This post authored by John Coonrod originally appeared on the ROG Blog hosted by Microwave Journal. You can read part 1 of the series here.

Achieving high reliability for a high-frequency circuit or system starts with the printed circuit board (PCB). The PCB material must deliver consistent performance over time and changing conditions, such as temperature. As explained in the previous blog post, it is possible to spot PCB materials that are “built to last” by assessing a number of their key performance parameters, such as coefficient of thermal expansion (CTE). In fact, PCB materials such as Rogers RO4835™ laminates can be engineered for high reliability through a careful combination of material components resulting in specific performance characteristics.

RO4835 laminates are thermoset materials like FR-4. They are part of Rogers RO4000® family of PCB materials and can be processed with the standard epoxy/glass methods used with low-cost FR-4 materials. RO4835 laminates exhibit a typical dielectric constant of 3.48 in the z direction at 10 GHz with low dissipation factor of 0.0037 in the z direction at 10 GHz. They offer x- and y-direction CTEs of 11 and 9 ppm/°C, respectively, that are relatively compatible with the 17 ppm/°C CTE of copper; the CTE is typically 26 ppm/°C in the z direction. RO4835 laminates have a glass transition temperature (Tg) of greater than +280°C to handle effects of high-temperature circuit processing.

As detailed in the previous blog, a number of material parameters can point to potential reliability issues, including a material’s CTE, its resistance to oxidation, and its heat- and power-handling capabilities. The CTE characteristics of RO4835 laminates represent stable mechanical and electrical behavior at higher power levels and across wide temperature ranges. In addition, the material has been engineered to be resistant to the effects of oxidation. In general, the material has been formulated for demanding applications where long-term reliability is a concern.

Oxidation can impact all thermoset laminate materials over time and at elevated temperatures. It is essentially caused by the absorption of oxygen atoms to form a carbonyl group within the material, leading to small increases in its dielectric constant and dissipation factor which are not reversible. The electrical impact of oxidation can also be affected by elevated temperatures. Physically, oxidation can also result in a “darkening” effect on the exposed dielectric surfaces of the laminate. The oxidation begins on the surface and slowly penetrates into the dielectric as the oxygen diffuses through the material. Copper metallization on a laminate greatly reduces the effect of oxidation on the dielectric material beneath the copper.

Where oxidation may be a concern, it might be necessary to store a circuit in an oxygen-free environment or enclosure, such as in a vacuum or nitrogen environment. Where such an option may not be available, RO4835 laminates are less affected by oxidation than most high-frequency circuit materials. RO4835 laminates were developed to combat the effects of oxidation and, in so doing, to promote better long-term reliability. They are composites formed of fused silica and woven glass fabric. They are bound with a highly cross-linked hydrocarbon polymer matrix and include an anti-oxidant additive, to be more oxidant resistant than traditional thermoset PCB materials. The RO4835 laminates provide electrical and mechanical properties quite similar to those of Rogers RO4350B™ laminates, with heightened resistance to oxidation because of the anti-oxidant additive.

Elevated temperatures are a threat to any PCB’s long-term reliability, especially when coupled with the need to handle high RF/microwave power levels. When subjected to the combination of high temperatures and high RF/microwave power levels, it is not just the amount of material expansion (as characterized by the CTE) but the rates of expansion (and contraction) of the different materials comprising a PCB that can result in stress junctions, such as between copper conductors and dielectric materials. Ideally, manufacturing processes support optimum thermal management of a PCB, such as proper implementation of plated through holes (PTHs). A through hole in a PCB with poor quality copper plating, for example, can result in undue stress on that portion of the circuit at elevated temperatures. Similarly, manufacturing flaws such as starved thermal viaholes can lead to hot spots and stress points on a PCB.

