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02/24/2012
01/09/2012
Choosing A Laminate That Matches The Model
January 9, 2012
John Coonrod is a Market Development Engineer for Rogers Corporation, Advanced Circuit Materials Division. John has 23 years of experience in the Printed Circuit Board industry. About half of this time was spent in the Flexible Printed Circuit Board industry doing circuit design, applications, processing and materials engineering. The past ten years have been spent supporting circuit fabrication, providing application support and conducting electrical characterization studies of High Frequency Rigid Printed Circuit Board materials made by Rogers. John has a Bachelor of Science, Electrical Engineering degree from Arizona State University. www.rogerscorp.com/acm. This blog is part of Microwave Journal's guest blog series.
Choosing a substrate material for use as the printed-circuit board (PCB) in a new design can be a stressful experience, unless it is a matter of sticking with that “tried-and-true” material that has worked so well in the past. But with ever-evolving and improving dielectric and laminate materials, and increasing demands to achieve high performance levels at reduced costs, most design engineers are wise to consider the cost-versus-performance benefits of different types of commercial PCB materials. Previous blogs have detailed how selecting a PCB material can be influenced by different performance requirements. This blog will explore the role that a computer-aided-design software tool plays in choosing the most suitable PCB material.
The technical design literature is filled with articles that document differences between simulated and measured results, with authors often blaming the unforeseen electrical effects of test fixtures or problems in fabricating the PCB prototype as the reasons for the discrepancies. The computer models are assumed to be accurate because they have been developed by experienced RF/microwave engineers and are based, as was seen in the last blog, on industry-standard test methods to characterize different parameters of the PCB substrate. But a CAD simulation is only as accurate as its models, and models are rarely perfect representations of the original.
One basic question to ask when considering different PCB materials for a new design is “How well is a material of interest represented in terms of models in circuit and electromagnetic (EM) simulation software tools?” For example, RT/duroid® 5880 from Rogers Corporation has often been cited in technical articles as a basis for modeled-versus-measured comparisons of new design circuits. Models for this substrate material have been developed by various sources, and simulated performance levels achieved with models of the material have been well documented as being in close correlation with measured results. Such a background tends to build confidence in this PCB material in terms of both simulation and prototype fabrication stages of a design project.
But what about other PCB materials? Most suppliers of CAD software tools can provide lists of the PCB materials that they support in terms of models. And if their models are not exact, a software user can typically create one to their liking by modifying the key defining characteristics of a PCB material model, such as relative dielectric constant, dissipation factor, coefficient of thermal expansion (CTE), and coefficient of dielectric constant. All software models of PCB material are based on one or more values of relative dielectric constant (εr), derived from one of the measurement approaches covered in the last blog for one or more test frequencies. These values may or may not agree exactly with the data-sheet values for εr provided by a material manufacturer. Rogers Corporation, for example, provides two sets of values for εr on its data sheets: one is a value meant for use with substrate manufacturing processes and one is for use with CAD simulators for circuit design. Experience has shown that having design values yields closer agreement between simulated and measured prototype results.
Quite simply, the accuracy of a PCB model is improved by accounting for as many electrical effects as possible. In the case of predicting insertion loss for a circuit design based on a particular PCB material, for example, many different factors are involved and one or two can easily be overlooked in a simulation. Insertion loss is affected not only by conductor losses, dielectric material loss, radiation losses, and leakage losses, but also by such factors as copper surface roughness, solder mask, plated finishes, and circuit configurations.
Solder mask is often added to a PCB to prevent unwanted connections between conductors. But it tends to be lossy at microwave frequencies. It is also high in moisture absorption and can impact PCB performance in high-humidity environments. Different types of solder mask can even have an effect on the εr and dissipation factor of a PCB. In short, a circuit with solder mask will tend to exhibit higher insertion loss than the same PCB material without solder mask, and this is an effect that must be included in an accurate PCB material model.
The type of surface finish used on a PCB, such as an electroless nickel immersion gold (ENIG) finish, can also play a role in the insertion-loss performance of circuits fabricated on that PCB. While an ENIG finish can add a level of protection to a circuit, it can also add to the insertion loss of that circuit, although the amount of additional loss is also a function of the type of circuit. A broadband circuit will tend to suffer higher losses due to surface finish than narrowband circuit.
Yet another factor that can impact the accuracy of a model used to predict loss in a circuit design is copper surface roughness. The effects of copper surface roughness can be difficult to predict, since they depend on the surface roughness profile and the operating frequency. Yet, as with the effects of solder mask and surface finish, copper surface roughness must be included in any PCB material model in hopes of achieving accurate simulations.
As a last word on selecting a PCB substrate, consider first whether a material meets all the electrical requirements for a design, how it rates mechanically, and where it fits within a budget. Then review how well the material is represented in terms of software model in the same commercial CAD simulator that will be used to help design a new circuit. Do these models account for as many electrical effects as possible? If not, is it possible to modify the model for a PCB material of interest to include such effects as solder mask, surface finish, and copper surface roughness? An accurate PCB material model should include as many variables as possible, to better ensure that those CAD simulations closely match measurements on a fabricated circuit prototype.
