Barry Matties: Travis, Isola recently announced some big changes within the company organization and its distribution. What changes did you make, what’s driving those changes, and what should the industry be made aware of?

Travis Kelly: We’ve had many discussions here relative to the momentum with printed circuit boards in the U.S. and at Isola, as well as the Printed Circuit Board Association of America (PCBAA), and through our customers and suppliers. We’ve spent the last two years aligning to the shifting market, and there are many changes.

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Fifth Generation (5G) networks offer extensive wireless services with even faster data rates to come. Most of the systems have been based on signals below 6 GHz. Once network coverage has been achieved at higher, millimeter-wave (mmWave) frequencies of 24 GHz and higher, 5G networks will support multi-gigabit upload and download speeds. Distributed network coverage at those higher frequencies will require many small cells with printed-circuit boards (PCBs) capable of RF, microwave, mmWave, and high-speed-digital (HSD) operation. Those small cells will demand PCB materials with high performance and reliability through mmWave frequencies, and with characteristics well suited to the operating environments of both indoor and outdoor 5G small cells.

Small cells in 5G networks are being designed for use at mmWave frequencies to take advantage of available frequency spectrum for wideband communications, including streaming video and fast data. By using mmWave spectrum and transmission bandwidths as wide as 2000 MHz, 5G small cells are being designed for upload speeds as fast as 10 Gb/s and download speeds to 20 Gb/s. But operating at those higher frequencies and bandwidths small cells will be much smaller than standard 5G cell sites or base stations, with far fewer simultaneous users and providing much smaller coverage areas because of the short-wavelength signals that provide coverage at mmWave frequencies. While base stations or macrocells may cover circular service areas several miles in circumference, dedicated small cells may be needed for each block of a city’s 5G service area.

5G small cells will be constructed in three basic sizes for indoor and outdoor use: microcells, picocells, and nanocells. The largest of the three, microcells, may be in shoebox-sized enclosures on every street corner,  mounted on lamp posts to give 5G service providers as much as one mile of indoor and outdoor coverage from about 2 W (+33 dBm) maximum transmit power at mmWave frequencies. Smaller and with about 0.25 W (+24 dBm) transmit power, picocells will provide short-range coverage (about 600 ft.) when mounted outdoors or indoors. The smallest of the small cells, a femtocell, is for indoor use only and typically for a single user. It transmits at about +20 dBm power for a maximum coverage range of about 30 ft.

Densely Packed PCBs

As 5G expands, thousands of small cells will provide new wireless capabilities to millions of 5G users in every major metropolitan area. The small cells will be invisible to most users, mounted at reasonable heights on lamp posts to avoid obstacles that might block mmWave signals. And they will lack the attention-grabbing presence of 4G base stations, with their multiple towers and large antennas. But, to consistently perform the many functions required of small cells for dependable 5G wireless communications, especially within the uncharted domain of mmWave frequencies, they will contain a full share of technology within those small enclosures, calling on densely packed PCBs with many integrated circuits (ICs) mounted on multilayer circuit assemblies.

What types of challenges exist for circuit materials in these 5G small cells? Consistency is a key term for many material characteristics, especially across wide operating temperature ranges and changing conditions such as temperature and humidity. Circuit materials for PCBs in 5G small cells must maintain electrical consistency, for such parameters as dielectric constant (Dk) and dielectric loss or dissipation factor (Df), as well as mechanical consistency capable of holding the tight dimensional requirements for multilayer PCB assemblies that pack a great deal of functionality within a small space. Whether at the prototype stage, where innovative designs may be implemented in as many as 16 circuit layers, or in final production that might require highly integrated circuit assemblies with as many as 40 circuit layers (as some PCB fabricators are claiming), the mechanical consistency is critical for maintaining the alignments of transmission lines that impact signal amplitudes and phases as mmWave frequencies. Alignments between circuit layers are also important, in the form of blind and buried via-hole interconnects, and mechanical circuit material deviations because of environment effects such as temperature, humidity, even vibration of the 5G small cell can degrade small cell electrical performance.

In general, circuit materials for 5G small cells, should have low Dk values. While those Dk values may not be totally constant with operating frequency, since circuit material Dk tends to gradually decrease with increasing frequency, they should be predictable so that a measured Dk value at frequencies below 6 GHz can be accurately projected for frequencies above 24 GHz in 5G small cells, to aid the accuracy and effectiveness of computer-aided-design (CAD) software tools used in circuit modeling.

Circuit materials for 5G small cells should also exhibit low Df values, which will rise as a function of frequency, to minimize dielectric circuit losses. Circuit materials with minimal moisture absorption, especially in outdoor cells, can minimize the dielectric losses caused by water absorption in high humidity environments. Specifying circuit materials with smooth conductors can also help minimize conductor losses at mmWave frequencies. The combination of low conductor and dielectric losses greatly aids hybrid mixed-signal circuits which may include radio transceivers operating at mmWave frequencies, microprocessors, data converters running at multigigabit rates, and the wide assortment of high-frequency, high-speed components and surface-mount-technology (SMT) devices employed on 5G small-cell PCBs, such as low-noise amplifiers (LNAs), power amplifiers (PAs), and digital attenuators.

Given the need to transmit signals from such small enclosures, 5G small cells also flirt with the effects of internally generated heat, from active components such as PAs and resulting from excessive circuit losses. PCBs for 5G small cells should have thermal management capabilities that can safely conduct heat away from heat-generating components and the components and circuit surrounding them and to outside the small cell enclosure. A critical material parameter, coefficient of thermal expansion (CTE), provides insight into how a circuit material changes physically as a function of temperature, in all three axes, so that it can also provide a means of evaluating different circuit materials for the durability of plated thru-holes used to interconnect multiple circuit layers.

