The frequency spectrum is being consumed quickly for many wireless applications from popular cellular communications systems to more esoteric safety and medical systems. For the available spectrum, circuit designers are reaching beyond traditional microwave frequencies and into the millimeter-wave (mmWave) frequency ranges from 30–300 GHz. Those designers often look to a relatively mature technology, flexible circuit technology as the PCB materials for those high-frequency circuits. Although such materials may not typically be a first substrate choice for high-speed, high-frequency circuits, the thinner, flexible circuit materials are well suited for the shorter wavelengths at mmWave frequencies.
Such high-frequency signals are being planned for global communications in fifth-generation (5G) wireless systems, automotive driver assistance system (ADAS), and self-driving autonomous vehicle systems, among other applications. Thinner substrate materials have several advantages for high-frequency circuits. For example, thinner dielectric materials between copper circuit planes can help minimize internal resonances in the circuits. Thicker circuit materials can suffer higher electromagnetic (EM) radiation losses to neighboring circuits and can lose energy as EM interference (EMI) at higher frequencies where signal wavelengths are smaller and signal power can be harder to come by. Circuits designed for mmWave frequencies usually start with thinner circuit materials for optimum performance.
The smaller wavelengths of mmWave frequencies tend to highlight circuit material anomalies at those higher frequencies—anomalies that can also influence the radio frequency (RF) performance of the circuit. Such material anomalies include variations in dielectric thickness, dielectric constant (Dk), copper conductor width and spacing, and copper conductor plated thickness. For mmWave circuits, even small variations in these key material properties can impact electrical performance. Fortunately, flexible circuit technology is typically well controlled for thickness compared to thicker, more rigid circuit board materials.
As a simple example, a microstrip circuit using 5-mil circuit material with a Dk of about 3 and a thickness variation of 1 mil will exhibit an impedance difference of about 6Ω at microwave frequencies. A 6Ω impedance difference may impact some applications; however, at mmWave frequencies, even small thickness variations can cause impedance differences that cause more reflections and many related issues. The 1-mil difference could be from a material with a thickness tolerance of only ±0.5 mil (±10% thickness tolerance). This is considered a very tight tolerance for a rigid circuit material; however, the thickness tolerance of flexible circuit materials can be much better controlled. Flexible circuit materials tend to provide highly consistent Dk that is nearly isotropic, as a function of the material formulation. This is generally true, although exceptions do exist.
To read this entire column, which appeared in the February 2019 issue of Design007 Magazine, click here.