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.
12/21/2011
12/15/2011
Measurements Help In Sorting Materials
December 15, 2011
John Coonrod is a Market Development Engineer for Rogers Corporation, Advanced Circuit Materials Division. John has 23 years of experience in the Printed Circuit Board industry. About half of this time was spent in the Flexible Printed Circuit Board industry doing circuit design, applications, processing and materials engineering. The past ten years have been spent supporting circuit fabrication, providing application support and conducting electrical characterization studies of High Frequency Rigid Printed Circuit Board materials made by Rogers. John has a Bachelor of Science, Electrical Engineering degree from Arizona State University. www.rogerscorp.com/acm. This blog is part of Microwave Journal's guest blog series.
Selecting a high-frequency PCB laminate from the many commercially available choices may sometimes seem like an impossible task. But it can be simplified by sorting materials by their key parameters, such as dielectric constant, dissipation factor, thermal conductivity, and CTE, and using those parameters to help match a material to an application. Of course, this also assumes confidence in the values of those key parameters as published by different materials suppliers, and such confidence comes from an understanding of the measurement methods used to determine the values of those key parameters.
As detailed in an earlier blog, a number of different measurement methods are used by materials suppliers to characterize one of the most basic of laminate parameters, relative dielectric constant. Commercial high-frequency circuit materials are generally available in roughly three values--3.0, 6.15, and 10.2—and various additional values and, in all cases, designers count on the accuracy and stability of those values when computing such things as circuit dimensions and impedances. The amount of variations in these values also tell a designer how suitable a particular circuit laminate might be for a broadband design, since broadband circuits require relative dielectric constant that is as flat as possible as a function of frequency.
Because the values determined by different measurement techniques can differ, Rogers Corporation provides two values of dielectric constant for their laminates—one that applies during processing and one that can be used for predictions with design equations and computer-aided-engineering (CAE) software tools. More information on the specific dielectric-constant measurement methods, such as the clamped stripline resonator test (IPC test method IPC-TM-650 2.5.5.5c) and the full sheet resonance (FSR) test (IPC-TM-650 2.5.5.6), can be found in the article by John Coonrod and Allen Horn III, “Understanding Dielectric Constant for Microwave PCB Materials,” available for free download from the Rogers Corporation web site.
Test frequency is often as important as test method, especially if using a material characteristic such as dielectric constant as a sort parameter. Accurate dielectric constant is essential for stable, predictable filter performance, for example, and the test method used to determine dielectric constant may or may not be suited to the target application for a laminate. Many of the methods for determining dielectric constant use a test frequency of 10 GHz. Some, such as the split post dielectric resonator (SPDR) test, are run at 1 GHz. The effects of laminate copper roughness, for example, can be easily overlooked at the lower frequency. For a meaningful comparison of different materials based on dielectric constant, their dielectric-constant values should be determined by the same test method and at the same frequency.
A laminate’s thermal characteristics, such as thermal conductivity and coefficient of thermal expansion (CTE), can be useful parameters for choosing a material for circuits that must handle high RF power levels and readily dissipate heat while providing stable performance. As with other material parameters, a number of different test methods are used to determine laminate thermal conductivity, depending upon manufacturer. Rogers Corporation, for example, applies method C518 from ASTM International (www.astm.org) to determine the thermal conductivity of its laminates. The method is based on the use of a heat flow meter to register the steady-state heat flow through a material. It is considered a secondary method since the test equipment must be calibrated by means of a material with known properties, referred to as a primary standard. But this is a method that has been approved by ANSI and adopted by the US Department of Defense (DoD), and has shown to be a fast and reliable measurement method for determining the thermal conductivity of production quantities of materials. Another means of determining high-frequency circuit laminate thermal conductivity is by ASTM method E1461, which is considered a flash measurement method that relies on the use of infrared (IR) sensors for detecting temperature changes.
Various test methods are used by different suppliers to determine a circuit laminate’s CTE across a range of operating temperatures, such as ASTM method D3386-94 and ASTM method E831, which supersedes it. These test methods are based on measuring the linear expansion of solid materials as a function of temperature, using thermomechanical analysis. Such test methods can also be used in many cases to determine a laminate’s glass transition temperature. In comparing CTE values when considering different laminate choices, values in the x and y dimensions should be closely matched, with z-axis values kept as low as possible to ensure plated-through-hole (PTH) integrity. While not always possible, ideally all laminate CTE values in a comparison will have been determined by the same standard test method.
In short, a choice of high-frequency circuit laminate should be steered by the requirements of an application, and different material characteristics used to sort from among the circuit material choices. And when sizing up performance parameters from different materials, the most meaningful comparison will come when those parameters have been determined by similar test methods. The next blog will take one last look at the challenge of making a choice among the many commercially available high-frequency circuit laminates, by considering how different circuit materials are represented within modern RF/microwave computer-aided-design (CAD) software tools, and the role of the
Technical Service Engineer (TSE) in guiding a laminate user through a successful circuit fabrication procedure.
