Decipher Glass-Weave Effects at Millimeter-Wave Frequencies
Glass is often added to dielectric materials to boost the strength of circuit substrates. It can fortify the thinnest of printed circuit boards (PCBs), but at what costs? What are the tradeoffs in performance? Glass has its own material characteristics and, when combined with the dielectric and copper materials that form high-frequency circuit materials, what does it do to the electrical performance of a circuit on that blend of materials? This blog will attempt to “see through” the effects of glass on high frequency circuit boards, especially on the millimeter-wave circuits that are becoming so important in emerging automotive radar systems (at 77 GHz) and Fifth Generation (5G) cellular wireless communications systems.
Woven through various dielectric resins which form circuit materials, fiberglass fabric can boost the strength and durability of a PCB. When high mechanical strength is needed in a circuit board, it can be achieved by forming dielectric substrates with a woven-glass layer or layers and adding ceramic material as a filler, as done with RO4830™ laminates from Rogers Corp. But the glass fibers that make up the fabric, usually have a higher dielectric constant (Dk) than the dielectric material (and the ceramic filler), and the blend of materials with different Dk values may not be consistent throughout, resulting in a circuit material with small, isolated Dk variations. The Dk variations may not be so critical at RF and microwave frequencies but can have an impact at millimeter-wave frequencies with their smaller wavelengths.
This impact of glass fibers on circuit boards is known as the glass-weave effect (GWE) or the fiber-weave effect (FWE). Glass is a strengthening component within a PCB material and does contribute to circuit materials that are extremely thin but durable. Thin materials have obvious advantages for applications with tight packaging requirements, and they are well suited for higher frequency, small wavelength circuit applications, such as millimeter-wave circuits of 28 GHz and higher in frequency.
Ideally, the components of a PCB material, including its glass and copper, will combine for consistent performance. The glass can be a concern for millimeter-wave applications, but it can also impact high-speed digital circuits, effecting propagation delay and skew or timing differences between adjacent signals (resulting in increased bit-error rate). This blog will focus on how the GWE impacts 77 GHz and other millimeter-wave circuits.
At millimeter-wave frequencies, even small variations in a circuit material’s Dk can result in electrical performance variations, such as differences in the signal delays and phase of transmission lines. For thin circuits, glass adds strength, but it also adds a much higher Dk than the dielectric material surrounding it, about 6.0 for glass compared to a Dk of about 2.1 to 2.6 for dielectric materials with a targeted overall Dk of 3.0. The glass cloth that is used to form a high frequency PCB is typically not a perfect grid and can be distorted by shipping and handling prior to circuit material manufacturing.
Circuit routing on a high frequency PCB material can also contribute to how much or how little the glass weave effect impacts performance. The glass cloth has a pattern which results in more or less glass or no glass in small discrete areas of the circuit material. Transmission lines are formed on circuit materials where there are differences in glass content, known as knuckle-bundle areas with more glass and bundle-open areas with less glass. The Dk values are higher for knuckle-bundle areas than for bundle-open areas with less glass. Transmission lines that align with areas of high glass concentration, pass over areas without glass, or zig-zag across different areas can experience quite a range in Dk due to the combinations of materials.
Due to the increasing impact of the glass weave effect at increasing frequency or higher digital speeds, developers of circuit materials have sought to minimize those effects through different glass types and patterns. Several different glass types are used in circuit materials for millimeter-wave circuits with three example glass types being type 106 open-weave balanced glass, 1080 open-weave unbalanced glass, and 1078 spread-weave balanced glass. The three glass fabric patterns are thin and the term “balanced” refers to the density of glass on the x-axis of the glass fabric compared to the y-axis of the fabric. The glass bundles and open areas between the bundles may have different geometries, but it is the density of the glass that defines whether it is balanced or not. Circuit materials with 1078 glass have a weave evenly spread across the material and do not have openings between the glass bundles as do materials with the 106 and 1080 glass.
