Microstrip transmission lines are widely used throughout the high-frequency industry, for both active and passive circuits. They are building blocks for many components, including couplers, filters, resonators, and power dividers/combiners, along with various coupled features formed from microstrip lines that help transfer energy from one point in the circuit to another. Of course, the printed circuit board (PCB) material also plays a major role in how these microstrip transmission lines perform their duties in these RF and microwave circuits, and it can be helpful to understand how certain PCB material characteristics contribute to the ways that microstrip transmission lines and their coupled features perform in these different high-frequency components.
Circuit board materials are selected by designers for a number of reasons, but usually with dielectric constant (Dk) at the top of the list. Maintaining consistent impedance for microstrip lines depends on consistent Dk for a PCB material since a change in PCB Dk at any point in the material will result in a change of impedance for the microstrip transmission lines at that point in the material. Using microstrip coupled features can complicate the choice of circuit materials since such coupled features typically exhibit different, even- and odd-order, wave modes as a function of the PCB material and circuit design. For electric fields between microstrip coupled features, the even-order modes use mainly the thickness or z-axis of the material, while the odd-order modes of the electric fields are mostly in the planar or length-width dimensions (x and y axes) of the PCB material as well as using some z-axis properties.
Ideally, PCB materials would exhibit tightly consistent Dk values in their x, y, and z dimensions, and modern computer-aided-engineering (CAE) software tools typically assume that they do. But in the real world, circuit materials more typically have differences between the Dk value through the thickness (z axis) of the material and the Dk value across the length and width (x and y axes) of the material. PCB materials are referred to as anisotropic in nature when they have different Dk values in the different axes of the material. In contrast, a PCB material with consistent Dk values in all axes is considered isotropic in nature.
Why the differences in Dk values through the material, and what effects can they have on different circuit designs? Most commercial PCB materials are at least slightly anisotropic in nature, due to the composition of those materials. They are formed with dielectric resin materials and some filler material, such as a glass or a ceramic filler, used for reinforcement and attribute adjustments, but which contribute to Dk variations. The manner in which fillers can orient within a substrate during the laminate manufacturing process accounts for the isotropic or anisotropic behavior for some laminates. Other laminates may have glass weave for reinforcement, which can cause Dk variations; the type of glass weave can impact the anisotropic behavior of the laminate. Combining the effects of these potential filler orientation variations with the effects of the glass weave can cause some laminates to exhibit higher variations in Dk in the different axes of the material, making them more anisotropic.
Designers working on microstrip circuits with coupled features often lean towards the use of PCB materials with higher Dk values, since those materials provide more efficient coupling of electric fields than their lower-Dk counterparts. In addition, since circuit dimensions required for a given impedance shrink on PCB materials with higher Dk values, smaller components can be developed with these higher-Dk materials. Unfortunately, the higher-Dk circuit materials also tend to be more anisotropic in nature than lower-Dk circuit materials, adding to the challenge of designing filters, directional couplers, resonators, and other high-frequency circuits based on microstrip transmission lines with coupled features.
PCB materials with high Dk values, typically 10 or more as measured in the z-axis of the circuit material at 10 GHz, can suffer from serious anisotropic characteristics that can challenge even the best of CAE simulation and design software programs. Circuit materials with a high degree of anisotropy may have, for example, a Dk of 10 through the thickness (z-axis) of the material as measured at room temperature and 10 GHz, but the Dk in the x-y plane of the same material may be different by 10% or 15%. When designing a microstrip circuit with coupled features, such as a filter, and using a CAE program, these variations in Dk values can be accounted for as a form of statistical approximation, but specific differences in Dk values, as might occur at a critical microstrip coupled feature, may not be precisely predicted in the CAE program. The variations in Dk typically result in performance variations in the final circuit, which yield differences between performance parameters predicted by a CAE program and performance parameters measured for a prototype with test-and-measurement equipment.
For designers of microstrip circuits with coupled features, variations in PCB material Dk can be detrimental to achieving expected performance results, fortunately commercial PCB materials with high Dk values are available with relatively isotropic natures. For example, the TMM® 10i circuit materials from Rogers Corp. are quite isotropic compared to other circuit materials with Dk value around 10.0. These are ceramic hydrocarbon thermoset polymer composite materials well suited for both microstrip and stripline high-frequency circuits. The TMM 10i circuit materials exhibit a Dk of 9.80 which remains within +/- 0.245 of 9.80 in all three axes of the material. (The Dk measurements are performed at 10 GHz in the z-axis of the material according to IPC-TM-650, method 184.108.40.206.) For designers in need of a PCB material with even higher Dk value, the TMM 13i circuit material offers a Dk of 12.85 +/- 0.35 as measured at 10 GHz in the z axis using the same test method.