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.
11/29/2011
Match Material Specs To Application Needs
November 29, 2011
John Coonrod is a Market Development Engineer for Rogers Corporation, Advanced Circuit Materials Division. John has 23 years of experience in the Printed Circuit Board industry. About half of this time was spent in the Flexible Printed Circuit Board industry doing circuit design, applications, processing and materials engineering. The past ten years have been spent supporting circuit fabrication, providing application support and conducting electrical characterization studies of High Frequency Rigid Printed Circuit Board materials made by Rogers. John has a Bachelor of Science, Electrical Engineering degree from Arizona State University. www.rogerscorp.com/acm. This blog is part of Microwave Journal's guest blog series.
Selecting a PCB laminate for a high-frequency application is like picking a foundation for a new building: the strength of the whole project relies on the right choice. The previous blog introduced a strategy to help simplify the PCB laminate selection process, by relating the requirements of an application to laminate specifications. Each RF/microwave application is unique, with its own requirements. But at least one laminate will usually offer the right set of specifications to best meet those requirements.
A glance at the “High Frequency Materials Product Selector Guide” from Rogers Corporation shows the breadth of circuit-board materials available from this single supplier. The materials have been developed over time in response to customers and their different requirements. Picking the optimum laminate for a given application requires understanding how different material capabilities match up with different types of high-frequency applications.
Previous blogs detailed many of these laminate specifications, such as dielectric constant, the consistency of the dielectric constant across the material, coefficient of thermal expansion (CTE), and thermal conductivity. Each parameter describes a specific material characteristic, although each parameter may not be critical for a given application. No one material is ideal for all purposes, so finding the best fit for an application involves pairing a material’s capabilities with the needs of that application. To apply this selection process, and the technique of “filtering” material specifications to focus on the main parameters for an application, it may help to review a few sample applications and types of materials that might work best for them.
The number of parameters listed for each laminate in the Selector Guide can seem overwhelming. But they all describe the behavior of a material in a different way or under different conditions. To make selecting a laminate a practical process, the different material parameters must be prioritized for a given application. The one parameter not listed in the Selector Guide is cost and, for a fair share of applications, cost is important. But if cost is the only guiding parameter, it is unlikely that all of the other requirements of an application will be met.
To determine the key laminate requirements for an application, the application should be defined in its broadest terms, such as frequency range, broadband or narrowband, small signal or large signal, large circuit or small circuit, controlled or “hostile” operating environment, even the size of a production run and the importance of unit-to-unit repeatability. By defining the needs of an application in terms of broad requirements first, it can be easier to identify key material parameters that should be used to guide the search for an optimum laminate for that application.
For example, describing an intended application circuit as “small signal,” which can include a wide range of active and passive components such as amplifiers, filters, and oscillators, can help determine which material parameters should be considered first. A small-signal circuit will typically operate with no more than a watt or two of RF/microwave power, compared to a “large-signal” circuit which may handle tens or hundreds of watts of RF/microwave power. The thermal requirements for the two types of circuits, and for their laminates, are dramatically different.
Someone designing a small-signal circuit is likely less concerned with a material’s thermal parameters, such as thermal conductivity and CTE, than they are with minimizing loss and in comparing materials for lowest possible dissipation factor. And where frequency stability is critical, the consistency of the dielectric constant is also important. One possible solution is RT/duroid® 5880, which combines low dissipation factor of typically 0.0009 at 10 GHz with a low dielectric constant of 2.20 at 10 GHz, maintained to a tight tolerance of ±0.02.
Of course, the laminate selection process is also about tradeoffs. In spite of its exceptional small-signal performance, RT/duroid 5880 is not engineered for applications where thermal management is critical. It has relatively low thermal conductivity of 0.20 W/m/K, unimpressive z-axis CTE, and lackluster thermal coefficient of dielectric constant (typically -125 ppm/°C from -50°C to +150°C).
In contrast, someone designing circuits with critical thermal-management requirements would give greater weight to a laminate’s thermal characteristics, and perhaps be more willing to tolerate a somewhat higher dissipation factor. RO4350B™ laminate, with outstanding thermal conductivity of 0.69 W/m/K, excellent z-axis CTE, and controlled thermal coefficient of dielectric constant (typically +50 ppm/°C), is well suited for many applications where thermal-management requirements are a concern, such as power amplifiers. There are tradeoffs associated with increased thermal conductivity, including a higher dielectric constant of 3.48 ± 0.05 and higher dissipation factor of 0.0037. The higher dielectric constant will shrink the dimensions of microstrip circuitry somewhat compared to the low 2.20 value of RT/duroid 5880, but RO4350B laminate with enhanced thermal characteristics will help effectively dissipate heat in higher-power circuits.
