This post authored by John Coonrod originally appeared on the ROG Blog hosted by Microwave Journal.

The use of circuit defects is a growing trend in high-frequency circuit design

Successful microstrip circuit design requires a great many factors to fall neatly into place, not to mention high consistency and performance from the printed-circuit-board (PCB) material. Maintaining tight microstrip circuit tolerances helps ensure predictable impedances from those transmission lines, assuming a consistent relative dielectric constant across the PCB. With so many challenges in designing and fabricating high-performance microstrip circuits, why would so many RF and microwave circuit designers currently be interested in adding “defects” to their circuits? As strange as it may sound, the use of circuit defects is a growing trend in high-frequency circuit design, especially for passive circuits such as filters. More precisely, the trend is in the increased use of defected ground structures (DGSs) and defected microstrip structures (DMSs) to alter the responses of microstrip circuit designs.

What are DGS and DMS forms?

Just what are these DGS and DMS forms, and does incorporating them into a high-frequency circuit change the way the PCB material should be specified? Although both types of structures are referred to as “defects,” they are well planned and calculated, essentially resonant gaps or slots that are placed in a PCB’s ground plane or transmission lines, respectively, to achieve modifications in impedance. By inserting a gap in the ground plane underneath a narrow microstrip transmission line, for example, a much higher impedance can be achieved than with a traditional microstrip transmission line having the same dimensions. Impedance transitions are useful in a number of different RF/microwave circuit designs, including lowpass filters which can be formed with a cascade of high- and low-impedance sections.

A DGS allows a circuit designer to insert a transmission zero or notch anywhere in the transfer function of a microstrip transmission line. This type of added attenuation can sharpen the rolloff of a bandpass filter or deepen the stopband attenuation of a lowpass filter. Designers have employed this phenomenon in both active and passive circuits. In active circuits, DGS circuit elements have helped improve the efficiency of power amplifiers. In passive circuits, they are often used to fine-tune a filter’s response or improve the performance of a patch antenna. Even simple DGS and DMS forms can help extend a filter’s stopband by adding attenuation at the resonant frequencies represented by the physical gaps in the PCB’s ground plane or microstrip transmission lines. In recent years, designers have become quite creative in exploring the possibilities of DGS and DMS forms and shapes in high-frequency circuits, eschewing simple slots or gaps in a ground plane, for example, for dumb-bell shapes and meander lines in an attempt to achieve higher-impedance transitions in smaller PCB areas.

DGS and DMS circuit elements must be treated with care

As part of distributed circuit design, DGS and DMS forms are treated as distributed circuit elements, more or less as inductors. They can replace a conventional distributed transmission-line inductor while possibly minimizing circuit loss. In terms of practical PCB fabrication, however, DGS and DMS circuit elements must be treated with care. While they can be used to add selective notches in a circuit’s frequency response, they will not improve passband insertion loss, which will largely be a function of the PCB’s material parameters and the transmission-line characteristics. The best way to achieve low passband insertion loss in a filter is still by choosing a low-loss laminate, such as RO4000® or RO3000® circuit materials, which are available with a wide range of dielectric constants and dissipation factors as low as 0.0013 at 10 GHz. In addition, by using circuit materials with a high relative dielectric constant, such as RO3010™  or RT/duroid® 6010.2LM laminates, both with dielectric-constant value of 10.2 at 10 GHz, the benefits of DGS and DMS circuit elements can be readily applied to edge-coupled filter designs.

DGS and DMS circuit elements can increase the size of a circuit, especially when these structures are fabricated as more element forms, such as meander lines. A DGS essentially acts like a resonant circuit in parallel with the transmission line above it, and the dimensions of the DGS and its position relative to the transmission line will determine the impact of the DGS on the circuit’s response. The precision with which the DGS or DMS form can be fabricated can also affect the circuit’s response. If the DGS is thought of as an equivalent circuit element, such as an inductor, in a distributed circuit design, then the value of that inductor will change according to the dimensional tolerances of the DGS and its alignment to the transmission line. Even the consistency of the PCB’s dielectric constant can play a role in the inductance of a DGS within a circuit. When applying these types of “defects” to a high-frequency circuit, they should be designed well within microstrip fabrication tolerances and taking into account the consistency of the PCB material’s relative dielectric constant.

How do you integrate DGS and DMS into practical RF/microwave circuit designs?