Proper thermal management of a PCB can also help control the effects of temperature swings on a laminate’s electrical performance. For example, a laminate’s variations in dielectric constant as a function of temperature are defined by a parameter called the thermal coefficient of dielectric constant, and typically evidenced as variations in the impedance of transmission lines. The value of the parameter is different for each laminate, but the amount of change in the dielectric constant due to this effect can be minimized by properly dissipating heat from a PCB.

Of course, starting with a circuit material that is designed for wide temperature ranges can help overcome even manufacturing/production shortcomings such as these. For applications where it may be necessary to handle both higher power levels and operating temperatures, the RO4835 laminates are based on dielectric material with CTE values in the x and y dimensions that are very closely matched to that of copper, to minimize stress junctions at elevated operating temperatures and power levels. In addition, the CTE through the thickness of the material (the z axis) is engineered for stable and reliable PTH quality, even when subjected to elevated thermal conditions.

In fact, the RO4000 family of materials, including RO4350B laminates, is formulated to deliver consistent performance even under more challenging operating conditions, such as high temperatures and power levels. The RO4000 series circuit materials feature low dielectric losses as well as high Tg, to maintain stable mechanical and electrical characteristics over a wide range of material processing temperatures. They are also characterized by excellent thermal conductivity, a parameter which indicates a circuit material’s effectiveness in dissipating heat.

The RO4000 series laminates are affected by oxidation, like all thermoset materials and unlike PTFE materials. But RO4000 materials, such as RO4835 laminates, are RoHS compliant and do not require special viahole preparation like PTFE materials. The RO4000 circuit materials can be processed using standard FR-4 production techniques and, in the case of RO4835 laminates, were formulated for minimal effects of oxidation and with thermal and mechanical properties which support excellent long-term reliability.

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

High reliability is a goal and desire for all designers and end-users of high-frequency printed-circuit boards (PCBs). Since all of the components mounted on the PCB depend on it, it is expected to deliver dependable and consistent performance over time. But, depending on the operating conditions, it can sometimes be difficult to achieve. A high-temperature environment or conditions of high humidity can challenge the stability and reliability of the best-engineered PCB materials.

Of course, some PCB materials hold up better than others under challenging conditions and some deliver outstanding reliability for a wide range of circuits. It can be helpful to know what to look for in a PCB material when high reliability is critical, and what types of operating conditions can put the reliability of a PCB material to the test. When considering different PCB material choices, it can be useful to know which material parameters and characteristics can provide insight into the expected reliability. In an attempt to help, we will explore PCB material reliability: this post, Part 1, will review some of the general obstacles for a PCB material to achieve good long-term reliability. The next post, Part 2, will take a close look at how the characteristics of one particular PCB material add up to good long-term reliability.

Nothing lasts forever, not even a PCB. But a well-engineered PCB material can provide reliable and consistent performance for a long time and across a wide breadth of operating conditions. For many applications, such as for circuits and systems in medical, military, and space applications, PCB material reliability is critical. Anything that can cause a change in the PCB material’s performance or behavior, such as a change in dielectric constant, can be considered a threat or challenge to the long-term reliability of the PCB material.

Elevated temperatures, and the high power levels that can produce them in a high-frequency circuit, may pose the greatest challenges to the long-term reliability of any PCB material. Any PCB laminate consists of different materials with different properties, including dielectric materials and conductive metals, such as copper. These materials expand and contract at different rates with temperature, respectively, resulting in thermally induced stresses on junctions between different materials.

Thermal expansion occurs at different rates through the different dimensions of a PCB material, described by a parameter known as coefficient of thermal expansion (CTE). It is not unusual for a PCB material to have different CTE values through its x and y directions—the width and length—compared to its z dimension or thickness. To improve reliability, PCB materials are often formulated for a dielectric with CTE in the x and y directions matched to that of copper (about 17 ppm/°C), so that the conductive metal and dielectric materials expand and contract similarly with temperature, avoiding stress from thermal junctions.

