The question of when to transition from FR-4 material to a higher performing material is typically not as straightforward as one may assume. Also, when the transition is obvious, a lot of unexpected issues may need to be addressed. Some of these issues can be related to the circuit design and others are related to the PCB fabrication process.
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. From my experience, I would categorize materials by Df as the following: high-loss material has a Df of 0.015 or greater and that is a pretty common value for many FR-4 materials. There are some high-performance FR-4 materials which have better loss, and some of these have Df value of 0.010. I would consider these materials to be categorized mid-loss materials. Low-loss materials typically have Df values of 0.004 or less and extremely low-loss materials have a Df of 0.002 or less. There is a grey area between low-loss and mid-loss material where the range of Df is from about 0.004 to 0.010.
Besides the differences in loss, significant design changes may be necessary when switching to high-frequency materials. These changes are mostly due to FR-4 materials having a dielectric constant (Dk) value of about 4.2 to 4.5, while many high-frequency laminates have considerably lower Dk values. The change in Dk can cause impedance differences, so design changes to the conductor routing are often necessary. Additionally, the high-frequency laminate may not have the same substrate thickness offerings as the FR-4 materials and the thickness difference can require design changes in order to maintain the desired impedance and other critical electrical properties.
Another issue is related to PCB fabrication processing. Some high-frequency materials are PTFE-based, and those types of materials can require very different processing parameters as compared to FR-4 materials.
To read this entire column, which appeared in the August 2017 issue of The PCB Design Magazine, click here.
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.
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