What if a circuit application is physically large, such as an antenna? This type of application calls for circuit materials with consistent dielectric constant across a large laminate panel, but also with low loss. Rogers RO4730™ laminate, a glass-reinforced, ceramic-filled hydrocarbon-based laminate, features a composition that is relatively low in cost, yields a dielectric constant of 3.00 ± 0.08 and that can translate into consistent impedance for transmission lines across a large panel and predictable, repeatable antenna patterns at RF/microwave frequencies. The material’s low dissipation factor and good thermal conductivity also mean that it can be used in some higher power levels.
Some applications may involve circuits and systems operating in environments with wide temperature ranges, high moisture levels, and other factors that can affect electrical performance. For example, water has a dielectric constant of about 80. Any water absorbed by a laminate will change the dielectric constant of the material. In these cases, laminate selection might be guided by environmentally related material parameters, such as thermal coefficient of dielectric constant and moisture absorption. As noted, RO4730 laminate is excellent for antennas, and it exhibits typical moisture absorption of 0.13%, generally considered quite good. In comparison, RO3730™ laminate with a dielectric constant of 3.00 ± 0.06 is also ideal for antennas, but has somewhat lower loss and improved moisture absorption of 0.04% for those applications where conditions like water vapor may have an impact.
These are just a few examples of how an application’s requirements can help guide the selection of a high-frequency laminate. Obviously, there are many factors to consider in any application, including cost, performance, repeatability, and even the size of the final circuit. Additional factors include the thickness of the dielectric material and the type and weight of the conductive cladding.
The next blog will examine how these different material choices fit into the selection process, and also explore the role of the PCB laminate supplier in terms of how they determine their material parameters. For some material specification, two different types of measurements can yield two different results. Selecting a laminate with the more accurate specification can save a great deal of grief during both design and fabrication stages of a product development.
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.
11/11/2011
Do You Have an Award Winning Application?
We want to hear from YOU! Tell us how Rogers’ materials helped you design your award-winning application and you could WIN! From now until May 1, 2012, The ACM ROG Contest is collecting entries in six categories: Best Digital Application, Most Extreme Conditions, Most Challenging Board Build, Most Innovative Design, Longest Product Life, and Most Unique and Creative Use of Material. One winner will be selected from each category to receive: a full-page ad featuring your story and team photo; a free Ad in Microwave Journal for your company; and a cool ROG Display Trophy. Winners will be announced at the Rogers Customer Appreciation Event during IEEE/IMS in June 2012. Check out this link and enter today! http://www.rogerscorp.com/acmcontest/. Follow ROG on Twitter at @Rogers_ACM for contest updates.
11/03/2011
Selecting A Suitable High-Frequency Laminate
November 3, 2011
John Coonrod is a Market Development Engineer for Rogers Corporation, Advanced Circuit Materials Division. John has 23 years of experience in the Printed Circuit Board industry. About half of this time was spent in the Flexible Printed Circuit Board industry doing circuit design, applications, processing and materials engineering. The past ten years have been spent supporting circuit fabrication, providing application support and conducting electrical characterization studies of High Frequency Rigid Printed Circuit Board materials made by Rogers. John has a Bachelor of Science, Electrical Engineering degree from Arizona State University. www.rogerscorp.com/acm. This blog is part of Microwave Journal's guest blog series.
Choosing the right circuit-board material can be a frustrating experience. After all, every manufacturer of printed-circuit-board (PCB) materials promises outstanding performance for their products, offering an array of dielectric formulations to achieve improved stability, or power-handling capability, or lower loss. Wouldn’t it be simpler to choose one material, such as FR-4, for all applications? In an ideal world, a single PCB material could serve all applications. But requirements for two applications may vary so widely, that no single PCB material can provide optimum performance for both.
Previous blogs examined some of the key material parameters pertaining to high-frequency laminates, such as dielectric constant, thermal conductivity, coefficient of thermal expansion (CTE), and even flexibility when used in conformal circuits. But how does an engineer combine all this information about a material’s electrical and mechanical properties when trying to choose the perfect substrate for a particular application? It can be a complex process, but it may be possible to simplify that process.
Perhaps any search for suitable PCB material for a given application should start with the application itself. It is often a customer’s application, and its particular requirements, that have driven high-frequency-laminate suppliers such as Rogers Corporation to develop particular material formulations for optimum performance in one type of application, such as a high-frequency, high-power amplifier. The better an engineer understands their target application, the easier the process is for selecting a circuit-board laminate for that application. Defining which performance parameters are the most critical for an application can help guide an engineer to the best choice of circuit-board laminate.
A high-frequency circuit design may have a list of required specifications that can fill a page or more, but typically a handful of those specifications are the critical ones that call for special design or fabrication approaches. For a PCB laminate, that can provide the foundation for meeting the most critical requirements. Where a bandpass filter is defined by such parameters as center frequency, percentage bandwidth, rejection, and passband insertion loss, a laminate is characterized by a completely different set of parameters, such as dielectric constant, thermal coefficient of dielectric constant, and CTE. As explained in an earlier blog (Blog #5), even the thickness of a PCB laminate can impact the high-frequency performance of a circuit. So, evaluating a particular application to help choose the right circuit board material is a matter of finding which of the application’s key performance parameters relate to which of the PCB laminate’s characteristics, and what possible tradeoffs may exist.
