Dk) of the material. Columnist John Coonrod explains.

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Lightning Laminates: Efficient Simulation Using High-Frequency Printed Circuit Materials

07-31-2013

Many different types of circuit simulation software are available, and each one is tailored to meet a different need. Dielectric constant and dissipation factor are two of the more important substrate properties to consider during simulation. The Dk value is provided on material datasheets, but Dk value can vary substantially depending on the test method.

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High-Frequency Flexible Circuits

07-03-2013

Flexible circuits come in many variations. When considering a flexible circuit application which requires good high-frequency performance, there are more material choices available besides the traditional flexible circuit material. John Coonrod of Rogers Corporation explains.

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2012

Lightning Speed Laminates: High-Frequency Laminates - Why the Wide Range of Dielectric Constants?

10-29-2012

In the world of high-frequency PCB circuit materials, dielectric constant (Dk) ranges typically from two to 10. Multiple reasons for this range exist, and it is highly dependent on the application of the circuit.

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2011

Should You Use a High-Frequency Laminate Instead of FR-4?

08-02-2011

They may be reliable and affordable, but there are several reasons why epoxy-based FR-4 circuit board materials are not the answer for every circuit, especially for a wide range of high-frequency designs. For designs that must perform over temperature, FR-4 and high-frequency laminates can provide very different results.

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