These circuit materials are more isotropic than most, with differences of typically 3% or less between the Dk value in the z-axis of the material and the x-y plane of the material. Compare this to the circuit materials noted earlier with differences of 10% or more. In addition to their isotropic natures, the TMM 10i and TMM 13i materials feature coefficients of thermal expansion closely matched to that of copper, supporting production of such circuit features as plated through holes (PTHs) with high reliability.
Of course, CAE design and simulation software continues to advance, and get better at anticipating such variations as found in anisotropic PCB materials. But for some designs, such as those with microstrip transmission lines and coupled features, even small variations in Dk can be disruptive. For designers working with microstrip coupled features and hoping to avoid surprises, the right choice of PCB material can help.
Do you have a design or fabrication question? John Coonrod and Joe Davis are available to help. Log in to the Rogers Technology Support Hub and “Ask an Engineer” today.
Dr. Xinhe Tang delivered two new papers on hybrid substrates at PCIM Asia 2014 and PCIM Europe 2014. Download your copy today from the Power Electronic Solutions Design Support Hub.
PCIM Asia 2014: Baseplate Cooling of Power Modules with Hybrid Substrate
The demand for fluid cooling of high power semiconductors increases continuously. Baseplate cooling with PinFin or “ShowerPower®” (patented technology by Danfoss) has attracted more attention. Previously, a hybrid substrate of directly bonded copper and adhesively bonded aluminum was introduced and the manufacturing technology developed. The hybrid substrate has been further developed and characterized in the present work.
The material combination and back side structure of Al-baseplate have been optimized to reduce thermal and mechanical strain. A power module has been built using the hybrid substrate and direct baseplate cooling of the power module has been carried out.
This paper reports on the results relating to thermal shock cycling, heat treatment, and flatness at various temperatures, as well as performance testing and reliability against chemical media involved in the manufacturing process. Finally, power module cooling using the hybrid substrate is discussed.
PCIM Europe 2014: Hybrid Substrate: A Future Material for Power Semiconductor Modules
The increasing level of integration in semiconductor power modules has led to higher power densities that, in turn, result in an increased need for better thermal management. Lately, liquid cooling is attracting more attention as a method to ensure the higher performance and reliability of these modules.
This paper introduces a hybrid substrate of copper, ceramic, and aluminum that combines the thermal and electrical performance of the copper with the corrosion resistance of aluminum using a ceramic dielectric. In this manner, the aluminum can directly contact the cooling water, improving cooling efficiency without the worry of corrosion. The copper portion can be used for the circuit, leveraging its superior current carrying and heat spreading capabilities.
This paper reports on the work of developing such a hybrid substrate by bonding aluminum to the back side of DBC using an adhesive, and explores its application in power electronics.
Evaluating the cost of a product throughout its lifetime isn’t new to the rail industry. Whether you call it Cost of Ownership, Product Life Cycle Cost Analysis, Life Cost Analysis (LCA), Whole Life Cost, or “Cradle to Grave” Costing, establishing a cost of ownership is important.
Many municipalities and governments require a cost of ownership as part of a tender offering and several transit authorities have expanded the criteria to include end-of-life costing and the triple bottom line of the cost impact upon environmental sustainability.
If you’re looking for a quick way to calculate the cost of ownership of rail seat cushions, this free Seat Cushion Cost of Ownership Tool allows you to simulate a specific scenario by entering the size of the seat cushion, number of seats per car, and number of cars in the fleet as shown below.
For more advanced options, click show advanced options and the below specification will appear. The use will be allowed to enter in the average number of years between refurbishments, cost of competitive materials, labor rates, estimated number of replacements, need for fire barriers, and revenue loss. The calculator presents a picture of the total cost of ownership for each material along with metric tons of material that will be designated for landfill over the time period.
Rogers Corporation, the manufacturer of BISCO MF1 open cell silicone foam specially formulated for rail car seating cushions launched the Seat Cushion Cost of Ownership Tool. It will be up to the user to add in the hidden costs (if applicable).
Now you can access Rogers’ PCB materials resources with the ROG Mobile App. Quick and easy access to calculators, literature, technical papers. You can even request samples on your smartphone or tablet
- The app has tools and technical information to assist you with Rogers printed circuit board materials.
- The Microwave Impedance Calculator assists with microwave circuit design in predicting the impedance of a circuit made with Rogers High Frequency circuit materials and also provides capabilities for predicting transmission line losses.
- The ROG Calculators assist RF engineers with thermal and mechanical simulations for microwave PCB designs.
- Data sheets and fabrication guides can be downloaded and material samples can be ordered.
ROG Mobile for iPhone and iPad devices:
Available for the iPhone and iPad in the Apple App Store
ROG Mobile for Android devices:
Available for Andoid devices in Google Play
In this video, John Coonrod discusses why there are so many different dielectric constants (Dk) that are used in the microwave printed circuit board industry.
Send us questions/comments by tweeting us @Rogers_ACM!