Defining the needs of an application can be as simple as a series of “filtering” processes, sorting by means of larger issues and working down to performance tradeoffs. For example, will a PCB laminate ultimately be used in a military system, for commercial use, for industrial use, in space, for a medical application, or in a combination of these application areas? Knowing where the circuit-board material is going, such as in a military electronic-warfare (EW) system, will eliminate some PCB materials from consideration, since they won’t meet the basic electrical and mechanical requirements for military use. Of course, if a designer is hoping to sell their circuit across commercial and military markets, the PCB material must be suitable for military environments. For circuit-board materials that will be used for products across multiple market areas, the market with the most rigorous set of requirements (usually military or aerospace) will set the requirements for the PCB material.
Reviewing the requirements of an application can also help to define necessary and unnecessary tradeoffs. One of the more obvious tradeoffs is cost versus performance. Compared to a high-performance PTFE-based laminate, FR-4 can save a bundle. But it may not provide the high-frequency performance needed for an application, and it may not provide much frequency or amplitude stability even if it does reach the right frequency. Even within such an obvious tradeoff are finer points for comparison: part of the overall price of using a given laminate includes processing costs—some materials are simpler and less expensive to process than others, depending upon the composition of the laminate. Circuit size can also contribute to lowering costs. Choosing a laminate with a higher dielectric constant can yield more circuits per laminate panel, provided that the electrical effects of the higher dielectric constant are acceptable for that application. These and other factors must be considered when making a “simple” tradeoff evaluation between costs versus performance for different laminate materials.
Most high-frequency laminate specifiers start with relative dielectric constant when comparing products from different suppliers, and then check other parameters, such as dissipation loss and CTE. Laminate manufacturers specify their products with a specific value and some amount of variation, such as 3.48 ± 0.05 in the z-direction at 10 GHz for RO4350B™ laminate from Rogers Corporation. But as noted in an earlier blog (Blog #16), this may not be the best value to use in a computer simulation. Choosing the right laminate material requires confidence in how the material has been characterized, so that simulations will represent final results. The next blog will detail some of the methods that laminate suppliers use to determine material parameters such as dielectric constant, typically by fabricating a circuit structure with known characteristics on the PCB material.
The next blog (blog #25) will also go into greater detail on how different PCB laminate specifications relate to the performance levels of different high-frequency circuits. For example, for a high-power microwave amplifier, a laminate’s thermal conductivity will certainly be one of the first parameters to compare among different substrates under consideration. But if a laminate has a high dissipation factor, it contributes to high circuit insertion loss. The higher loss results in more heat generated through the amplifier circuit, in turn requiring higher thermal conductivity. In this example, these two parameters (and possibly others) must be balanced and compared from laminate to laminate to make the best choice for a particular power amplifier circuit. To be continued in blog #25………
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.
10/17/2011
Bending and Forming RF/Microwave PCBs
October 17, 2011
John Coonrod is a Market Development Engineer for Rogers Corporation, Advanced Circuit Materials Division. John has 23 years of experience in the Printed Circuit Board industry. About half of this time was spent in the Flexible Printed Circuit Board industry doing circuit design, applications, processing and materials engineering. The past ten years have been spent supporting circuit fabrication, providing application support and conducting electrical characterization studies of High Frequency Rigid Printed Circuit Board materials made by Rogers. John has a Bachelor of Science, Electrical Engineering degree from Arizona State University. www.rogerscorp.com/acm. This blog is part of Microwave Journal's guest blog series.
Bending and forming RF/microwave printed-circuit boards (PCBs) around a curved shape are sometimes part of the design process, such as when fabricating conformal antennas. While this may not be commonplace, for those times that it is necessary, it is important to know several things about the high-frequency PCB material for the project. This includes the correct type of material to use, by how much the material can be flexed without damage, and what types of mechanical and electrical effects are to be expected by bending and forming an RF/microwave PCB. Quite simply, picking the wrong PCB material for bending and forming applications can result in mechanical cracks and damage to the circuit board.
When a PCB material must be bent or formed around a mandrel to conform to a certain shape, circuit materials without glass reinforcement are preferred; materials with glass fabric tend to be more rigid and lack the flexibility needed for bending without cracking. Of course, the glass reinforcement provides mechanical stability, so some tradeoff in stability must be accepted as part of having the flexure capability in a PCB material.
Similarly, thinner PCB materials lend themselves more readily to bending and forming than thicker materials, although there is also a tradeoff in selecting a thinner material. Thicker materials offer improved dimensional stability over thinner versions of the same material, and thicker materials are usually specified when attempting to minimize insertion loss in a circuit.
PCB materials suppliers incorporate a range of different filler materials to improve electrical and mechanical stability, so choosing a circuit material for an application that requires bending and forming calls for a careful choice. For example, RO3200™ series PCB materials from Rogers Corp. are ceramic-filled laminates reinforced with woven fiberglass for structural stability, while the company’s RO3000® series laminate materials are fabricated without the woven fiberglass reinforcement.
