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The Rog Blog is contributed by John Coonrod and various other experts from Rogers Corporation, providing technical advice and information about RF/microwave materials.

Taking A Measure Of Copper Surface Roughness

Conductor surface roughness in printed-circuit boards (PCBs) is a material parameter that should not be overlooked. As detailed in the previous Blog in this series, the surface roughness of a PCB’s conductor layer can have a great deal of impact on signal losses through the conductors. If the effects of conductor surface roughness are not accounted for at the design stage, when using a commercial computer-aided-engineering (CAE) software simulation program, the predicted performance results of the simulations can deviate. These deviations can be significant from the actual performance measured from a designed prototype circuit. The differences can add up to lost design time, added design iterations, and added time and expense when creating a new circuit.
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Sizing Up PCB Laminate Surface Roughness

Designing an RF/microwave circuit requires some knowledge of printed-circuit-board (PCB) qualities, especially when selecting a PCB material for a particular application. Modern computer-aided-engineering (CAE) simulation tools can help predict the electrical performance of circuits on different PCB materials, using material parameters such as relative dielectric constant in the calculations. But one PCB material parameter that is often overlooked in the design process is the surface roughness of the conductors. In the past, conductor surfaces were assumed to be perfectly smooth. What happens when they are not?
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Modeling A PCB’s Thermal Behavior

Temperature effects on a printed-circuit board (PCB) can make it difficult to achieve target performance goals, even with the best PCB substrate materials. Modeling these effects takes imagination—to visualize different sources of heat, for example, and thermal paths where the heat might travel. It also requires an understanding of both thermal-mechanical and electromagnetic (EM) relationships to account for the assortment of variables that can influence PCB performance with changing temperatures. As a result, modeling thermal effects on PCB performance combines predictions provided by the heat diffusion equation as much as from Maxwell’s equations for EM fields.
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Picking the right PCB for lead-free processing

The goals of the Restriction of Hazardous Substances (RoHS) directive by the European Union (EU) for lead-free printed-circuit-board (PCB) processing are worthwhile, but have impacted how electronic circuits are designed and manufactured. For one thing, lead-based solders had lower melting points than lead-free solders. The peak solder temperature for lead-free solders is typically +260°C. As a result, PCB materials must handle higher temperatures during lead-free-solder processing and rework. Understanding what happens to a PCB at those elevated temperatures can help guide the task of selecting PCB materials for lead-free-solder processing.
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Planar Resistors Build On Reliability

Planar resistors can be fabricated along with circuit patterns on selected high-frequency laminate materials. By executing processing steps not unlike those that form circuits on printed-circuit boards (PCBs), embedded planar resistors can be added to a high-frequency PCB. They can be made with precise values, with tight tolerances, and with reasonable power-handling capabilities. They can replace discrete resistors with their associated assembly and reliability issues, in many high-frequency applications. Planar resistors are well suited to compact designs, including in multilayer circuits where they can minimize or eliminate plated through holes for resistors.
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Learn To Apply Design Dk

Dielectric constant (Dk) is one of the most essential of printed-circuit-board (PCB) material parameters. Circuit designers rely on it for determining such things as impedances and the physical dimensions of microstrip circuits. Yet, it is not unusual to see a laminate data sheet with different values of Dk for the same material, such as a process Dk and a specification Dk. A material supplier may even recommend an additional value of Dk, to be used in computer-aided-engineering (CAE) software simulators. Why all the different numbers and is there one value of Dk that is the one to trust when designing a circuit?
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Test Dielectric Constant With Microstrip Circuits

Circuit designers select laminates for printed-circuit boards (PCBs) by merit of relative dielectric constant (Dk), among other parameters. Suppliers of laminates furnish Dk values on their data sheets and web sites, but designers often prefer the reassurance of knowing the Dk value as it relates to their specific application. The last blog explored the way that materials manufacturers typically use four techniques to evaluate the Dk of a dielectric material in its “raw” form, meaning without circuits. This blog will explore some common methods that materials users employ for determining a laminate’s Dk value and focus on a practical method.
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Detecting The Dk Of “Raw” Circuit Boards

RF/microwave designers have a wealth of circuit-board materials from which to choose, which can be good and bad. Having so many options can make the selection process difficult, so that many designers start with relative dielectric constant--Dk for short--as a key sorting parameter. As was pointed out in the last blog, the value of Dk can depend as much on the material composition as the type of test used to measure it. Tests for Dk can be performed either on “raw” laminate material, without circuits formed on it, or by making use of test circuits that have been fabricated on the laminate and measuring the electrical responses of those circuits. This blog will address the four most widely used Dk tests in the first group; the next blog will examine four popular Dk tests in the second group.
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Measuring Performance Of Microwave Substrates

Circuit-board material parameters are printed on every laminate data sheet. They describe the electrical and mechanical characteristics of a PCB material, including such parameters as relative dielectric constant, dissipation factor, coefficient of thermal expansion (CTE), and thermal conductivity. Design engineers count on these values to be accurate, since their circuits depend on them. But the accuracy often depends on the test method used to measure a material parameter. Even when different laboratories perform the same test on the same material, they can obtain different results. This blog will provide a brief overview of the different tests used to evaluate a printed-circuit material’s characteristics; the next several blogs will go into greater details on specific tests, and will explain how various test results impact the way PCB materials are modeled with modern computer-aided-engineering (CAE) software tools.
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Comparing RF Circuit Material Processing Costs & Performance

Performance requirements typically guide the selection of a PCB material. Some applications may also be cost-sensitive, and require evaluation of the total costs of choosing a circuit material. This includes the cost of the material as well as costs associated with processing the material. For example, FR-4 is a low-cost material with minimal processing costs. However, its performance is also low relative to some higher-costing materials, such as PTFE- or hydrocarbon-based circuit materials, although these materials can have considerably different processing requirements and associated costs. By considering the costs of the material as well as its processing requirements, it’s possible to determine if “you get what you pay for” truly applies to circuit materials.
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