Growing demand for mobile wireless communications services has quickly eclipsed the capabilities of Fourth Generation (4G) Long Term Evolution (LTE) wireless networks and created a need for a next-generation mobile wireless network solution. Fifth Generation (5G) wireless networks promise more capacity and capability than 4G LTE systems, using wider channel bandwidths, new antenna and modulation technologies, and higher carrier frequencies even through millimeter-wave frequencies. But before 5G wireless networks can become a reality, systems and circuits will be needed for higher frequencies than current 2.6-GHz 4G LTE wireless networks.
Standards are still being formulated for 5G wireless networks, with goals of achieving data rates of 10 Gb/s and beyond with low latency, using higher frequencies than in traditional wireless communications systems. In the United States, for example, last year the Federal Communications Commission (FCC) approved the use of frequency bands at 28, 37, and 39 GHz for 5G.
PCB Materials for Millimeter Waves
For circuit designers, one challenge will be in knowing where to start, which means, for millimeter-wave frequencies, knowing what types of printed-circuit-board (PCB) material characteristics are the most important at higher frequencies. Millimeter-wave frequencies (above 30 GHz) were once used almost exclusively by the military and for research experiments, but 5G represents an opportunity to “popularize” millimeter-wave frequencies and make them part of everyday life, not just for exotic electronic devices in the limited quantities used in research and by the military, but for potentially billions of electronic devices for people and things, as in how Internet of Things (IoT) devices will use 5G networks for Internet access.
Designing circuits at millimeter-wave frequencies starts with the right PCB material, and knowing how different PCB characteristics affect circuit performance at millimeter-wave frequencies. Variations in certain circuit material parameters, such as dielectric constant (Dk), can have greater impact on performance as the operating frequency increases. For example, signal power is a valuable commodity at millimeter-wave frequencies, requiring circuit designers to minimize loss in their circuits as much as possible. This begins with the choice of PCB material, since a PCB material not meant for use at millimeter-wave frequencies can result in excessive signal losses when operated beyond its intended operating frequency range.
PCB materials can degrade signal power in three ways: radiation losses, dielectric losses, and conductor losses. Losses through radiation of EM energy largely depend on the circuit architecture, so even the lowest-loss PCB material may not save a circuit configuration that has a tendency to radiate energy.
A thoughtful choice of PCB material can help minimize dielectric and conductor losses at millimeter-wave frequencies. A circuit material’s dielectric loss is closely related to its dissipation factor (Df) or loss tangent, which increases with frequency. The Df is also related to a material’s dielectric constant (Dk), with materials that have higher values of Dk often have higher Df loss, although there are exceptions. Attempts to minimize dielectric losses for millimeter-wave circuits can be aided by considering circuit materials with low Df values.
Controlling Conductor Loss
Finding a material with low conductor losses at millimeter-wave frequencies is not as straightforward, since conductor losses are determined by a number of variables, including the surface roughness and the type of finish. As the name suggests, millimeter-wave signals have extremely small wavelengths, mechanical variations in a circuit-board material can have significant effects on small-wavelength signals. Increased copper surface roughness will increase the loss of a conductor, such as a microstrip transmission line, and slow the phase velocity of signals propagating through it. In microstrip, signals propagate along the conductor, through the dielectric material, and through the air around the circuit material, so the roughness of the conductor at the interface with the dielectric material will contribute to the conductor loss. The amount of loss depends on frequency: the loss is greatest when the skin depth of the propagating signal is less than the copper surface roughness. Such a condition also degrades the phase response of the propagating signal.
The impact of copper surface roughness on conductor loss depends on the thickness of the PCB material: thinner circuits are more affected than thicker circuits. The effects of copper surface roughness on loss become apparent at millimeter-wave frequencies. For example, two circuits based on 5-mil-thick RT/duroid® 6002 circuit material from Rogers Corp. but with two different types of copper conductor and surface roughnesses were tested at 77 GHz. The circuit with rolled copper and root mean square (RMS) conductor surface roughness of 0.3 μm exhibited considerably lower conductor loss than the same circuit material with electrodeposited (ED) copper conductor having 1.8-μm surface roughness.
Propagation of the small wavelengths at millimeter-wave frequencies can also be affected by the type of finish used on a PCB’s conductors. Most plated finishes have lower conductivity than copper, and their addition to a copper conductor will increase the loss of the conductor, with loss increasing as the frequency increases. Electroless nickel immersion gold (ENIG) is a popular finish for copper conductors; unfortunately, nickel has about one-third the conductivity of copper. As a result, ENIG plating will increase the loss of a copper conductor, with the amount of loss increasing as a function of increasing frequency.
Environmental conditions can also impact the amount of loss exhibited by a PCB material, especially at millimeter-wave frequencies. Many network scenarios for 5G predict the need for many smaller wireless base stations than used in earlier wireless network generations, in part because of an increased number of expected users and the use of millimeter-wave frequencies and their shorter propagation distances than lower-frequency carriers. Where 5G base stations cannot be maintained in climate-controlled environments, circuits may be subject to changing environmental conditions, such as high relative humidity (RH). Water absorption can dramatically increase the loss of a PCB material, and the loss of circuit materials with high moisture absorption will be greatly affected under high RH conditions.
Testing on 5-mil-thick RO3003™ circuit material from Rogers Corp. for two different operating environments showed how loss at millimeter-wave frequencies can increase with RH. One circuit was maintained at room temperature and the other was subjected to +85ºC and 85% RH for 72 hours. At 79 GHz, the room temperature material had about 0.1 dB/in. less loss than the material subjected to higher humidity and temperature. When testing was performed on a third, thermoset circuit material from a different supplier, the increase in circuit loss at 79 GHz was even more dramatic.
For those interested in learning more about the nuances of selecting PCB materials and designing circuits for 5G, in particular at millimeter-wave frequencies, Rogers has created a number of tutorial videos in their “The Road to 5G” series. The videos guide viewers on what different circuit material parameters mean at millimeter-wave frequencies, and which material characteristics make the most difference at those higher frequencies. The videos offer quick and easy ways to learn how to specify PCB materials for 5G, and to get ready for this next revolution in wireless communications.
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.