Both series of materials are available in various thicknesses and in versions with dielectric constants of 3.00, 6.15, and 10.2 in the z-axis at 10 GHz. Both series of materials offer excellent electrical performance with temperature and outstanding structural stability. For example, RO3003™ material from the RO3000 series, with a dielectric constant of 3.00 in the z-axis at 10 GHz, has a coefficient of thermal expansion (CTE) of 17 ppm/°C in its x and y axes, closely matched to that of copper for good stability with temperature. In the z-axis, the CTE is 24 ppm/°C for good PTH reliability. In comparison, RO3203™, with a dielectric constant of 3.02 in the z-axis at 10 GHz, also features x- and y-axis CTE values closely matched to copper, at 13 ppm/°C, and z-axis CTE of 58 ppm/°C that also ensures good PTH reliability. Both materials are similar in thermal conductivity, with 0.5 W/m/K for RO3003 laminates and 0.48 W/m/K for RO3203 laminates. Both materials are available with electrodeposited (ED) copper foil in 0.5, 1.0, and 2.0 oz. options. But, while the data sheets for the two series of materials do not point this out, the absence of fiberglass reinforcement in the RO3000 series laminates makes them the more suitable of the two material series when considering high-frequency PCB materials for applications that require bending and forming.
Another consideration when bending and forming a circuit laminate is the amount of stress that will be imposed on the material’s copper layers. Because the copper will be stressed along the material’s x and y directions when a laminate is bent or formed around a mandrel, a PCB material should be specified with rolled wrought or rolled annealed copper, which has a grain structure that is well suited for the elongation in the x-y plane that occurs with bending and forming a PCB substrate. The type of plating for a PCB substrate that must undergo bending and forming is also a concern. Electroless nickel/immersion-gold (enig) plating is often used to plate high-frequency laminate materials. But nickel is extremely brittle and subject to cracking with flexure. Similarly, to minimize reliability problems in a PCB that must be bent or flexed, viaholes or plated-through holes (PTHs) should be avoided in the area of the PCB undergoing the greatest amount of flexure.
Once a PCB material has been selected for bending and flexing, it is important to determine practical limits for the bending and flexing, by means of a parameter known as a PCB material’s minimum bend radius. The minimum bend radius for a given material refers to two different types of flexures in a PCB material: for a one-time bend (often referred to as a “flex-to-install” application) and for an application in which multiple flexures will occur. In the first case, a laminate may be bent around a mandrel to create a particular conformal shape, and this occurs only once during the manufacturing process. In some cases, a PCB may be required to withstand dynamic flexing as part of an application, such as printed-wire connections in old-style clam-shell cellular telephones.
A general industry rule of thumb is to use a minimum bend radius that is 10 times the thickness of the PCB material for one-time bends and a minimum bend radius that is 25 times the thickness of the PCB material for dynamic PCB bending. Returning to the RO3003 material as an example, it is available in a range of standard thicknesses, from 0.005 in. (0.13 mm) through 0.060 in. (1.52 mm). For a RO3002™ PCB laminate with thickness of 0.010 in. (0.25 mm), the minimum bend radius for one-time bends is 0.1 in. (2.5 mm) while the minimum bend radius for dynamic bends is 0.25 in. (6.25 mm). Further details on minimum bend radius for a variety of PCB materials can be found in Document IPC-2223 from the IPC (www.ipc.org). The document also includes recommendations on copper layer elongation for flexible circuits, such as 3% for one-time-bend applications and 0.3% for dynamic bend applications.
RF/microwave design engineers often consider PTFE-based PCB materials as flexible substrate materials, and such materials can be suitable candidates for bending and forming applications. For example, RT/duroid® 5880 from Rogers is a glass-microfiber-reinforced PTFE composite material with extremely low loss and low dielectric constant of 2.2 in the z-axis at 10 GHz. Because it does not use woven-glass-type reinforcement, it is suitable for both single-bend and dynamic bend applications, providing high reliability with the electrical performance associated with PTFE substrates.
09/27/2011
Aiming For The Perfect Wire Bond
September 27, 2011
John Coonrod is a Market Development Engineer for Rogers Corporation, Advanced Circuit Materials Division. John has 23 years of experience in the Printed Circuit Board industry. About half of this time was spent in the Flexible Printed Circuit Board industry doing circuit design, applications, processing and materials engineering. The past ten years have been spent supporting circuit fabrication, providing application support and conducting electrical characterization studies of High Frequency Rigid Printed Circuit Board materials made by Rogers. John has a Bachelor of Science, Electrical Engineering degree from Arizona State University. www.rogerscorp.com/acm. This blog is part of Microwave Journal's guest blog series.