But this may not be the same in the z direction, where plated through holes (PTHs) are drilled and metalized through the thickness of the material to form signal and ground interconnections as well as to connect different layers of multilayer circuits. The metal/dielectric junction for these structures must also endure thermal stresses, but may not enjoy the same matched CTE values as in the other two PCB dimensions. High reliability depends on the durability of these different metal/dielectric junctions over time and temperature. For that reason, a PCB material engineered for reliability usually features a CTE in the z direction for minimal dimensional changes in PTHs with time and temperature.

Design engineers typically sort through potential PCB material choices by their performance levels, such as the dielectric constant or loss at a target frequency. But when picking a PCB material for high reliability, it is necessary to compare another set of parameters. In its simplest terms, a PCB material built for good long-term reliability should exhibit stable mechanical and electrical characteristics with temperature. A PCB material’s parameters with close ties to reliability include CTE, dissipation factor (Df), and glass transition temperature (Tg).

The Df, which is the loss attributed to the dielectric material, can impact reliability at high operating temperatures and power levels, since high loss will contribute to high material temperatures and added thermal stress. The glass transition temperature, Tg, is something of a warning point for reliability, since it represents the temperature at which dramatic changes can occur in a PCB material’s CTE behavior. Above this temperature, a material can become mechanical and electrically unstable. Although it may be necessary to exceed the temperature for short-term processing steps, it is a temperature that should not be exceeded for any length of time to ensure good long-term reliability.

Another concern for the long-term reliability of some PCB materials is the effect of oxidation, which can cause a small increase in the dielectric constant of a PCB material over time, especially for thermoset PCB materials. The dielectric material covered by copper is protected against oxidation, but the uncovered dielectric material is subject to oxidation and “darkening” in color over time, especially at elevated temperatures. Some thermoplastic PCB materials, such as polytetrafluoroethylene (PTFE), are relatively immune to oxidation, but can be impacted by CTE and other reliability concerns. Oxidation, which is accelerated at higher temperatures, can also cause a small increase in dissipation factor and the loss of transmission lines.

Oxidation can also cause an increase in the loss of transmission lines, although this change is relatively small over time and at elevated temperatures. It has also caused small increases in dielectric constant over time and temperature of a few percent or less, which can be a concern for some circuits where dielectric constant is critical, such as resonators and filters. Of course, oxidation will not occur where oxygen is not present, such as in space in satellite-communications (satcom) applications. When oxidation is a concern, a circuit can be stored in an oxygen-free environment, such as under nitrogen or in a vacuum.

Numerous standards laboratories, including Underwriters’ Laboratory (UL), have studied PCB reliability. UL’s long-term thermal rating for PCB materials is known as the relative thermal index (RTI). It refers to the maximum temperature that a material can handle indefinitely, without compromising performance or critical material properties. It is backed by another UL rating, the maximum operating temperature (MOT), which refers to a particular PCB construction. In that particular PCB configuration, it is the highest temperature that the circuit can withstand without enduring changes in performance or material properties. The MOT can never exceed the RTI for a given material.

Design engineers must review the requirements of an application when sorting through their PCB material choices. Some applications may not have the environmental/performance demands of others and not require the same PCB materials to ensure high reliability. Because PCB material is a complex issue, the next post, Part 2 on PCB reliability, will explore how these general guidelines for reliability in Part 1 can be met by a specific PCB material, Rogers  RO4835™ laminate. This laminate was formulated specifically to improve the long-term aging characteristics of the company’s popular RO4350B™ laminate.

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.

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

Transmission lines are akin to electronic roadways, routing signals along different paths of a printed circuit board (PCB). At RF/microwave frequencies, circuit designers often create PCBs based on three popular planar transmission line approaches: microstrip, stripline, or coplanar waveguide (CPW). Each uses circuit-board materials in a different way, with different results in terms of insertion-loss performance. By getting a grasp on the insertion-loss mechanisms for these different transmission-line formats, circuit designers can better match the mechanical and electrical characteristics of their circuit substrates to their intended applications and transmission lines when choosing PCB materials.

