This post authored by John Coonrod, Technical Marketing Manager, and team originally appeared on the ROG Blog hosted by Microwave Journal.

VIDEO: High Frequency Circuits Which Bend and Flex

Flexibility can be an important feature for printed-circuit boards (PCBs). Not all circuits are planar; some may need to be bent once to fit a particular product design while some might need to undergo continuous flexing as part of an application. Not all circuit materials are created equal, and some are more mechanically flexibile than others and can survive a certain amount of bending and flexing without damage. Understanding what makes a circuit material capable of bending and flexing, and what happens to it when it is bent or flexed, helps when specifying circuit materials for such uses.

Circuit boards are composites of different materials, such as conductive metals and dielectric materials, each with its own mechanical properties. The material stackup will depend on the type of circuit and the number of circuit layers. As more different materials are combined to form a PCB, especially in multilayer PCBs, the task of predicting the effects of bending and flexing becomes more complex. A key material parameter in determining how well a particular material will bend and flex is the modulus or stiffness of the material, with some of the composite materials of a PCB significantly stiffer, or with much higher modulus values, than others.

For example, the metallization in an RF/microwave PCB, primarily copper, will essentially determine the limits of flexibility in a circuit board since it has the highest modulus value of the material stackup, at 17,000 kpsi. Compare this to the much lower modulus values of dielectric materials, such as polytetrafluoroethylene (PTFE) with ceramic filler, at 300 kpsi, or PTFE with microfiber glass filling, at 175 kpsi. In a typical microstrip circuit, with conductor layer, dielectric, and ground-plane layer, the dielectric layer offers great flexibility but the top and bottom metal layers will set the limits of bending and flexibility for the composite structure.

Since high-frequency circuit boards are composite structures, the differences in flexibility of the component materials must be considered to determine how much bending and flexing a circuit board can withstand without damage to the stiffest of its material components, the metallization layers. This can be done by treating a PCB as if it were a beam being bent, with a certain bend radius depending upon the stiffness of the beam. A rubber beam will bend much more easily than a higher modulus metal beam, and be capable of enduring a much smaller bend radius without cracking. A PCB considered as a beam will also have a certain bend radius depending upon the overall stiffness of the composite group of materials, with the metallization layers setting the limits on the flexibility and minimum bend radius of the circuit board.

As with a beam, when a PCB is bent into a section of an imaginary circle, with a bend radius for that circuit, strain is placed on different parts of the beam and the PCB, with tension on the outer side and compression on the inner side of the bend radius. Between the areas of tension and compression lies an almost infinitely thin transition area or neutral axis with no strain. The strain increases as the distance from the neutral axis to the tension or compression plane increases. In a balanced circuit board, the neutral axis would lie at the geometrically center of the circuit board.

Stress from tension and compression works in different ways on a PCB’s materials, with tension pulling materials apart and compression squeezing them together. For a PCB with microstrip circuitry and copper conductors on the outer bend radius, this means that the stiffest or highest-modulus material in the composite PCB is being subjected to a certain amount of tension that will increase as the bend radius is made smaller. At the same time, the bottom ground plane is also being stressed and subjected to compression. Both forms of stress, if excessive, can lead to cracks in a microstrip circuit’s metallization layers. In addition, stress occurs at the interfaces of materials with different modulus values, such as the intersection of the copper conductor layer and the dielectric layer. Cracks from stress can start at the interface and work through the copper layer. To minimize damage to the metallization layers and ensure reliability in bent and flexed circuit boards, the key is to determine the amount of stress that a particular PCB can endure without cracking the metal layers.

The amount of stress on a PCB from bending and flexing is not simply a matter of knowing the modulus of the stiffest material component but in knowing how the PCB is constructed. For example, in a multilayer circuit board, differences in the thicknesses of the dielectric layers can cause increased amounts of strain when the circuit is bent. Each layer of a multilayer circuit structure will have its own modulus, and the structure will have a modulus as a whole. Since copper is the stiffest material component of most microwave circuits, the thickness of the copper and the percentage of copper in the entire PCB material stackup will contribute a great deal to the overall modulus and flexibility of the PCB as a whole.

Even the type of copper can determine the flexibility of a microwave circuit. Due to differences in the grain structures of rolled copper and electrodeposited (ED) copper, rolled copper is typically better than ED copper for PCBs that must be bent or flexed. For applications that may call for ED copper, some special types of ED copper are available for better bending and flexing than standard ED copper.  In addition, finishes added to copper conductors, such as electroless nickel/immersion gold (ENIG) plating, can add a high modulus to the overall PCB modulus, limiting the amount of bending and flexing that a PCB can safely endure.

