Choosing Circuit Materials for Millimeter-Wave PCBs
by Alexander Ippich, Technical Director, Signal Integrity and Advanced Technology, Isola
Printed circuit boards are being developed with more bandwidth, typically at higher frequencies, to meet growing demands in the millimeter-wave region. In the commercial world, materials for those PCBs are usually sorted by cost, while in industrial and medical markets circuit materials may be selected according to performance, and in aerospace and military applications, minimum size, weight, and power (SWaP) often guide the choice of a PCB material.
But when designing and manufacturing millimeter-wave circuits, critical circuit material features and characteristics must be considered when choosing a circuit material that will perform well at those higher frequencies while providing the best support for interconnected functions in a complex electronic system.
Wired and wireless millimeter-wave signal frequencies provide broad bandwidths for a growing number of applications, with military use of millimeter-wave bandwidth expected to increase rapidly during the next decade (Figure 1). From narrow-bandwidth systems such as secure line-of-sight (LOS) communications links to applications requiring much wider swaths of bandwidth, such as electronic warfare (EW) systems, high-resolution radar, and high-definition video surveillance, military electronic systems are starving for bandwidth and the millimeter-wave frequency range contains a great deal of it.
The added frequencies will especially help emerging applications on the battlefield, such as a growing number of IoT unmanned sensors that must be constantly monitored and recorded. For many applications not associated with them, such as aircraft radar systems and weapons detection, millimeter-wave frequencies have already been in use for military applications for several decades.
The millimeter-wave frequency range is nominally 30 to 300 GHz or where signal wavelengths are measured from 1 mm to 1 cm. Specifying a circuit material for any PCB involves several choices in dielectric thickness, copper thickness and surface smoothness, and in the types of fabrication and manufacturing processes that will be needed to produce practical PCBs with repeatable performance. At millimeter-wave frequencies, the circuit dimensions required to achieve consistent circuit amplitude and phase behavior are microscopic and require manufacturing processes capable of achieving tight tolerances within a single circuit board and from board to board.
Selecting a circuit material for millimeter-wave circuits is as much a matter of choosing material characteristics supporting millimeter-wave performance as it is finding a material that can be properly processed to transform the material into a millimeter-wave PCB. Extremely tight manufacturing control is needed for circuit dimensions at millimeter-wave frequencies often when meeting other production requirements, such as when assembling hybrid, mixed-signal circuits that also include “general-purpose” circuit materials such as FR-4 or those for RoHS-compliant, lead-free PCB fabrication methods. Circuit materials are defined by several characteristic temperatures, notably the glass transition temperature (Tg), above which the thermomechanical behavior of the material is affected, and the decomposition temperature (Td). PCB processing for a material usually is performed between these two temperatures.
Finding a circuit material well suited to any high-frequency, high-speed application starts by defining the requirements for a PCB, including frequency, bandwidth, function, power level, and the maximum size of the circuit board. Meeting those requirements is a matter of design, such as choice of transmission-line technology, components and component technologies, and circuit layout. Meeting performance expectations will depend on how well each component in a PCB contributes to the overall behavior of the board.
The millimeter-wave frequency range, although challenging for circuit designers to provide consistent performance, offers massive bandwidth for a wide range of applications. Available bandwidth will depend upon occupied spectrum, with bandwidth requirements varying widely among applications. The bandwidth needs of simple LOS radio links will be much less, for example, than the instantaneous bandwidth required for EW or surveillance systems transmitting high-definition video from remote locations.
At millimeter-wave frequencies, the single most important circuit material parameter to consider is loss. Lower loss in a PCB-based antenna circuit means higher sensitivity and greater range for the antenna. Available signal power tends to diminish as signal frequency increases, requiring extremely efficient PCB board layouts, low-loss components, and extremely low-loss circuit materials to conserve as much signal power through the PCB as possible.
Signal loss through a PCB occurs according to the materials that compose the circuit board, the circuit layout, and leakage losses from interconnections among components. When selecting a circuit material for millimeter-wave circuits, material-based losses stem from its conductor and dielectric materials.