If DGS and DMS circuit elements are so finicky, how is it possible to integrate them into practical RF/microwave circuit designs? For the most part, both types of structures are modeled by means of electromagnetic (EM) simulation software, based on a number of different analytical methods, such as the method of moments (MoM) or the finite-element method (FEM). Through EM simulation, it is possible to estimate the effects of DGS and DMS placement, dimensional tolerances, and even how using circuit laminates with different values of permittivity affect expected performance. However, it is only fair to note that DGS elements can act as radiators as well as resonators. A slot in a laminate’s ground plane essentially forms a slot antenna. Fortunately, most of the incident energy of a DGS slot at its resonant frequency is reflected back into the circuit’s transmission lines, although depending upon the circuit topology, some of this energy can be coupled into different circuit features. Such coupling effects are difficult to model in a practical EM simulation.

This post has only touched upon some of the unconventional structures being used by RF/microwave circuit designers in search of improved performance at higher frequencies. A future post by John will return to this topic with a closer look at some of the other stripline and microstrip circuit structures that are finding application in high-frequency circuits, including torroidal inductor structures using holes in the ground plane, substrate integrated waveguide (SIW), and split-ring resonators as used in terahertz-frequency metamaterials. And for those involved in automotive electronics, John’s next post will explore the role of PCB materials in helping automotive manufacturers meet their performance and cost budgets for automotive electronic systems.

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.

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John Coonrod has written so many useful blog posts over the past couple of years.  We wanted to pull it all together with this index to make it easy to find his information as it relates using advanced materials in PCB/microwave/RF design.  Here we go:

May 1, 2012:  PCB Advances Drive Automotive Applications

April 4, 2012: PCB Considerations For Defected Structures

March 20, 2012: Learning To Launch Onto Different Circuit Thicknesses

March 5, 2012: Matching Materials To Millimeter-Wave Circuits

February 8, 2012 Digging Out The Details On Embedded Capacitance

January 23, 2012 Choosing A Laminate That Matches The Model

December 15, 2011 Measurements Help In Sorting Materials

November 29, 2011 Match Material Specs To Application Needs

November 11, 2011 What is Design Dk?

November 3, 2011 Selecting A Suitable High-Frequency Laminate

October 17, 2011 Bending and Forming RF/Microwave PCBs

September 27, 2011 Aiming For The Perfect Wire Bond

September 9, 2011 Taking A Measure Of Copper Surface Roughness

August 23, 2011 Sizing Up PCB Laminate Surface Roughness

August 3, 2011 Modeling A PCB’s Thermal Behavior

July 13, 2011 Picking the right PCB for lead-free processing

June 28, 2011 Planar Resistors Build On Reliability

June 5, 2011 Learn To Apply Design Dk

May 26, 2011 Test Dielectric Constant With Microstrip Circuits

May 6, 2011 Detecting The Dk Of “Raw” Circuit Boards

April 25, 2011 Measuring Performance Of Microwave Substrates

April 1, 2011 Comparing RF Circuit Material Processing Costs & Performance

March 14, 2011 Controlling Conductor Losses In Coplanar Transmission Lines

February 24, 2011 When Digital Signals Reach Microwave Frequencies

January 27, 2011 Transmission-Line Modeling Tool: Free Downloadable Software

January 11, 2011 Substrate Anisotropy Affects Filter Designs

December 20, 2010 Microstrip Versus Stripline: How To Make The Choice

November 9, 2010 What Is Outgassing And When Does It Matter?

November 4, 2010 Thinner Materials Help Target Higher Frequencies

October 13, 2010 Selecting Substrates For Printed-Circuit Antennas

September 24, 2010 Picking PCB Materials for Power Amplifiers

August 30, 2010 Understanding the true meaning of dielectric constant

August 3, 2010 FR-4 Versus High Frequency Laminates

Stay tuned, as we will keep adding to this index over time.  Thank you!

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

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This post authored by John Coonrod originally appeared on the ROG Blog hosted by Microwave Journal.

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.

A main concern for circuit laminates at high temperatures is maintaining a strong copper/dielectric bond. That bond can weaken when exposed to elevated temperatures or when subjected to the thermal gradients that occur during processing. Strong copper/dielectric bonds are essential for PCB reliability. Another concern, particularly in multilayer-board (MLB) assemblies, is the electrical and mechanical stability of the PCB materials with exposure to elevated and changing temperatures.

Often, mixed dielectric materials may be used to form a MLB assembly. In a high-temperature lead-free-solder process, RO4000® PCB materials from Rogers Corporation, for example, are popular laminates when high performance is required, and are often combined with high-temperature FR-4-type materials for handling less critical signal chores in the MLB. The RO4000 series of materials includes RO4003C™ and RO4350B™ laminates as well as companion bondply materials.

What are the dangers when mixing and matching different materials such as RO4000 and FR-4 laminates using a lead-free-solder process? The answers can be found by evaluating how temperature affects different PCB materials.