Different microwave circuit constructions will present different bending and flexing capabilities. Stripline, with copper conductors sandwiched between upper and lower dielectric layers, is inherently better equipped for bending and flexing than microstrip. The signal conductor layer in a typical stripline construction is at or close to the neutral axis for minimum stress; however, the outer ground planes will typically have high stress.

General guidelines to avoid damage when bending or flexing circuit materials pertain to single-bend and dynamic flexing situations. When a single bend is required, the bend radius should be at least 10 times the thickness of the circuit so that the strain on the circuit layer is 2% or less. For dynamic flexing, strain should be held to less than 0.2% for more than 1 million flex cycles and less than 0.4% for 1 million or less flex cycles.

Readers wishing to learn more about how to model stresses placed on PCBs from bending and flexing are invited to view John Coonrod’s MicroApps presentation, “High Frequency Circuits Which Bend and Flex,” from the 2016 IEEE International Microwave Symposium (IMS). The presentation provides circuit bending prediction models and includes a microstrip example using ½-oz. rolled copper on 5-mil-thick RO3003™ laminate material from Rogers Corp.

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

Spurious modes can occur in printed circuit boards (PCBs) in spite of the best-laid plans. These modes support extra, unwanted signals, in addition to the intended signals, that can wreak havoc on a PCB and its application, causing interference and degradation of the intended signals. Although minimizing spurious modes in PCBs is largely a result of careful design practices, the choice of PCB material can have some bearing on the final spurious mode behavior, especially at higher frequencies. Understanding how these spurious modes originate can help in keeping them under control, especially on PCBs operating at millimeter-wave frequencies.

PrintAt RF, microwave, and millimeter-wave frequencies, numerous transmission-line technologies are fabricated on PCB materials, stripline and microstrip are two popular transmission-line methods at higher frequencies. The transmission-line structures propagate electromagnetic (EM) waves in different ways, with stripline supporting transverse-electromagnetic (TEM) wave propagation while microstrip supports quasi-TEM propagation. Quite simply, the mechanical structures of these transmission lines are different, with stripline employing a metallic conductor surrounded by dielectric material while microstrip fabricated the conductor on the top of a dielectric layer with a ground plane on the bottom of the dielectric layer. Coaxial cables, where the conductor is also surrounded by dielectric material, also operate in a TEM propagation mode like stripline.

Spurious waves can be surface waves that propagate through a high-frequency PCB or they can be produced by resonant effects within circuits fabricated on a PCB. Microstrip transmission lines offer very little design freedom for minimizing spurious mode propagation. In terms of physical changes to the PCB, using a thinner microstrip PCB material can diminish the amount of spurious mode propagation in a high-frequency circuit, and this is one of the reasons that thinner circuit materials are used at higher-frequencies.

Of course, many of the PCBs designed with microstrip transmission lines must also make a transition to coaxial cables at a launch point, and this represents a transition from the TEM mode of the cable to the quasi-TEM mode of the microstrip transmission lines. But simply because a PCB has been fabricated with microstrip transmission lines and circuitry does not mean that other modes cannot propagate on that PCB; spurious signals represent one of these other propagation modes. These unwanted spurious or “parasitic-mode” signals can interfere with the desired quasi-TEM-mode signals of the microstrip transmission lines and circuitry.

The quality of the signal launch to a microstrip PCB can affect the amount of spurious mode suppression. For example, EM waves propagating from a coaxial connector to a microstrip PCB will not only make a transition from the TEM mode of the connector to the quasi-TEM mode of the microstrip, but the EM waves from the connector to the microstrip will also make a transition from the polar orientation of the cable and connector to the planar orientation of the microstrip. Even the most ideal coaxial-connector-to-microstrip PCB can suffer stray electrical reactances as a result of the transition of the propagating EM waves across an interface that will have some mechanical variations. Even minor impedance mismatches at the connector-microstrip transition can result in signal reflections and radiation at the transition. In addition, variations between the signal path and the ground return path in the transition area can lead to EM wave skew and additional “interruptions” in the intended propagation path and additional sources for spurious mode propagation.