Sorting Solutions

With the high device densities and hybrid natures of PCBs in 5G small cells, practical circuit materials must be formulated for stable but exceptional electrical behavior as well as outstanding mechanical behavior even in outdoor operating environments. In addition to performance, there is the manufacturing aspect and favorable circuit materials for 5G small cells should also be process-compatible with commonly used circuit materials, such as FR-4, which may also be within a 5G small cell enclosure and part of the production line for 5G small cell PCBs. Fortunately, I-Tera® MT40, Astra® MT77, and Tachyon® 100G circuit materials from Isola-Group (www.isola-group.com) are low-loss substrates with high dimensional stability across temperature and humidity. All three materials feature less loss than traditional FR-4 circuit materials at higher frequencies, but they are process-compatible with FR-4 materials to help speed and simplify the steps needed to fabricate multilayer circuit assemblies for 5G small cells.

All three substrates employ low-profile copper conductors which contribute to low loss in HSD circuits and low passive intermodulation (PIM) in antennas and other analog mmWave circuits. The three materials feature good dimensional stability with temperature, which minimizes physical changes with temperature for good soldering accuracy. And all three exhibit extremely low moisture uptake so that even in high-humidity operating environments, increases in signal loss due to moisture absorption will be minimal, even at mmWave frequencies. For processing purposes, all three are characterized by a glass transition temperature (Tg) of +200°C.

More specifically, I-Tera® MT40 (RF/MW) laminate and prepreg materials (Fig. 1) have a typical Dk of 3.45 at 10 GHz as measured through the z-axis (thickness). The materials have typical Df of 0.0031 at 10 GHz. With a low z-axis CTE, the circuit materials suffer 2.8% or less total variation in thickness at temperatures from +50 to +260°C. This enables reliable plating of thru-hole interconnections with the tight impedance control needed at mmWave frequencies. High thermal conductivity of 0.61 W/m-K helps channel heat away from a source, such as a PA, while low moisture absorption of 0.1% means loss characteristics that are unaffected even in high-humidity operating environments.

When less loss is required at a lower Dk, Astra® MT77 laminates and prepregs (Fig. 2) exhibit typical Dk of 3.0 through the z-axis at 10 GHz, with Df of 0.0017 at 10 GHz. The Dk through the z-axis remains constant at 3.0 from -40 to +140°C, with thermal conductivity of 0.45 W/m-K to aid in the transfer of heat. Astra® MT77 circuit materials are very stable in high-humidity conditions, with low moisture absorption of 0.1%.

The third of the three circuit materials, Tachyon® 100G laminates and prepregs materials (Fig. 3), are well suited for HSD circuits of 100 Gb/s and faster in 5G small cells. They have a Dk through the z-axis o 3.02 at 10 GHz which remains constant for operating temperatures from -55 to +125°C and Df of 0.0021 at 10 GHz. With a thermal conductivity of 0.42 W/m-k, they can support densely packed PCBs with high heat sources and, with low z-axis CTE, they z-axis variations with temperature of only 2.5% result in dependable plated thru-holes for multilayer circuit assemblies. They share the low moisture absorption of 0.1% of the other two mmWave circuit materials.

PCB assemblies within 5G small cells will be densely packed with components to maximum analog and digital functionality while delivering the consistency and reliability needed for modern 5G wireless network communications service. Circuit materials for those PCB assemblies will serve as foundations for the HSD and mmWave signals moving to and from the 5G small cells and they must perform in all seasons and at all temperatures. The choice of circuit materials for many of the circuit layers in those assemblies will be critical to determining the successful operation of 5G wireless networks at mmWave frequencies.

by Alexander Ippich, Technical Director

Signal Integrity & Advanced Technology, Product Manager RF/Microwave

OEM Marketing Europe, Isola

References

“Isola’s Role in the Deployment of 5G”

“The Importance of Circuit Board Material for Millimeter-Wave Circuits”

Figure captions

1. I-Tera® MT40 circuit materials support high-performance mmWave, HSD, and mixed-signal circuits in 5G small cell circuit assemblies. [Image of I-Tera MT40]
2. Astra® MT77 laminates and prepregs meet the most demanding low-loss circuit requirements at mmWave frequencies to 110 GHz and higher. [Image of Astra MT77]
3. Tachyon® 100G circuit materials are well suited for HSD circuits operating at rates to 100 Gb/s and beyond as parts of hybrid circuit assemblies in 5G small cells. [Image of Tachyon 100G]

The post Materials Meet Needs of Millimeter-Wave 5G Small Cells appeared first on Isola Group.

Thousands of satellites surround the Earth in their own orbital slots and even their own constellations, at distinct altitudes. They help communicate with distant places, track weather, and, with the Global Positioning System (GPS), find distant places here on Earth. Closest to Earth, low Earth orbit satellites (LEOS) are in the greatest numbers with totals growing rapidly. Circuit designers and manufacturers in need of circuit materials for RF/microwave and high-speed-digital (HSD) circuits for LEOS must seek substrates that can handle the rigors of a long, orbiting lifetime while meeting rigorous size, weight, and power (SWaP) requirements to keep a LEOS flying. LEOS (as well as their Earth stations) have a special set of needs for circuit materials and knowing those requirements can prevent a LEOS from getting lost in space!

