I started my PCB industry career in 1987 in the Flexible Interconnect Division of Rogers Corporation. That division of Rogers was sold many years ago, but I learned a tremendous amount working there. My first responsibility was learning flexible circuit design and I was in the ideal surroundings to learn.
I had responsibility for a new product from beginning to end of life. I designed flexible circuits, many times from scratch using schematics and mechanical drawings. In the beginning phase of the product life cycle, I designed the circuit and panel layout, designed the tooling, and established the build sequence for the prototype. In my learning stage and after all the tooling arrived, I went to the prototype department and helped them build the circuits from beginning to end. I cannot express how valuable that experience was, seeing the tooling I designed, working with the various process steps in the build sequence, and finally shipping the parts.
After multiple revisions of prototype circuits, the customer would usually go into high-volume production, and it was a very different circuit design discipline for designing the circuits and tooling for mass production than it was for prototype circuits. High-volume production requires the designer to consider all customer circuit properties and specifications, and perform CpK analysis to ensure the circuit will have good manufacturing yield at high volumes.
Over the years, I’ve taught several engineers circuit design. I am convinced that if the designer has a very good understanding of the different process steps it takes to make a circuit, they will be a much better designer. As an example, if a designer encounters a need for exception when designing for annular ring, the designer who knows the circuit fabrication process well will make a much better design decision than a designer who only knows the annular ring design theory.
My circuit design experience also included rigid board design, which was different than flex circuit design, but the basics were the same. After my circuit design experience, I was a technical support engineer for the flex circuit materials division of Rogers and later a technical support engineer for the high frequency circuit materials division of Rogers. In the technical support role, I worked directly with our circuit fabricator customers and that caused me to work with many processes for making flex circuits and rigid high-frequency PCBs. Also, because I worked at a materials company, I had to understand the material engineering aspects of the circuit material, how it interacted in different PCB processes, and its end-use. Additionally, I was a part-time student in electrical engineering at ASU during most of my engineering experience in the industry. My diverse engineering experience was influenced by the engineering theory while I was working in the industry. The overall diversity of my engineering background has proven to be extremely beneficial for me at Rogers Corporation.
My point is that having a very diverse engineering background is extremely beneficial for understanding PCB designs. I think most engineers today probably do not have the opportunities I had. I would strongly recommend design engineers get very involved in circuit processing, manufacturing, and circuit materials engineering. When a PCB designer understands these different aspects of PCB technology, they will certainly be able to deliver circuit designs that will be more robust and have better manufacturing yields. The high-volume manufacturing yields of a PCB are directly related to profit and that is the connection between good circuit design and a successful circuit-product.
An aspect related to PCB design is software. There are many different software programs that the circuit designer needs to work with, and some of these programs allow the designer to run simulations. This is another tool for designers to build on their design expertise. The simulation software can be used to model a circuit design, change different features of the circuit, run the simulation again, and learn what the design change will do for circuit performance. The benefit of running many different simulations can be very educational for a circuit designer.
Another aspect of engineering that can be beneficial for the PCB designer is having a good understanding of practical statistics. Since PCB design is focused on the customer’s specifications, it is beneficial for the designer to have a good understanding of standard deviations, CpK analysis, histograms, box plots, etc. These statistical tools help with the circuit design process, but they can also help with optimizing yields, troubleshooting, failure analysis, and design modifications.
In summary and in my opinion, the following topics need to be well understood for a PCB design engineer to gain expertise: circuit materials, major processes in PCB fabrication, defining stackups, understanding the PCB build sequence, statistics, and simulation software. Additionally, there are many resources for PCB design at IPC.org, IEEE.org, and the websites for companies that make PCB software for routing and stackups.
This column originally appeared in the August 2021 issue of Design007 Magazine.
08-12-2021
When a PCB designer understands the different aspects of PCB technology, they will certainly be able to deliver circuit designs that will be more robust and have better manufacturing yields. The high-volume manufacturing yields of a PCB are directly related to profit and that is the connection between good circuit design and a successful circuit-product.
View Story06-17-2021
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.
View Story04-21-2021
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.
View Story02-18-2021
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.
View Story12-14-2020
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.
View Story10-20-2020
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.
View Story08-31-2020
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.
View Story07-22-2020
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.
View Story04-30-2020
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.
View Story02-15-2020
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.
View Story12-26-2019
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.
View Story10-18-2019
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.
View Story06-27-2019
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.
View Story05-10-2019
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.
View Story04-04-2019
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.
View Story12-27-2018
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.
View Story11-29-2018
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.
View Story08-29-2018
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.
View Story07-16-2018
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.
View Story05-11-2018
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.
View Story08-30-2017
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.
View Story07-10-2017
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.
View Story05-17-2017
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.
View Story01-11-2017
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.
View Story11-02-2016
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.
View Story08-29-2016
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.
View Story07-14-2016
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.
View Story06-01-2016
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.
View Story01-20-2016
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.
View Story11-05-2015
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.
View Story09-02-2015
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.
View Story07-15-2015
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.
View Story05-06-2015
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.
View Story03-04-2015
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.
View Story01-07-2015
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."
View Story10-29-2014
"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.
View Story09-03-2014
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.
View Story09-03-2014
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.
View Story07-02-2014
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.
View Story07-02-2014
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.
View Story05-14-2014
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.
View Story05-14-2014
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.
View Story03-26-2014
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.
View Story03-26-2014
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.
View Story01-29-2014
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.
View Story12-11-2013
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.
View Story11-06-2013
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.
View Story10-09-2013
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.
View Story07-31-2013
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.
View Story07-03-2013
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.
View Story10-29-2012
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.
View Story08-02-2011
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.
View Story