A grounded coplanar-waveguide (GCPW) launch, which is also known as conductor-backed coplanar waveguide (CBCPW), is capable of a fairly smooth transition to a microstrip transmission line, with minimal spurious signal generation. When even more spurious mode suppression is required, for example at millimeter-wave frequencies, GCPW or CBCPW transmission lines can be used on the PCB in place of microstrip transmission lines. This provides more design freedom to minimize spurious mode generation, with a tradeoff being in added design complexity.

GCPW circuits are often used at millimeter-wave frequencies rather than microstrip transmission lines for better suppression of spurious modes at those higher frequencies. The physical configuration of these circuits helps suppress the resonances that can lead to spurious signals. In addition, the use of grounding viaholes in GCPW circuits can help suppress the propagation of resonance modes between the signal and ground planes. The pitch of these viaholes is important, and related to the wavelength of the operating frequency. The pitch of the viaholes should be 1/8 wavelength or less of the highest intended operating frequency for the circuit.

For a PCB, particularly based on microstrip transmission lines and at higher frequencies, resonances in a circuit and its transmission lines can lead to unwanted spurious signals. Resonances can develop between the transmission line’s signal conductor and the PCB ground plane, with resonances occurring between opposite edges of the signal conductor and paving the way for spurious signal propagation. Such resonances can generate their own EM waves in a circuit or transmission line, especially in microstrip circuits at higher frequencies.

The resonances occur according to the dimensions of the transmission-line conductor and the wavelength of the frequency of interest for the circuit. For example, if the physical width of a microstrip conductor is equal to ½ or ¼ the wavelength of the circuit’s operating frequency, resonances will occur. These resonances can lead to EM waves that can interfere with the intended quasi-TEM waves that are meant to propagate through a microstrip circuit. As with the pitch of the grounding viaholes in the GCPW circuits, a design goal that can help avoid the generation of circuit-based resonances (and their accompanying spurious modes) in microstrip circuits is to make certain that no transmission line or circuit features are greater than 1/8 wavelength of the intended operating frequency.

What does the choice of PCB material or PCB material characteristics have to do with spurious mode rejection? The quest for increased spurious mode rejection typically becomes more difficult at higher frequencies, notably at millimeter-wave frequencies, and is not highly dependent on the choice of PCB material, although the dielectric constant (Dk) of a circuit material is one parameter that can have an impact on spurious mode rejection. When a circuit material with higher Dk value is selected, it results in shorter wavelengths for a given operating frequency, which in turn can affect the target size of the microstrip transmission lines when trying to ensure that these transmission lines and circuit features are no greater than 1/8 wavelength of the intended operating frequency.

Screen shot 2014-08-08 at 1.33.54 PMAlthough the thickness of a PCB material can be a concern at higher frequencies, such as millimeter-wave frequencies, the particular conductor width (as noted earlier) is more of a concern at these higher frequencies (with their smaller wavelengths). Still, thinner circuit laminates can help minimize spurious modes at millimeter-wave frequencies, and thinner laminates are also beneficial for reducing radiation losses in higher-frequency circuits. A tradeoff in selecting thinner PCB materials is that they tend to have higher losses than thicker circuit materials. Fortunately, advances in modern circuit materials, such as the lower insertion loss exhibited by RO4000® LoPro™ laminates from Rogers Corp., make it possible to achieve good spurious mode suppression at higher frequencies without necessarily compromising circuit loss performance.

Download the ROG Mobile appto access Rogers’ calculators, including the popular Microwave Impedance simulation tool, literature, technical papers, and the ability to order samples of the company’s high performance printed circuit board materials.

Do you have a design or fabrication question? Rogers Corporation’s experts are available to help. Log in to the Rogers Technology Support Hub and “Ask an Engineer” today.

In this video, John Coonrod discusses why there are so many different dielectric constants (Dk) that are used in the microwave printed circuit board industry.

Send us questions/comments by tweeting us @Rogers_ACM!

There are a number of test methods to determine the dielectric constant of circuit materials used in the microwave or high frequency industry.

In this video, “Common Test Methods for Measuring Dielectric Constant,” you will learn about the most common test methods like Clamped Stripline Resonator Test, Split Post Dielectric Resonator, Full Sheet Resonance (FSR), and Microstrip Differential Phase Length Method.

For additional information and technical tools, join us at rogerscorp.com/techub

Everything changes with time and printed circuit boards are no different! Watch this video to learn about four items that contribute to the aging of high frequency PCBs and the impact on electrical performance.

Once you’ve watched the video, send us your thoughts on Twitter at @Rogers_ACM. We love hearing from you!

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