Printed-circuit boards (PCBs) are typically tasked with carrying analog, digital, and even photonic signals within a LEOS under the extremely hostile operating conditions of space. Due to its orbiting motion, a satellite is facing the sun or shielded from it by the Earth, constantly going through thermal cycling as a result. Any circuit materials for LEOS should be thermally robust, over a wide temperature range and for a prolonged period. Circuit materials for space are typically fabricated to withstand a temperature range of -55 to +125°C without mechanical change or degradation in electrical performance.

Circuit materials integrated in an orbiting satellite, such as a LEOS, must be capable of handling greater thermal stress than the circuit materials supporting HSD or mmWave signals back on Earth because of the way temperature changes while in orbit. Temperatures drop rapidly while in the shadow of Earth and rise quickly when a satellite returns into direct sunlight. Circuit materials with good thermal conductivity characteristics can help dissipate the heat produced by direct sunlight.

Circuit materials in LEOS must be prepared for the vacuum of space since dielectric materials tend to lose mass in a vacuum. Outgassing is a phenomenon during which air or other gas trapped within a circuit material during PCB production is released in an ultrahigh vacuum (UHV). It can have unwanted side-effects, forming condensation on PCBs which increases the insertion loss of transmission lines, especially at the increasing number of millimeter-wave (mmWave) frequency bands that will be used for communications in LEOS.

Circuit materials well suited for LEOS should have low outgassing rates, for example, a loss of mass of less than 1% when tested according to ASTM standards. Outgassing can occur during PCB production when circuit materials are subjected to elevated temperatures during bonding and soldering. Standards such as IPC-1601 define methods for baking circuit materials to eliminate or at least minimize outgassing.

LEOS and mmWaves

As the numbers of LEOS increase in orbit, with growing use of mmWave frequencies, reliable, long-lasting circuit materials become even more essential components in the production of low-maintenance LEOS. Whether on Earth, at sea, or in space, circuit materials are characterized by a list of key parameters which indicate their expected behavior under certain conditions. Especially critical for dependable mmWave circuit performance is the consistency of a circuit material’s dielectric constant, Dk. It should not change with temperature, frequency, or time since even the slightest changes can mean variations in the characteristic impedance of the transmission lines that carry a circuits high-frequency and HSD signals. A circuit material with low temperature coefficient of Dk (TCDk) provides the consistent Dk with temperature needed when installed within a LEOS, such as for antennas.

Circuit materials capable of supporting high-speed, high-frequency transmission lines typically exhibit a Dk in the range of 3.0 to 3.5 which yields fine circuit dimensions for a given characteristic impedance. Because signal power is limited in HSD and mmWave circuits, loss from the circuit material’s conductors and dielectric content, as signified by the material’s copper surface roughness and dielectric dissipation factor (Df), should be as low as possible for mmWave circuits, where loss of signal power can be even more critical than for HSD circuits.

LEOS orbit the Earth at altitudes below the radiation belts that can haunt satellites at higher altitudes, such as medium Earth orbit satellites (MEOS) and geosynchronous Earth orbit satellites (GEOS), so radiation resistance is not as critical for circuit materials in LEOS compared to MEOS and GEOS. But circuit materials in LEOS must support low-distortion signal signals in antennas, as indicated by low passive intermodulation (PIM) numbers. Circuit materials for LEOS must also be capable of managing the high voltage corona fields that can form in space. It is this combination of parameters that provides a quick starting point when specifying circuit materials for the PCBs in a LEOS. The selection process is hardly automatic but, with the proper choice of circuit material, a LEOS can serve those beneath it with a long and useful lifetime.

Examples of commercial circuit materials suitable for various configurations of PCBs in LEOS, including single-sided, double-sided, and multilayer boards, are Tachyon® 100G, I-Tera® MT40, Astra® MT77, and 370HR from diversified materials supplier Isola Group. Tachyon® 100G laminates and prepregs support digital speeds to 100 Gb/s and faster. They maintain low Dk of 3.04 at 2 GHz and 3.02 at 5 and 10 GHz with low loss tangent (Df) of 0.0021 at 2, 5, and 10 GHz. I-Tera® MT40 and higher-frequency version, I-Tera® MT40 (RF/MW),  are available with typical temperature-stable Dk values ranging from 3.38 to 3.75 and Df values from 0.0028 to 0.0035.

Astra® MT77 is also extremely temperature stable, with typical Dk of 3.00 at 2 and 10 GHz and Df of 0.0017 at 2 and 10 GHz. And 370HR, with higher Dk of 4.04 and Df of 0.021, is well known for its reliability for lower-frequency circuits where loss is less critical. All the materials are formulated for a wide operating temperature range of -55 to +125°C and low 0.1% moisture absorption which supports reliable use in LEOS. A closer look at any of these materials would require time spent with its data sheet, available for speedy download from the Isola-Group website (www.isola-group.com).

by Fandy Wei, Senior Director, OEM Marketing in Asia, Isola Asia Pacific (Taiwan) Inc.

The post Circuit Materials Meet Critical Needs for LEOS appeared first on Isola Group.

For expanding 5G network coverage and performance, 5G cells will be much smaller than their lower-frequency counterparts (Figure 1), while 5G macro cells employ distributed antenna systems to bring signals at 6 GHz and below to many users over large areas. The same approach does not work effectively at millimeter-wave frequencies where the small signal wavelengths have limited propagation distances and signals can be blocked by nearly anything.

Such small cells will require densely-packed circuitry with high functionality and printed-circuit-board (PCB) materials that can support the performance needed at millimeter-wave frequencies. Fortunately, ultra-low-loss materials have been developed to support the circuits needed for these small cells as some 5G telecommunications equipment designers expect operating frequencies to ultimately reach 140 GHz. Raw circuit materials such as I-Tera® MT40, Astra® MT77, and Tachyon® 100G from Isola Group meet the needs of 5G small cells that can economically and reliably provide the coverage and capacity for present and future 5G wireless cell networks.