Suppliers of high-frequency PCB materials provide numerous data-sheet parameters to describe the electrical and mechanical characteristics of their materials. Some of these parameters can also be used as “predictors” to determine the suitability of a circuit laminate for lead-free-solder processes. Some of these lead-free predictors include the glass transition temperature (Tg), the coefficient of thermal expansion (CTE) in the z-axis of the material, the decomposition temperature (Td), and the time for the material to delaminate at a specific temperature. What must be known about a PCB material is how many reflow cycles it can withstand at higher temperatures and whether multiple temperature cycles have additive detrimental effects on the material, for example, by reducing plated-through-hole (PTH) reliability.

The Tg of a material is the temperature at which the material undergoes a transition from a rigid state to a soft state. While it is a useful parameter for evaluating the thermal robustness of a material, it is not by itself a good indicator of a material’s suitability for lead-free-solder processing. Typically, circuit-board materials for lead-free-solder processing have a Tg of greater than +175°C, although Tg values for FR-4 like materials can range from +125°C to +220°C. In contrast, both RO4003C and RO4350B laminates have Tg values in excess of +280°C, as determined by differential scanning calorimetric (DSC) analysis.

The decomposition temperature, Td, of a material is defined as that temperature during a steady-state ramp at which the material’s mass has been reduced by 5%. This temperature marks the permanent thermal degradation of the material, and it may be accompanied by other effects to the material, including blistering, measling, delamination, and even loss of the copper/dielectric bond. For high-Tg FR-4-like materials intended for lead-free-solder processing, the Td is typically in the range of +300°C to +350°C. In contrast, for RO4000 materials, the Td ranges from +390°C to +425°C.

The CTE in the z-axis of the material is yet another temperature-dependent parameter which can shed light on the expected behavior of a material in a high-temperature, lead-free-solder process. The CTE in the z-axis refers to the rate at which the thickness of the material changes in parts per million (ppm) with changes in temperature, over a specified temperature range. A material’s z-axis CTE directly correlates to the PTH reliability of the material. Lower CTE values translate to increased PTH reliability. The RO4000 materials, for example, are characterized for z-axis CTE of 46 ppm/°C or less from -55°C to +288°C. In a lead-free-solder process, the major concern for CTE is at temperatures approaching or exceeding a material’s Tg value, where the CTE value is considerably higher than for its specified temperature range.

Not only are high temperatures a concern for a PCB material to be used in a lead-free-solder process, but also the manner in which the temperatures change or cycle. High temperatures and temperature cycling both contribute to weakening of a laminate’s copper/dielectric bond, often resulting in delamination. PCB materials are characterized in terms of their time to delaminate at different temperatures, such as T-245 for the time at +245°C, T-260 for the time at +260°C, and T-288 for the time at +288°C. The onset of delamination is usually evidenced by blistering, measling, and voiding in a material. For materials to be used in a lead-free-solder process, typical goals for T-260 might be 30 minutes and for T-288 about 10 minutes.

To evaluate their suitability for high-temperature lead-free-solder processes, 0.125-in.-thick RO4000 materials were subjected to T-288 tests. Tests included 90 minutes at +288°C, with the sample then removed from the furnace for evaluation. The samples were cross-sectioned, and no defects were found. They were then returned to the furnace for another 90 minutes at +288°C, removed, and again cut into cross-sectioned slices for examination. Again, no defects were found. For comparison, high (+175°C) Tg FR-4-like materials were placed in the furnace at +288°C to determine their T-288 values. Although these are nominally high-temperature materials, with Td of +300°C to +350°C, they showed clear signs of delamination in less than 10 minutes.

The high-Tg FR-4-like materials did not fare well in various studies of temperature cycling as might be found in PCB rework processes. For example, the material’s copper peel strength dropped dramatically at temperatures above +150°C. This is a fairly standard test for evaluating the reworkability of a material, even though lead-free-solder rework may be performed at temperatures exceeding +370°C.

In contrast, evaluation of RO4000 series materials for reworkability was conducted through simulated rework with typical circuit elements, such as capacitors and resistors, on circuit boards populated with Sn/Ag/Cu solder paste with a peak reflow temperature of +365°C. The boards were reworked through three cycles, using a variety of extraction techniques, including  solder tips, solder tip fixtures, hot air, infrared (IR) heating, and a convection oven. Reattachment of circuit components was performed at +371°C using a solder tip. In short, the RO4000 materials survived through three cycles. When high-Tg FR-4-like materials were subjected to the same sequence of events, some failed during the first cycle and many during the second cycle.

Finally, the RO4000 circuit materials were subjected to different series of thermal shock tests, with pressure to simulate the fabrication of MLBs.  Various temperature gradients and dwell times between temperatures were applied. In all cases, MLBs fabricated with these materials produced PTHs that passed with flying colors mechanically and electrically even after as many as 1000 temperature shock cycles from -55°C to +125°C, regardless of the core thickness, prepreg, copper type, MLB thickness, and hole diameter.