Achieving low loss in an RF/microwave circuit is more critical for some applications than for others, and many excellent low-loss commercial PCB materials such as RO4350B™ laminates from Rogers Corporation are available to help optimize a circuit’s loss performance. But the choice of transmission line for a design can also impact the insertion-loss performance of that circuit. The insertion loss of a PCB’s transmission lines is actually the sum of a number of contributing losses, such as losses attributed to the conductors, to the dielectric material, and due to radiation from the PCB. Microwave transmission lines can also suffer leakage losses, although these tend to be associated more with semiconductors than with PCB materials.

Conductor losses are related to the type of metal (and possible finish on the conductor metal) in the PCB’s conductor layer as well as the operating frequency. Signal propagation at higher frequencies tends to use less of the conductor’s metal as the frequencies increase, with signal “skin depth” becoming very shallow at the highest operating frequencies and only the outer surface of the conductor used for signal propagation at the highest frequencies.

An ideal electrical conductor would exhibit minimal resistance and high conductivity for signals of interest. Of course, real conductors do exhibit loss and have imperfections, including surface roughness, which can contribute significantly to a conductor loss. At RF/microwave frequencies, a rough conductor surface represents a longer propagation path than a smoother conductor surface, with higher loss. A PCB’s dielectric loss is related to the material properties of the circuit substrate, in particular its dissipation factor (Df). Selecting circuit materials with low Df can help minimize this component of transmission-line insertion loss.

Radiation loss is due to energy passed by a PCB’s transmission lines into the surrounding environment. This insertion-loss component can be affected by a number of factors, including the choice of transmission-line topology, the PCB’s dielectric constant, the operating frequency, even the circuit-board thickness. It tends to decrease with thinner PCB materials and for circuit materials with higher dielectric constants. Radiation losses are most noticeable at junctions in a circuit, including impedance transitions and signal launch areas, such as the transition from a transmission line to a coaxial connector’s center pin. Of the three popular RF/microwave transmission-line formats, microstrip is particularly susceptible to radiation loss.

Each of the transmission-line technologies suffers some insertion loss, no matter how good the PCB material. Understanding how loss occurs for the different transmission-line approaches can help guide a circuit designer when choosing a PCB material for a given loss budget. As mentioned, microstrip can suffer more from radiation loss than stripline or CPW, requiring additional shielding for some microstrip circuits. But microstrip is the most popular of the three transmission-line formats, since it is the simplest and least expensive to fabricate. It is basically a metal conductor on the top of a dielectric layer with a metal ground plane on the bottom of the dielectric layer. Factors that can influence performance include the type and weight of the metal for the conductor and ground plane, the width of the conductor lines, the relative permittivity or dielectric constant of the dielectric material, and the thickness of the dielectric layer.

In contrast, stripline transmission lines are sandwiched between top and bottom dielectric layers, which in turn have metal ground planes on the top and bottom of the dielectric materials. Plated through holes (PTHs) are machined through the metal and dielectric layers to electrically connect the top and bottom ground planes. Stripline presents difficulties in adding discrete circuit elements and active devices, which require viaholes to connect components on the outside of the circuit to the internal circuitry and transmission lines. This is in contrast to the simplicity of top-mounting components on a microstrip board. CPW circuits offer the simplicity of top-mounting components, since these circuits are formed with top-layer conductors surrounded by a top-layer ground plane, and with an additional bottom-layer ground plane separated by a dielectric layer. As with stripline, the top and bottom ground planes are electrically linked by PTHs machined through the substrate material. The additional ground planes help improve electrical performance but also add size, complexity, and cost to the stripline and CPW circuits compared to microstrip circuits, which are among the tradeoffs that circuit designers must weigh when choosing a transmission-line format for a particular circuit application.

