When Comparing Data Sheets, the Devil is in the Details

Many PCB fabricators and designers will often compare the properties listed on material data sheets to assist with the material selection process. Of course, this is a wise thing to do when choosing materials, however data sheets need to be reviewed carefully. Comparing information between data sheets may be misleading since there are many variables that go into collecting the data.

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. So it is possible to test the same piece of material, using a test method that evaluates the z-axis of the material for Dk, and get a different answer when testing that same material using a procedure that tests the x-y plane of the material. Both answers could be correct, but this can cause confusion when comparing data sheets. It is very important to ensure the test methods are the same when comparing similar materials.

Electrical properties can be misleading since the Dk and dissipation factor (Df) of all circuit materials is frequency-dependent. These properties will naturally change with a change in frequency. Again, when comparing data sheets from similar materials and ensuring the test methods are the same, the subtle fact that the test frequency is different can alter the comparison. All materials will have a lower (better) Df at lower frequencies, and if a comparison is done with material A at 2.5 GHz and material B was tested at 10 GHz, then it is not an equal comparison because the lower-frequency test will naturally have a lower Df when compared to testing that same material at a higher frequency.

To read this entire column, which appeared in the June 2017 issue of The PCB Design Magazine, click here.

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2017

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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