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This post authored by John Coonrod originally appeared on the Rog-Blog hosted by Microwave Journal.

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?

As detailed in the last several blogs, there are more than a few ways to determine the Dk of a microwave laminate, and these different measurement methods often yield different results for the same material. Some of the measurement techniques are based on the use of “raw” PCB materials—without circuits on them—while some of the methods use a well characterized circuit with predictable performance to then determine the Dk for the material. Materials suppliers may use terms like “process Dk” to refer to the target value for the material when it is being processed, and “specification Dk” to mean a value determined by means of one or more of the measurement methods described in the two previous blogs. Often, the process and specification Dk values are the same for a given laminate.

A more meaningful version of Dk is the “Design Dk” that is currently published in the Rogers’ Product Selector Guide and serves as the values for Dk in the MWI-2010 Impedance Calculator, available for free download (note that it does require sign-up). The Design Dk is a value that provides the most accurate and repeatable results when used for circuit design purposes, notably in commercial CAE circuit and system simulation programs.

For some materials, the process or specification Dk may have the same value as the Design Dk. For Rogers’ popular RT/duroid® 6002 microwave laminate, for example, the process Dk and the Design Dk are both 2.94 in the z-axis. One difference is that the process Dk is specified at 10 GHz on the data sheet, while the Design Dk is given for frequencies from 8 to 40 GHz. The values were determined using two different test methods.

At the same time, the Dk values may differ appreciably. Rogers RO3010™ laminate has a process Dk of 10.2 in the z-axis at 10 GHz, but a Design Dk value of 11.2 is recommended for use with commercial CAE software simulators for more accurate modeling purposes.

If process and specification Dk values are determined by measurements, why should there be a need for a “Design Dk” value? As mentioned in the previous two blogs, there are many test methods for determining the Dk of a laminate. As an example, the global trade organization IPC lists 13 different test methods to determine a material’s Dk. Materials suppliers use any number of these measurement methods for their own determinations of Dk, while laminate users may have their own, and different, methods for determining the Dk of a laminate before using it for design purposes. In the two materials mentioned above as examples, a different measurement was used in each case to find the process/specification Dk and the Design Dk: the clamped stripline method was used for the process/specification Dk and the differential phase length method was used for the Design Dk.

Within Rogers, for example, the X-band clamped stripline resonator test is used for standard quality assurance (QA) testing of specification or process Dk, although the full-sheet-resonator (FSR) measurement method may also be used for QA testing. The split post dielectric resonator (SPDR) method may also be used to characterize materials within Rogers. For determining the Design Dk, the microstrip differential phase-length method will be used for all materials.

While none of the test methods is ideal, the differential phase-length method is elegant in its simplicity. It relies on fabricating two microstrip circuits of significantly different lengths on the same laminate material, using the same connectors or test fixture to determine the phase angle differences between the circuits for a given test frequency. A value of Dk can be determined from simple calculations based on the differences between physical lengths and phase angles. The process is repeated for as many frequencies as is practical. It is not a fast method, but it does provide accurate results for Dk in the z-axis, with anisotropic material effects (Dk values in the x and y axes) having little impact on the measurements.

This test method uses microstrip circuits commonly used in actual applications. It is also performed at the high frequencies often used in applications, to account for “copper effects,” in which a laminate with rougher copper surface can test for a higher apparent Dk value than a laminate with smoother copper surface. Test methods using lower frequencies may not reveal the effects of the copper roughness on measured Dk value.

The Design Dk values have been determined for all of Rogers’ high-frequency laminates and are being reported to all major developers of CAE simulation software tools. In addition, those values are now included in the MWI-2010 Microwave Impedance Calculator, the Product Selector Guide, and in the Slide Rule published in the November 2010 issue of Microwave Journal.

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This post authored by John Coonrod originally appeared on the RogBlog hosted by Microwave Journal

Microstrip or stripline? That choice has been faced by high frequency designers for decades. Both transmission-line technologies are widely used in both active and passive microwave circuits, with excellent results. Is one approach better than the other? Before tackling such a question, it might help to know how each transmission-line technology works and what kind of demands each place on a printed circuit board (PCB) material.

Microstrip is a transmission-line format in which the conductor is fabricated on a dielectric substrate which itself has a bottom ground-plane layer. Conductors are usually formed by etching away unwanted metal from a conductor layer, such as copper.

Stripline is often compared to a flattened coaxial cable in that, like the cable, it consists of an inner conductor completely surrounded by dielectric material which is itself surrounded by a ground braid or foil. Of course, stripline circuits are planar, so that they appear as a sandwich of conductors in the middle, surrounded by dielectric layers, which in turn have parallel ground planes on the top and bottom.

Continue reading the full article on the Rog-Blog

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