Circuit materials for 5G small cells must support high circuit densities and often a blend of analog, RF and microwave, high-speed-digital, and power circuitry within indoor and outdoor 5G base stations and repeaters often about the size of a shoe box. Whether for urban or rural areas, the local zoning codes will usually determine the size and appearance of a small cell and where it can be located. Although in several shapes and sizes with different power levels, the goal of all 5G small cells is the same: to fill in any gaps in coverage and capacity left by larger, more traditional cell towers and base stations.

Small cells are usually defined in ascending order as femtocells, picocells, microcells, and metro cells, with larger small cells transmitting at higher power levels and reaching greater numbers of users. For example, the two smallest 5G cells, femtocells and picocells, are designed for indoor use such as within buildings, as well as outdoor use, and can support as many as 32 and 128 users, respectively, at transmit power levels up to about 5 W. The larger small cells, microcells and metro cells, are meant for outdoor installation and for 256 or more users at transmit power levels as high as 20 W.

In addition to differences in size and power, the small cells may employ different types of antennas and backhaul interfaces to the 5G cellular network that may include wired, wireless, and fiber-optic interconnections. In most cases, the small cells are designed and assembled to be as inconspicuous as possible, adding millimeter-wave coverage and capacity to larger and more conspicuous 5G cell towers and repeaters.

Shrinking Circuits

For economical and efficient 5G small cell circuits capable of high performance through millimeter-wave frequencies, preferred circuit materials have a low dielectric constant (Dk), low dissipation factor (Df), and high glass transition temperature (Tg). Dk is a measure of the capacitance between two conductors on a dielectric material compared to those conductors at the same distance from each other in a vacuum. Materials with higher Dk values can store more electromagnetic energy than materials with lower Dk values.

However, the flow of energy through the conductors is slower for circuit materials with higher Dk values. Lower Dk values also allow for wider transmission lines for the same dielectric thickness and have reduced conductor losses. For these reasons, circuit materials with low Dk are usually preferred as the basis for PC boards with millimeter-wave circuitry.

Signal power is often difficult to maintain at millimeter-wave frequencies, requiring circuit materials with the lowest possible loss. Different transmission-line formats such as microstrip or coplanar waveguide (CPW) are typically used for circuits at millimeter-wave frequencies, and the loss of each transmission-line technology is affected by various circuit material characteristics.

Transmission-line loss is a combination of the PCB’s conductor loss and its dielectric loss. Circuit materials with low Df exhibit low dielectric loss. Conductor loss, especially at millimeter-wave frequencies, is a function of the surface roughness of the conductor material. Low conductor loss is usually achieved with the smoothest, low-resistance copper conductor. The effects of conductor skin depth on transmission-line conductor loss have the greatest impact at the highest frequencies. For example, at 1 GHz, variations in copper roughness of less than 2.1 μm usually have a minimal effect on loss performance. But at 10 GHz, copper treatment variations of only 0.7 μm are enough to disrupt the conductance of those small-wavelength signals.

A circuit material’s Tg is a measure of the temperature at which the material’s thermomechanical properties change, above which the material loses mechanical stability. Circuit materials with high Tg are attractive for millimeter-wave and other circuits in small 5G cells because they have better heat resistance and very often lower moisture absorption than materials with lower Tg values.

For 5G small cells, circuit materials should also provide stable, consistent performance over wide operating temperatures and under conditions of high humidity, especially when meant for use in outdoor locations, and they should be capable of supporting high circuit densities to achieve the number of circuit functions required for a small package. In many cases, the use of environmentally friendly circuit materials such as halogen-free types may be preferred to standard circuit materials, especially for applications deemed to be environmentally sensitive.

Circuits at millimeter-wave frequencies are sensitive to variations in material thickness and rough surfaces of copper conductors on laminates. Thickness variations and rough conductor surfaces can result in variations in amplitude and phase due to the small signal wavelengths at millimeter-wave frequencies.

For consistent, low-loss conductance of millimeter-wave materials, the circuit materials should have smooth copper conductor surfaces and minimal dielectric thickness variations, with minimal physical changes caused by environmental factors such as heat and humidity. Such materials are usually denoted by high glass transition temperature (Tg) and low coefficient of thermal expansion (CTE) while also exhibiting low water (H²O) absorption so the material’s Dk will not be affected by operation during rainstorms in outdoor small cells.

Mining the Materials

To support a growing market in 5G small cells, circuit materials capable of providing the performance needed at millimeter-wave frequencies should also be economical and “process friendly,” compatible with machining and manufacturing steps and techniques used in transforming widely used circuit materials such as FR-4 into PC boards. Such compatibility makes it possible to combine lower-loss, higher-frequency circuit materials with standard materials such as FR-4 as part of hybrid circuit constructions using FR-4 for power-supply circuits and other functions not requiring the low loss of the RF, microwave, and millimeter-wave circuit materials.

As an example, I-Tera MT40 (RF/MW) very-low-loss laminate and prepreg materials (Figure 2) exhibit the low conductor and dielectric losses needed for maintaining good signal strength at millimeter-wave frequencies, evidenced by a typical Df of 0.0031 at 10 GHz. The Dk is 3.45 at 10 GHz through the z-axis or thickness of the material. The circuit materials have a high Tg of +200°C and provide the low moisture absorption and excellent thermal stability needed for densely packed circuits in small 5G cells, with a coefficient of thermal expansion (CTE) and total expansion through the z-axis of 2.8% from +50 to +260°C.