How does the choice of PCB material impact the insertion loss of one of these high-frequency circuits? The loss characteristics of a microstrip circuit, for example, will change for different thicknesses of the same PCB material. A free personal computer (PC) software tool, MWI-2010, available for download from the Technology Support Hub on the Rogers Corp. web site, can show the influence of a circuit material on transmission-line loss. MWI-2010 contains models of different circuit board materials, permitting designers to explore the impact of different material parameters on performance.

The software was used to analyze the impact of substrate thickness on microstrip transmission-line loss, modeling simple 50-Ω microstrip transmission-line circuits on three different thicknesses (6.6, 10, and 20 mils) of RO4350B circuit material. The material has a process dielectric constant of 3.48 at 10 GHz and low dielectric loss, with Df of 0.0037 at 10 GHz. For microstrip transmission lines, the software shows that the insertion loss is the least for the thickest circuit board, with conductor and dielectric losses that were relatively low and similar in value. The thinnest circuit board had the highest insertion loss, with conductor loss the dominant of the three loss components. Conductor loss can be somewhat diminished by choosing a PCB material with smooth conductor metal, such as RO4000® LoPro™ circuit material from Rogers Corp. The dielectric loss changed little with the three thicknesses of RO4350B laminate, indicating it is an electrically stable PCB substrate.

When loss is critical for a circuit, a low-loss circuit material can help achieve design goals by minimizing dielectric losses. And conductor and radiation losses can be controlled through choice of transmission-line technology, although that choice will also depend on a number of other factors, such as required circuit size, complexity, and cost.

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.

This post authored by John Coonrod originally appeared on the ROG Blog hosted by Microwave Journal. You can view Bandpass Filters, Part 1 here.

Bandpass filters make many of our modern electronic systems possible, from dependable cellular telephones to secretive military surveillance and radar systems. The last blog – Part 1 of this two-part series on bandpass filters – highlighted the versatility of one circuit material from Rogers Corporation, RT/duroid® 6010.2LM laminate, for fabricating RF/microwave bandpass filters. But not all circuit materials are the same and there may be some advantages to designing bandpass filters on other materials, such as Rogers RO4000® family of printed-circuit-board (PCB) materials. This blog will examine different grades of these and other circuit materials and the impact they have on the design and fabrication of high-frequency bandpass filters, especially compared to filters formed on filled-PTFE-based circuit materials.

The first installment of this two-part blog on bandpass filters described different bandpass filter responses, including Chebyshev, Bessel, and Butterworth filters, and reviewed a number of the performance parameters for comparing bandpass filters, such as center frequency, passband, passband insertion loss, and stopband responses. As was noted in that first part, the choice of PCB material for a bandpass filter usually starts with the material’s dielectric constant (Dk). Materials with higher values of Dk, such as 10.2, have long been favored for RF/microwave filters because of the relationship of the Dk value to the size of the filter. Quite simply, higher Dk values result in shorter wavelengths and higher frequencies, enabling filter designers to occupy less PCB area for a given filter structure.

Circuit materials with a Dk of 10.2 are typically based on polytetrafluoroethylene (PTFE), which provides excellent electrical characteristics but tends to be more expensive than other circuit materials. As pointed out in the first part of this blog, circuit materials based on PTFE with some form of filler are also susceptible to moisture absorption, which can result in shifts in their Dk values in high-humidity environments. As that earlier blog suggested, a lower-cost material such as RT/duroid 6010.2LM laminate, which is a composite of PTFE and ceramic materials, can also provide the high Dk value without the concern for moisture absorption.

But are there benefits to designing and fabricating bandpass filters on materials with lower values of Dk? Rogers RO4000 circuit materials are widely favored by amplifier designers for their mechanical and electrical stability, but they are also excellent starting points for bandpass filters. The materials are based on reinforced hydrocarbon/ceramic laminates, not PTFE. RO4360™ laminates, for example, have a Dk value of 6.15 in the z-axis at 10 GHz, held to an impressive tolerance of ±0.15. The material is based on a ceramic-filled, thermoset-resin system reinforced by glass fiber for excellent mechanical stability. These lead-free-process-capable laminates exhibit dissipation factor (loss) of 0.0030 at 2.5 GHz and 0.0038 at 10 GHz, both in the material’s z-axis.