Loss from a circuit material’s dielectric content is typically defined by the material’s loss tangent or dissipation factor (Df), with low Df values identifying circuit materials with low dielectric loss for a referenced frequency. Although datasheet Df values for a material of interest may not be referenced to millimeter-wave frequencies, the rate of increase in Df at lower frequencies usually provides insight into the Df to be expected at higher frequencies. The percentage of dielectric material to conductive metal in a circuit material also accounts for how much loss is governed by each type of material. At millimeter-wave frequencies, for example, conductor losses tend to be more dominant in PCBs with thinner dielectric layers.
Conductor losses depend on the quality and weight of a circuit laminate’s copper, with a laminate formed by surrounding a dielectric prepreg material consisting of glass fabric and resin with top and bottom copper layers. Typically, electrodeposited or rolled copper is used, with the surface of the copper playing a significant role in electrical performance possible at millimeter-wave frequencies. Smoother copper foils contribute less conductor loss, especially at higher frequencies, than copper with a rougher surface.
At millimeter-wave frequencies, the signal path across a copper conductor with a large peak-to-valley surface profile can be longer than for a smoother copper surface, resulting in a longer propagation path with greater losses. For thin laminates with rough copper surfaces, high surface profiles have been found to alter the effective permittivity or Dk of the material and impact the modeling accuracy for the material when based on a particular Dk. At millimeter-wave frequencies, laminates with ultra-low roughness HLVP and HLVP3 copper foils provide minimal surface profiles for accurate modeling and reliable performance.
In addition to Df, Dk is a critical parameter in selecting a circuit material for an application. Dk refers to a circuit material’s capability to store EM energy. Materials with higher Dk values can store more EM energy than materials with lower Dk values, although the flow of energy through the conductors is slower for materials with higher Dk values. For that reason, circuit materials with lower Dk values are usually preferred over higher Dk materials for millimeter-wave circuits. Circuit materials with lower Dk values require transmission lines with wider line widths to achieve a characteristic impedance of 50 ohms compared to circuit materials with higher Dk values.
As important as the Dk value of a circuit material considered for millimeter-wave applications is the consistency of its Dk across various operating conditions, such as frequency, temperature, and humidity. Variations in Dk with thickness or with temperature, for example, can result in variations in the frequency and bandwidth of a circuit at millimeter-wave frequencies, requiring troublesome and expensive design modifications to compensate for the changes in Dk.
High overall consistency is a desirable characteristic of circuit materials for millimeter-wave PCBs. Minimal variations in the physical and electrical properties of circuit materials result in minimal variations in performance, especially at the smaller wavelengths of millimeter-wave frequencies. Physical variations, such as in copper conductor or dielectric thickness, promote variations in electrical performance, such as in signal amplitude, phase, and timing/synchronization, especially at increasing frequencies, leading to unreliable performance in circuits relying on amplitude, phase, and timing accuracy.
Consistent circuit material performance is typically indicated by a handful of specifications that indicate changes according to environmental conditions, such as temperature coefficient of dielectric constant (TCDk) and moisture absorption (the Dk and Df values of a circuit material with high moisture absorption will increase for circuits operating in high-humidity environments).
Circuit materials providing practical foundations for millimeter-wave PCBs in defense applications start by contributing as little loss as possible to a design, identified as having low Df values at high frequencies. As an example, Astra® MT77 laminate and prepreg materials (Figure 3) from Isola Group, with a Tg of +200° C and Td of +360° C, support PCB manufacturing processes over that wide temperature range with extremely low circuit loss. It has a Df of 0.0017 at 2 GHz which is constant with frequency, with the same Df value at 10 GHz.
It features a low Dk of 3.00 which is also stable with temperature, remaining constant when measured at 10 GHz. The Dk is stable with temperature, remaining constant at 3.00 across a wide operating temperature range of -40°C to +140°C. The circuit materials are RoHS compliant for lead-free manufacturing processes and are also compatible with FR-4 processing temperatures and conditions to ease the construction of multilayer hybrid PCBs built with different types of circuit materials.