Figure 2: A sequential I-Tera® MT40 stack-up

The materials have the thermal conductivity needed for long operating lifetimes in tightly enclosed small cell housings at 0.61 W/m-K. And for small cells that will be mounted next to streetlights in highly populated areas, low 0.1% moisture absorption signifies reliable operation even during rainy weather conditions.

For millimeter-wave circuits requiring even lower loss, Astra MT77 ultra-low-loss laminates and prepregs (Figure 3) offer a cost-effective alternative to PTFE circuit materials, with a Df of a low 0.0017 measured at 2 and 10 GHz. The Dk, which is 3.0 at 2 and 10 GHz, also remains stable with temperature when analyzed from -40 to +140°C. These circuit materials have a Tg of +200°C and exhibit excellent thermal electrical and dimensional stability with low moisture absorption. They have a somewhat lower thermal conductivity than I-Tera MT40 (RF/microwave) circuit materials, at 0.45 W/m-K, but like those materials are FR-4 process compatible.

Figure 3: Astra® MT77 ultralow-loss laminates and prepreg circuit materials are compatible with standard loss circuit materials used in hybrid circuits in 5G small cells

For high-speed data circuits in 5G small cells, Tachyon 100G ultra-low-loss laminate and prepreg materials are also defined by a high Tg of +200°C. As with the other circuit materials, they have a low Dk that remains stable over wide operating temperature ranges. Tachyon 100G has slightly higher Dk and Df than Astra MT77 materials, although with suitable characteristics for HSD circuits operating at rates more than 100 Gb/s. The Dk is 3.04 at 2 GHz, 3.02 at 5 GHz, and 3.02 at 10 GHz. The Df is consistently 0.0021 at 2, 5, and 10 GHz. The materials undergo 2.5% total expansion through the z-axis at temperatures from +50 to +260°C and, like I-Tera MT40, Tachyon 100G circuit materials have low moisture absorption of 0.1%.

These three materials feature the high-performance characteristics required for low-loss circuits at microwave and millimeter-wave frequencies and high-speed data circuits operating at Gigabit speeds. However, they may provide performance that is not needed for some of the circuits in a 5G small cell, such as for control or power distribution circuitry.

Hybrid circuit designs have emerged in which circuit materials are selected according to function, such as FR-4 circuit materials for control and power circuits, and they are combined with the higher performance (and more costly) circuit materials needed for higher-frequency circuits. In addition to low-cost FR-4 materials, 370HR circuit materials (Figure 4) from Isola Group provide the processing and thermal compatibilities with higher-frequency, higher-speed I-Tera MT40, Astra MT77, and Tachyon 100G circuit materials to form economical but reliable multiple-signal, multilayer circuit assemblies that mix and match the circuit functions according to the circuit material characteristics.

Figure 4: 370HR circuit materials in an automated manufacturing line at Isola

Compatible with FR-4 circuit processing, 370HR materials support similar circuit functions in 5G small cells as FR-4 materials, such as power distribution and control circuits. They have higher Dk and Df than the microwave/millimeter-wave circuit materials with good consistency for general purpose use. The Dk is 4.24 at 0.1 GHz, 4.04 at 2 GHz, and 3.92 at 10 GHz, while the Df is 0.0150 at 0.1 GHz, 0.0210 at 2 GHz, and 0.0250 at 10 GHz. The 370HR materials have a lower Tg at +180°C than the higher-frequency materials but undergo total z-axis CTE of 2.8% or less from +50 to +260°C. This provides thermal behavior consistent with the higher-frequency materials for minimal distortion caused by temperature when combined with the other materials in hybrid circuit assemblies.

Along with being physically inconspicuous, many 5G small cell manufacturers are setting new standards for zero carbon emissions and for circuit materials that will not emit toxic gases into the environment. TerraGreen® laminates are “environmentally friendly” while providing the properties needed for low-loss millimeter-wave circuit performance in 5G small cells.

These halogen-free circuit materials match the high Tg of the other materials, at +200°C with Dk that remains stable with temperatures from -55 to +125°C. The Dk is 3.45 at 2, 5, and 10 GHz with a Df of 0.0032 at those same test frequencies. As with the other circuit materials, TerraGreen is compatible with FR-4 circuit processing, is RoHS-compliant with lead-free processing, and is mechanically stable with temperature and consistent with the other materials, with total z-axis CTE of 2.9%.

Summary

The characteristics of circuit board materials are always important but millimeter-wave frequencies present additional challenges that aren’t as great a consideration at frequencies where, among other factors, signal traces are larger and easier to accommodate and achieving best performance is somewhat less difficult. Fortunately, Isola Group has developed families of materials designed specifically for use at frequencies that are increasingly used for both commercial and defense applications.

by Alexander Ippich, Technical Director, Signal Integrity & Advanced Technology, Product Manager RF/Microwave OEM Marketing Europe

The post The Importance of Circuit Board Material for Millimeter-wave Circuits appeared first on Isola Group.

A basic two-way radio relies on a variety of circuits, with RF, digital, power, control, and audio functionality. “One board fits all” would simplify the circuit design process, but the circuits have much diverse needs. Finding the best substrate material for each circuit is a matter of knowing as much as possible about  each circuit. A PCB substrate is a dielectric insulator that isolates and protects electrical conductors and components, like the jacket around a cable. Effective blends of material transfer the electromechanical (EM) force of the conductors with minimal loss. As circuits grow more complex, often combining functions in hybrid arrangements, the requirements for their substrate materials become more challenging. Knowing as much as possible about each circuit can ease the task of finding the right circuit material for it.