In terms of bandpass filter size, the lower Dk value of 6.15 for RO4360 laminates  compared to a Dk of 10 or higher for filled PTFE circuit materials translates into somewhat larger filter structures, with wider conductor widths. The higher loss of RO4360 laminates compared to filled-PTFE-based materials typically means somewhat higher passband insertion loss, but the differences in loss between filters formed on the two materials may not be so significant when factoring in RO4360 laminate’s wider conductors. RO4360 laminates offer lower material costs than filled-PTFE-based substrates, and circuits are easier and lower in cost to fabricate compared to filled PTFE. RO4360 laminates provide improved mechanical stability and consistency compared to filled-PTFE-based materials, especially in environments with high humidity. Essentially, the tradeoffs between the two materials involve cost versus the levels of performance required for a particular application, as well as the somewhat smaller size possible for high-frequency bandpass filters fabricated on materials with higher Dk values.

Larger filter dimensions may not always be a design goal but at times can be a benefit, especially for applications that involve higher power levels. When designing and fabricating filters on higher-Dk materials, transmission-line conductor widths must be reduced compared to lower-Dk circuit materials to maintain the typical 50-Ω impedance of higher-frequency designs. But those thinner conductor widths, along with the circuit material’s thermal properties such as its thermal conductivity, will serve as a limit for a filter’s power-handling capabilities on that particular material. Also, the thinner conductor widths can result in penalties in terms of production yields.

RO4360 circuit materials provide better thermal conductivity than many substrates based on filled PTFE, although with somewhat higher loss that can in part offset the enhanced thermal conductivity. RO4360 laminates provide typical thermal conductivity of 0.8 W/m/K, enabling it to dissipate heat produced by circuits handling high power levels. In addition, RO4360 laminates have coefficients of thermal expansion (CTE) of 16.6 and 14.6 ppm/°C, respectively, in the x and y directions, very closely matched to copper in support of good circuit reliability at higher power levels.

For some small sacrifice in passband insertion-loss performance compared to filled-PTFE-based materials, RO4360 circuit materials can deliver RF/microwave filters without complicated production processes. The thermoset material can be handled in much the same way as low-cost, epoxy-based FR-4 circuit materials, and even readily combined with these lower-cost materials as part of a multilayer circuit structure. Often, a circuit design is “segmented” by material type in a multilayer design, with higher-frequency circuits, such as RF/microwave bandpass filters, fabricated on materials such as RO4360 laminates and less-critical circuits, such as power supplies, formed on lower-cost circuit materials such as FR-4.

Compatibility in processing the different materials not only simplifies production, but also ensures reliability of the plated through holes (PTHs) used to electrically connect the different circuit layers in a multilayer circuit construction. The reliability of those PTHs in multilayer circuits with RO4360 laminates is also aided by the materials excellent CTE in the z-axis (30 PPM/ºC), also closely matched to that of copper to minimize stress of connections and circuitry across a range of operating temperatures.

In contrast to filled-PTFE-based materials, which require special processing measures, RO4360 laminates are compatible with standard PCB processing methods, as used with FR-4 materials. They boast a high glass transition temperature (Tg) of greater than +280ºC as assurance of handling high process temperatures. In addition to electrical and mechanical characteristics that make them attractive for fabricating RF/microwave bandpass filters, they are environmental friendly and RoHS compliant. For those bandpass-filter applications that can afford slightly higher passband loss and slightly larger circuit dimensions (than some filled-PTFE materials), RO4360 circuit materials offer a high-performance alternative that can cut both material and processing costs.

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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