Wire bonds keep everything in place on a printed-circuit board (PCB). They are used to attach passive and active components as well as integrated circuit (ICs) to a circuit substrate, and even to connect one circuit substrate to another. Wire bonds can be formed with a variety of different wire bonding machines, including manual and automatic models. In all cases, the goal is to achieve a low-resistance connection with good mechanical integrity and high reliability. But this seemingly simple goal depends not only on the type of substrate material and its parameters but numerous wire-bonding parameters, including the temperature, time, and applied force when making a wire bond.
Wire bonds can be formed by various methods, including ultrasonic bonders, in which the energy for the weld comes from ultrasonic force, and thermocompression bonders, in which thermal energy is applied to form a wire bond. In addition, a thermosonic bonder uses a combination of ultrasonic and thermal energy to form a wire bond. Types of wire bonds include ball bonds and wedge bonds. Interconnection bond wires are typically formed of gold (Au), copper (Cu), or aluminum (Al) wire.
Wire bonding machines used with high-frequency PCBs include ball bonders and wedge bonders. Ball bonders are faster, but wedge bonders tend to deliver higher-reliability bonds. Ribbon bonders are essentially wedge bonders in which flat ribbon wire is used instead of round wire to provide a large cross section at the heel of a bond and, presumably, a higher-reliability bond.
Establishing workable wire-bond parameters depends not only on the type and thickness of the substrate material, but on the type of metal plating and plating thickness on the substrate and even the dimensions of the bonding pad. For example, softer circuit-board materials such as PTFE can present challenges for forming wire bonds because a soft material can absorb more energy than a harder material and deform during the wire-bonding process.
When evaluating the quality of a wire bond, electrical testing is conducted to establish that a low-resistance path has been formed, while pull tests are typically performed to determine the strength of a wire bond. For example, making wire bonds on substrates with smaller bond pads can be more difficult than with a substrate having larger bond pads, since the vertical pressure from a wire bonder is much greater on a small pad than on a larger pad. If a substrate’s bond pad is too small, the force of attaching a wire bond can deform the bond pad, or even force it beneath the surface of softer substrate materials. Deformation of the bonding pad and/or substrate material can also occur as a function of the bonding temperature, when temperatures higher than the glass transition temperature (Tg) of the substrate material are used. Bond-wire suppliers often provide recommendations for the maximum bond pad size (in mils) for a given type and diameter of their bond wire.
Because the choice of substrate material plays a major role in the ultimate quality that can be achieved with a PCB’s wire bonds, high-performance materials supplier Rogers Corporation recently performed a study on wire bonds formed on high-frequency substrate materials. The results of this study helps material users better understand how different wire-bonding parameters are needed for different types of materials. Sample boards were manufactured with different Rogers’ materials as part of the study to better understand how such parameters as device finish, wire type, and wire diameter can impact the quality of a wire bond.
Two different wire bonders were employed in the study, an automated wire bonder and a manual wire bonder, both using thermosonic bonds with 1-mil gold bond wire. Substrates were plated with 50 microinches of nickel and 200 microinches of Type III grade A gold. FR-4 was used as the reference material in the study, which also included RO4000® hydrocarbon ceramic laminates and RT/duroid®5880 glass-microfiber-reinforced PTFE composite PCB material from Rogers. Low-cost RO4000 laminates are engineered to be processed like FR-4, while RT/duroid 5880 represents the challenge of forming wire bonds on a softer PCB material.
The study (available upon request from the author) establishes safe “starting points” for making low-resistance, reliable wire bonds on each of the materials. It details a number of different parameters for a wire bonder, including the temperature of the stage on which the substrate is mounted, the bonding power and force, and the time required to form the wire bond for each material.
For example, the reference material, FR-4, has the shortest processing time but the highest stage temperature (+130°C), highest applied force, and greatest amount of bonding power of the materials studied. The RT/duroid 5880 material, because it is a “soft” PTFE-based composite material, worked with the lowest stage temperature (+80°C), less bonding power, and considerably less bonding force. The study even cautions that a lower stage temperature may be required for PTFE-based materials, along with a stabilization period for the material to reach a level of thermal equilibrium once mounted on the wire-bonder stage.
The study points out that its results are to be taken as starting points for setting wire-bonding parameters. In handling PTFE-based materials, for example, more reliable bonds may come as a result of decreasing the bonding force while increasing the time required to form the wire bond. The type of plating used with soft PCB materials can also impact the reliability of the wire bonds and the PCB in general. The study offers tested starting points known to deliver good results in terms of wire bond electrical performance and reliability for these materials, and PCB users are invited to modify those bond-wire parameters in their quest for the perfect PCB wire bond.
09/09/2011
Taking A Measure Of Copper Surface Roughness
September 9, 2011
John Coonrod is a Market Development Engineer for Rogers Corporation, Advanced Circuit Materials Division. John has 23 years of experience in the Printed Circuit Board industry. About half of this time was spent in the Flexible Printed Circuit Board industry doing circuit design, applications, processing and materials engineering. The past ten years have been spent supporting circuit fabrication, providing application support and conducting electrical characterization studies of High Frequency Rigid Printed Circuit Board materials made by Rogers. John has a Bachelor of Science, Electrical Engineering degree from Arizona State University. www.rogerscorp.com/acm. This blog is part of Microwave Journal's guest blog series.