In competitive markets, modern circuits perform many functions, at higher speeds and frequencies, and in smaller sizes. An amplifier that boosts the levels of high-frequency signals for transmission by a microwave radio may contain circuitry for power, control, transfer of modulated signals, even high-speed digital conversion, and a single PCB for all functions could provide a small, lightweight radio design. Why not use a single substrate material for the PCB?

Numerous substrates are available, based on combinations of dielectrics and reinforcements. They can be supplied as prepregs—without conductive metals—and as laminates with copper films. They are characterized by a bewildering number of parameters, often difficult to compare from one material data sheet to another. A key parameter is permittivity. It denotes a material’s capability to store charge or serve as a capacitor. It is often described by dielectric constant (Dk or κ) which is the ratio of the material’s permittivity to the permittivity of free space. A material’s Dk determines the dimensions of a conductor at a particular impedance, with higher Dk requiring smaller dimensions for a given impedance.

More important than its nominal value is the consistency of a circuit material’s Dk, especially at higher analog frequencies and digital speeds. Variations in Dk cause variations in conductor impedance and these should be minimized, especially as analog frequencies increase and digital speeds race higher. Circuit materials can suffer variations in Dk due to temperature, moisture, and inconsistency in its thickness (z-axis). So, while the nominal Dk value of a circuit material is an important circuit design parameter, Dk consistency is essential for any circuits in which circuit impedance and stability is a concern, a concern which tends to grow at higher analog frequencies and digital speeds.

Knowing a Circuit

How a circuit is used, such as its operating temperature range, determines the circuit material characteristics best suited for that application. Operating limits to consider include a circuit material’s electrical, mechanical, and environmental limits. With strict limits for all three categories, such as for space-bound or high-reliability (hi-rel) circuits and systems, a list of candidate circuit materials for a project can grow shorter very quickly.

A material’s composition accounts for its behavior. Dielectric materials with woven glass cloth reinforcement can achieve mechanical and thermal stability not possible without it. The type of circuit will point to the material composition required. A circuit at higher power levels, such as a power amplifier, will best be served by a substrate capable of low loss and high thermal dissipation, typically formulated with a woven-glass-reinforced material with low dissipation factor (Df), that contains filler particles to increase heat transfer capability.

The composition of a circuit material can also have much to do with the fabrication and assembly costs of a circuit such as an amplifier. Whether that amplifier is mounted on a single-layer PCB or part of a multilayer circuit assembly, it will generate heat which must be dissipated with various structures, such as heatsinks and plated thruholes fabricated on the circuit material. By knowing the requirements of the amplifier, circuit material can be selected for optimum electrical performance but also “fabrication friendly” by requiring shorter processing time and lower processing temperatures than costlier material choices.

By knowing more about a circuit under development, it is possible to know more of what to expect from the circuit materials selected for that circuit. Requirements for circuit functions vary widely, with optimum circuit performance achieved with a laminate having properties best suited to the specific needs of a circuit. Since some circuit parameters, such as highest performance and lowest cost, may conflict, the best choice of a circuit material is made by understanding the needs of the circuit as fully as possible and then weighing the characteristics of dissimilar circuit materials and how they relate to the needs of a circuit to be designed and fabricated. Of course, the material selection process must consider the manufacturing side as well as design challenges and such things as processing temperatures

This series of blogs will examine the needs of distinct types of circuits, including HSD and millimeter-wave (mmWave) circuits, and how real-world circuit materials can best support them, not just at the prototype stage but in production. These blogs will attempt to make the circuit material selection process a little easier, even when the list of PCB materials looks long. By knowing what the circuits require, the numbers on the circuit material data sheets make more sense and are simpler to sort through. The blogs will explain how circuits are being combined, how various circuit materials perform, and how materials are processed to save time and cost.

Many commercial materials offer attractive electrical, mechanical, and environmental characteristics while supporting the automated machining and processing temperatures needed for cost-effective, high-volume production. By knowing the needs of their assemblies, designers and fabricators can match those requirements to the right materials. This blog series will help by exploring how certain material characteristics best serve distinct applications and operating conditions. In the meantime, advisors at Isola Group are available to offer guidance and advice on the selection of an optimum circuit material. It starts with knowing the circuit!

by Matthew Lake

Senior Director of Advanced Technology

The post Why So Many Circuit Materials? appeared first on Isola Group.

Construction of the network infrastructure for 5G wireless networks involves the addition of smaller cells to existing cell towers and radio equipment. Large-area cells known as macrocells are being supplemented by smaller femtocells, picocells, and microcells as part of what is being called “the densification of 5G network infrastructure.” More cells are needed to provide wireless coverage at mmWave frequencies. Signal wavelengths are shorter at higher frequencies and mmWave signals propagate across shorter distances than lower-frequency signals. They are also more attenuated through building materials than lower-frequency signals, requiring small indoor cells for optimum wireless coverage within indoor cell sites, such as shopping malls and office buildings.

Smaller Cells

In 5G network infrastructure, the large cell towers that have identified earlier wireless cellular network generations will be outnumbered by many thousands of smaller 5G cell stations as needed for dependable radio access at mmWave frequencies. Macrocells with those large cell towers will still provide coverage for many users, especially over long ranges, and lower frequencies, but smaller cells will need to “fill the holes” for extensive signal coverage at higher frequencies and densely populated areas with large numbers of users.

Smaller cells provide a ready means of adding users to a 5G network and existing macrocells. Each small cell functions as a cellular base station, sending and receiving signals from users. A small cell adds its own data capacity to a 5G network’s macrocells, increasing the overall 5G network capacity. Small cells have smaller coverage areas than macrocells and serve less users than macrocells.