With low moisture absorption of 0.1%, the Astra® MT77 circuit materials can be used in high-humidity environments without suffering significant additional signal losses due to water absorption. They are available with smooth HVLP copper in standard weights of 1/3 to 2 oz., and are well suited for millimeter-wave circuits operating through W-band frequencies (110 GHz), although the circuit materials have been evaluated for applications as high as 300 GHz.
When designing and producing hybrid circuits with millimeter-wave functionality, Tachyon® 100G laminates and prepreg materials (Figure 2) exhibit the characteristics needed for high-frequency and high-speed-digital (HSD) PCBs with excellent thermal management while compatible with the processing conditions of widely used commercial circuit materials such as FR-4.
As with the Astra® MT77 circuit materials, Tachyon® 100G materials are defined by wide processing temperature limits, with a Tg of +215° C and Td of +360° C. Tachyon® 100G materials have slightly higher loss, with Df of 0.0021 measured at 2 and 10 GHz, but the Dk remains constant and almost as low, at 3.02 through the thickness (z-axis) of the material using the Bereskin stripline method at both 2 and 10 GHz.
Both Df and Dk remain constant over a wide operating temperature range of -55° C to +125° C. Tachyon® 100G materials are a good match for fast digital circuits requiring synchronization/timing among multiple lines, such as in high-speed network interconnections, and support data rates to 100 Gb/s. Like Astra® MT77 circuit materials, Tachyon® 100G materials have a composition engineered to efficiently dissipate heat in high-power circuits. The thermal conductivity of Tachyon® 100G is similar to the 0.45 W/m-K for Astra® MT77 circuit materials.
For cost-competitive applications at millimeter-wave frequencies, Isola’s I-Tera® MT40 supports practical PCBs with repeatable performance at millimeter-wave frequencies for reduced cost compared to the Astra® MT77 and Tachyon® 100G circuit materials, although with a penalty in loss (Df) performance. The typical Df of I-Tera® MT40 is higher than the other two materials, at 0.0031 when measured at 2 and 10 GHz. Its Dk is stable with frequency and temperature, at 3.45 when measured at 2 and 10 GHz. But for circuits where thermal management is a consideration, lower-cost I-Tera® MT40 laminates and prepregs offer the most robust thermal conductivity of the three materials, at 0.61 W/m-K.
When protecting the environment is a concern, Isola’s TerraGreen® series (Figure 4) of circuit materials are halogen-free circuit foundations that can be formed into high-performance, low-loss PCBs that will not introduce dangerous substances, such as chlorine and bromine, into the operating environment.
The first two materials in the TerraGreen® halogen-free line, standard TerraGreen® and high-frequency TerraGreen® (RF/MW), have provided the conveniences of RoHS compliance and FR-4 processing compatibility along with environmental protection but may not have been ideal in their loss attributes for use at demanding millimeter-wave frequencies. Both standard TerraGreen®, with Dk of 3.44 at 2 and 10 GHz, and TerraGreen® (RF/MW), with Dk of 3.45 at 2 and 10 GHz, exhibit similar loss behavior as I-Tera® MT40 with Df of 0.0032 at 2 and 10 GHz.
As an alternative, the company recently introduced three ultra-low-loss members of the TerraGreen® circuit material family: TerraGreen® 400G, TerraGreen® 400GE, and TerraGreen® 400G2. TerraGreen® 400GE, with Dk of 3.4 at 10 and 20 GHz, improves only slightly in loss compared to the earlier halogen-free materials, with Df of 0.0028 at 10 and 20 GHz. But TerraGreen® 400G, with lower Dk of 3.1 at 10 and 20 GHz, trims loss with typical Df of 0.0018 at 10 and 20 GHz while TerraGreen® 400G2, also with Dk of 3.1 at 10 and 20 GHz, drops Df to 0.0015 at 10 and 20 GHz for truly ultra-low-loss millimeter-wave circuits without halogens.