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.
For those who may wonder how the roughness of a PCB conductor is measured, there are several techniques, including contact and non-contact methods. Measuring systems based on the use of contact gauges use a fine stylus to map a small area of a PCB’s conductor surface for its profile. This technique is limited in measurement speed and is typically confined to measuring a small area of the PCB, making it difficult to gauge the true conductor surface roughness across the full area of the circuit-board material.
Noncontact surface-roughness measurement approaches are usually based on optical spectroscopy and other optical techniques. At Rogers Corporation, for example, conductor surfaces have been characterized with a Wyko® NT1100 Optical Profiling System from Veeco (www.veeco.com). Designed for noncontact surface metrology of advanced materials, the small-footprint measurement system, which resembles an optical microscope, is based on white-light interferometry. The system can produce a three-dimensional image of the surface topology of a PCB’s conductor surface in an area as large as about 1 x 1 mm. The system has a vertical measurement range of 0.1 nm to 1 mm, with 1 Angstrom resolution and 0.01 nm root mean square (RMS) measurement repeatability. The measurement system also includes software which can be used to determine different statistical parameters, including RMS roughness and peak-to-valley roughness.
As mentioned previously, the work of S. P. Morgan in 1949 laid the foundation for much modern knowledge about the effects of conductor surface roughness. The “Morgan correlation,” a kind of correction factor for conductor surface roughness, is commonly used in calculators and programs designed to predict signal loss due to conductor surface roughness. Calculations usually involve a correction factor for surface roughness, Kr, which takes into account the relative roughness of a PCB conductor’s surface, as a ratio of a smooth surface to a rough surface. The Morgan correlation tends to predict the highest losses for conditions where the conductor surface roughness is the greatest and the highest frequencies of operation are being applied.
Still, in spite of its long track record, calculations based on Morgan’s work may be conservative in some cases, since those calculations predict worst-case loss for conductors with extremely rough surfaces that are about twice the loss of a perfectly smooth surface. More recent studies, including several performed by Rogers Corporation, indicate that losses may even exceed 3 dB more for a conductor with a rough surface compared to an ideal smooth conductor. And especially at frequencies above 10 GHz, these more recent studies have found the losses for particular rough conductor surface profiles to exceed the values predicted by the Morgan correlation.
Most recent studies on conductor surface roughness are targeted at developing accurate models to account for the electrical performance effects of surface roughness on both analog microwave and high-speed digital PCBs. In the search for an improved conductor surface-roughness model, the Morgan correlation is often used as a starting point. Calculations of conductor losses based on the Morgan correlation have traditionally agreed closely with measured results. It is important to note that those calculations are typically performed for microwave circuit materials that tend to be thicker than the thin substrates used in digital circuits. Any phase distortion, for example, induced by excessive conductor surface roughness, could translate into timing errors for a high-speed digital circuit. In addition, some studies have shown that the value of relative dielectric constant calculated for a thin substrate with rough conductor surface can be considerably higher than the value calculated for a PCB material with the same thickness, but with smooth conductor surface.
Because of possible deviations that can occur when basing a high-frequency or high-speed circuit design on conductor surface-roughness models, those models should include as much detail as possible about a PCB material’s attributes, including those that are frequency-dependent, such as the relative dielectric constant. Typically, three-dimensional (3D) full-wave electromagnetic (EM) field solvers as well as two-dimensional (2D) planar EM simulation software tools are used when modeling the effects of PCB conductor surface roughness on RF/microwave electrical performance. These tools should either include detailed models for a PCB material of interest, or provide the means of customizing a model of a PCB material, including a detailed profile of the PCB’s conductors.
Recent studies at Rogers Corporation on the effects of conductor profiles have shown that conductor surface roughness can impact the propagation constant of a PCB. When the differential phase length of a transmission line is used to calculate the effective dielectric constant, errors in the phase constant can lead to a misrepresentation of the dielectric constant of the material. In terms of CAE software tools, they must be able to account for the higher-than-expected conductor losses that can occur due to conductor surface-roughness effects, or calculated phase-constant results will be in error.
Some CAE software suppliers do provide tools for modeling the effects of conductor surface roughness, such as Sonnet 12.56.1 release and later from Sonnet Software (www.sonnetsoftware.com). It includes models that include the surface roughness effects of copper foils used in PCB materials from Rogers Corporation, and can calculate the surface impedance for either thin or thick copper conductors based on a ratio of smooth surface to rough surface.
More details on work being performed on analysis and model development of the effects of PCB conductor surface roughness can be found in several articles available for free download, “Effect of conductor profile on the insertion loss, phase constant, and dispersion in thin high frequency transmission lines” and “Conductor Profile Effects on the Propagation Constant of Microstrip Transmission Lines.” Both articles can be downloaded free of charge in PDF form from Rogers Corporation, at www.rogerscorp.com/acm.