While a microcell may serve many more than 2000 users at one time over a coverage distance as great as 25 km, a microcell may provide network access for as many as 2000 wireless users but only at a distance as far as 1 km and with much less transmit power. Picocells, designed for indoor and outdoor sites, may reach only 200 m, and connect as many as 100 users. Femtocells, designed only for indoor use (and more controlled environmental conditions), will transmit at the lowest power levels, and provide a range of only about 50 m, or about 30 users at one time, such as in an office building.

Obviously, for 5G mmWave frequency coverage in densely populated areas, many smaller cells will be needed, such as on every lamppost as some cellular carriers have suggested. The operating environment for 5G NR PCBs poses many challenges, not just within the confines of a miniature 5G cell housing but with all the electromagnetic (EM) noise sources in proximity. The requirements for circuit materials to reliably serve the PCBs in those small cells, including for transceivers and antennas, are quite demanding.

Materials That Fit

Substrates for the PCBs in 5G small cells must support the high precision for very small circuit features enabling not only low-loss signal transfer at microwave frequencies but closely spaced circuit lines and traces that will squeeze as many analog, digital, optical, and power functions into a limited housing size as possible. Prepreg materials for 5G small cell PCBs should be characterized by permittivity or dielectric constant (Dk) that is consistent across the length and width of the material and for a temperature range that compares to the expected operating conditions of a 5G small cell.

Consistent Dk ensures that circuit traces formed on the dielectric material will maintain the tightly controlled impedance needed for low-distortion analog mmWave signals and high-speed-digital (HSD) signals with good signal integrity (SI) and low latency. Consistent Dk and impedance also translates to higher-frequency circuits with less phase distortion, as essential for many of the phase-based modulation schemes of 4G and 5G networks and the timing of their HSD circuits.

Prepreg and laminate materials for 5G small cell circuits should also have minimal dielectric loss; for a laminate, low dielectric loss should be coupled with low loss for the conductive cladding. A laminate circuit material should have a smooth conductive surface, which benefits both higher frequency mmWave analog signals and HSD signals, supporting good circuit SI performance. A smooth conductor surface minimizes signal distortion at the small wavelengths of mmWave frequencies with minimal delays for HSD signals.

Low conductor loss is also a prerequisite for the conductive cladding of any circuit laminate specified for use in 5G NR small cells, to minimize signal attenuation at higher frequencies and limit the thermal dissipation of signal power as heat within such a small, confined volume. The type of dielectric material will also contribute to the thermal dissipation capabilities of a circuit laminate, important for densely packed 5G small cells operating with a maximum number of users and at its transmit power limits.

While few circuit materials can meet the challenges of emerging 5G wireless networks, especially those with multifunction, densely packed circuit assemblies that must fit within the tight confines of small cells, I-Tera® MT40 very low-loss laminate and prepreg materials from Isola Group may come close. The laminates feature typical Dk of 3.38, 3.45, 3.60, or 3.75 measured in the z-axis or thickness at 10 GHz. It remains within ±0.01 of that Dk value even across a wide operating temperature range of -40 to +140°C to provide the consistent impedance needed for low-distortion circuitry at mmWave frequencies.

I-Tera® MT40 very low-loss laminate and prepreg materials also feature low dielectric loss, detailed by a typical loss tangent or dissipation factor (Df) of 0.0028 at 10 GHz. The materials have the thermal conductivity needed for long operating lifetimes in tightly enclosed small cell housings, at 0.61 W/m-K.

These materials can be formed into HSD and mmWave circuits without elaborate micromachining or fabrication processes, allowing circuit manufacturers to create consistent low loss plated thruholes and microvias for interconnecting multiple circuit layers in multifunction circuit assemblies. After all, 5G small cells may be no larger than a shoebox, but users will expect full 5G voice, video, and data performance at all times and from 5G cells of all sizes.

by Alexander Ippich, Technical Director, Signal Integrity & Advanced Technology, Product Manager RF/Microwave

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During the tour, I met with Travis Kelly, Ed Kelley, Sean Mirshafiei, Jenny Inocencio, and Walt Niziolek. We covered a range of topics, including lab operations, hiring, the supply chain, R&D, and the concept of the new facility. The factory was designed to extensively utilize automation to improve efficiencies and, as Travis Kelly stated, “to de-risk the labor shortage issue.” As we walked through the facility, it was clear that automation efficiencies were in action as material moved from one process to the next with well-orchestrated choreography.
However, automation does not handle every step in the process. There is a vibrant workforce, and their contributions are highly valued. The following are excerpts from this wide-ranging, daylong tour

Read Full Article Here

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Circuit machining equipment has grown in sophistication. While mechanical drills are commonly used to form alignment holes and thruholes between circuit layers, laser drilling equipment provides precision machining without physical wear on drill bits. High-resolution, precision machining in combination with advanced photoresist systems are needed to form the shrinking dimensions of circuit geometries required for higher-speed digital (HSD) circuits, wireless networks, and applications at RF/microwave (MW) frequencies, especially for densely packed multilayer circuits with growing numbers of features and functions. As important as the precision of the circuit machining equipment is the availability of “process-friendly” circuit materials that lend themselves to strong lamination adherence, clean thruhole formation, and fabrication of the exact linewidths and spacings needed to support HSD and high-frequency RF/MW circuits.

Fitting Formulas
Modern requirements for multiple functions in small electronic products, such as smartphones or wearable medical devices, down-sized high-data-rate boards, or even miniaturized telecommunications boards, may no longer be increasing the layer counts of multilayer circuits but they are increasing the complexity of the processes required to fabricate the circuit boards. Working with circuit materials that must be processed with advanced techniques requires circuit fabrication facilities to maintain sophisticated equipment, such as laser drills, and personnel with the skills to use the equipment. Using circuit materials that are not “process friendly” requires additional time and processing steps compared to many of the materials offered by the Isola Group (www.isola-group.com), which have been engineered for ease of use.

Materials that require complex machining and processing can add to the total time and cost taken to form a complete multilayer circuit or high-density-interconnect (HDI) assembly. The number of layers refers to the conductive layers, which provide interconnections between ICs and other components. The conductive layers serve as planes for different functions, such as signal, ground, and power planes, with prepreg materials providing insulation and isolation between conductive layers. The placement and spacing of the conductive layers are parts of the design and layout of any multilayer circuit or HDI assembly, with good signal-to-ground connections, for example, resulting in good electromagnetic-compatibility (EMC) and electromagnetic-interference (EMI) performance.

Material quality, such as the smoothness of the copper foils used as conductive layers and the stability of the dielectric constant (Dk) for circuit laminate and prepreg materials, contributes a great deal to the performance levels possible with a given HDI assembly. But PCB fabrication and machining are also essential to extracting optimum performance levels from a set of circuit materials. And building PCBs and HDIs with materials that not only are well suited but engineered for modern fabrication processes can result in actual circuit performance that is as close as possible to the levels promised by computer circuit simulations.

The HDI buildup board starts with a double-sided core, with a layer-by-layer buildup by lamination with prepregs. The compatibility of the materials is important for reliability and electrical performance. For cost-effectiveness, for example, FR-4 circuit boards provide suitable stability and performance for power and ground planes as well as for lower-speed digital and lower-frequency RF signal planes. Higher-speed, higher-frequency signal planes require materials with higher electrical and mechanical (often environmental) stability than FR-4 circuit materials so that low-loss dielectric materials with low Dk and low dissipation factor (Df) have often supported the higher-speed, higher-frequency signal planes in HDIs, although some low-loss circuit materials require sophisticated processing methods for dependable, high-yield circuits. Since they are also combined with other circuit materials such as FR-4 in HDIs, interconnections between layers in these high-speed, high-frequency, multiple-material HDIs must be carefully formed for optimum impedance matching between signal and ground planes.

The Isola Group and its Asia Pacific division have worked closely with circuit designers and fabricators to understand specific requirements for multilayer circuits and HDIs with multiple materials, especially when they must be machined for HSD and RF/MW circuits. As an example, Astra® MT77 laminates and prepregs are usable for RF/MW circuits through W-band frequencies. Laminates are available with copper weights from 0.5 to 2.0 oz. (with custom foils available on request) and can be used with prepregs for multilayer circuits and HDIs with any number of layers.

Astra®MT77 circuit materials feature good flow to fill and FR-4 process compatibility to simplify multilayer circuit production. These circuit materials offer a cost-effective alternative to PTFE circuit materials without the machining complexity of PTFE or PTFE’s special thruhole treatments. They exhibit a low dielectric constant (Dk) of 3.00 with consistent performance to 100 GHz. The Dk for each production lot is controlled and measured from 2 to 10 GHz before shipping to customers. For good circuit stability with temperature, critical for automotive electronics applications, they maintain that Dk value from -40 to +125°C. They are compatible with lead-free, RoHS-compliant PCB manufacturing processes and are excellent circuit candidates for HDI assemblies and RF/MW antennas that can be machined easily and effectively.

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Nolan Johnson and Andy Shaughnessy visited with Jim Hartzell of Isola to learn about how the company has thrived in the past 18 months, despite some real supply chain backlogs. Jim also talks about new products, and what’s on the horizon for Isola.

Nolan Johnson: Jim, you’re the Vice President of Sales and OEM Marketing for North America. Business operations, after this very eventful year and a half, have shifted. Walk us through the market challenges you’re currently seeing?

Hartzell: Thanks, Nolan. We’ve been fortunate enough to stay extremely busy through these times and the operations team has been hitting record numbers of our shipments and so forth. From that standpoint, COVID hasn’t had a real big impact. Certainly, going and seeing customers has had a big impact due to COVID. Shows like this is where we’re getting a little bit of face time, but for the most part, we’ve been able to come through this shutdown pretty well.

We’re having some issues and some challenges on the logistics side of the business. A lot of our raw materials come in, either by ocean container or by air, but because of logistics delays, we’re having to ship a lot in by air. We’re seeing a significant number of delays coming in from ocean containers, particularly anything coming in from Asia.

To read the full article click here.

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Nolan Johnson speaks with Michael Gay of Isola and Chris Hunrath of Insulectro about the release of their new halogen-free, high-thermal reliability material, which they hope fills the gap in the market between epoxies and polyimides.

Nolan Johnson: I understand there’s something new on the market for us to talk about. Why don’t you tell us what it is and what the application is?

Michael Gay: Quite a few years ago, the automotive industry OEMs were looking for an alternative to ceramic-based materials for high temperature applications. They wanted something that was less costly. They wanted something that would fill the gap in between typical FR-4 applications and ceramics. A consortium called The Help Project was developed with several large OEMs and other industry participants in the automotive sector who wanted to work together and develop this material. We started with about a half-dozen different candidates and then whittled it down, making comparison to products like 370HR, which is a typical FR-4 lead-free compatible material. We started doing evaluations and we came up with the product we now call IS550H…

To read the full interview, click the link below!

Isola Releases IS550H Material (iconnect007.com)

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