77 GHz Differences
Studies performed on circuit materials with the different glass styles have found significant differences in how transmission-line circuits are oriented to different patterns of glass-knuckle-bundle and bundle-open areas. Network analyzer measurements were performed from hand-picked circuits representing the three glass styles mentioned, and with using rolled copper, for circuit glass orientations of knuckle-bundle and bundle-open. The study evaluated performance differences due to measuring group delay, propagation delay, and phase angle response of each circuit for insight into how glass weaves and different glass styles result in different Dk values across a circuit.
The study used a 4 mil thick, PTFE-based laminate with no filler and rolled copper with all three glass styles. Circuit materials with 1078 glass with a spread, balanced configuration kept differences between knuckle-bundle orientation to the circuit and bundle-open orientation to a minimum. Circuit materials with 1078 glass showed measured differences of only 20 deg. In phase-angle difference at 77 GHz.
How did the other two glass styles compare? The same 4 mil thick, PTFE-based laminate using 106 glass in its open weave, balanced configuration exhibited average differences between knuckle-bundle and bundle-open circuit orientation of 100 deg. phase angle difference at 77 GHz. The same circuit material with 1080 glass and its open weave, unbalanced configuration showed average differences between knuckle-bundle and bundle-open circuit orientation of 149 deg. in phase angle at 77 GHz.
What do these differences mean in terms of how the glass weave effect causes variations in the Dk of a circuit material? Measurements were made for the 4 mil thick PTFE circuits with the different glass types and with rolled copper conductors. For the circuits using material with 1078 glass, a difference between circuits oriented to the knuckle-bundle configuration as compared to the bundle-open configuration corresponds to a Dk difference of 0.02. For the material with 106 glass, there was a larger Dk difference of 0.09. The largest Dk difference of 0.14 was found when evaluating circuits on material using the 1080 glass style.
For circuit laminates with a single layer of glass fabric, the glass weave effect can be more pronounced than in laminates with more than one layer, in which averaging results in more consistent glass content. But millimeter-wave circuits, with their small wavelengths, are typically thin and often use only a single layer of glass reinforcement, and performance will be more impacted by the glass weave effect. Laminates with filler, such as ceramic, have this additional material with a Dk that is between the Dk of the glass and the Dk of the resin system. While a filler does not fully solve the glass weave effect, it does result in transitions and some smoothing of the Dk across the circuit material to lessen the impact of the glass-weave effect at high frequencies. As an example, RO4830 laminate from Rogers Corp. is a circuit material with 1078 spread glass weave and ceramic filler.
Additionally, RO3003™ laminate from Rogers Corp., with no woven glass fabric, is a popular circuit material choice for millimeter-wave circuits. It is a ceramic-filled PCB material that features a Dk of 3.00 that is tightly maintained within ±0.04; such Dk consistency is vital for millimeter-wave circuits and their small wavelengths as well as differential pairs in high-speed digital circuits.
Hold the Glass
One way to avoid the glass weave effect altogether is to use circuit material without glass. Especially for millimeter-wave circuits such as 77 GHz automotive circuits, circuit materials developed for higher frequency applications and without glass may be a better solution than circuit materials using glass reinforcement. Early measurements on the newly released RO3003G2™ circuit laminates from Rogers Corp., which does not have woven glass fabric, have indicated extremely consistent microstrip transmission-line impedance at millimeter-wave frequencies across each circuit board and from board to board.
Other material and/or circuit parameters, such as variations in conductor width, copper thickness, and substrate thickness, can cause variations in transmission-line impedance which masquerade as the glass weave effect. So, even circuit materials without glass may still exhibit some symptoms of the glass weave effect. But high frequency materials such as the newly released RO3003G2 materials eliminate at least one cause in performance variations which can be critical at 77 GHz and beyond.
Note: This blog is based on a webinar by the author, “An Overview of Glass Weave Impact on Millimeter-Wave PCB Performance,” which provides extensive details on the glass weave effect and how it influences circuits at 77 GHz and other millimeter-wave frequencies. The webinar also provides results of studies on different circuit materials with different copper conductor types, with and without filler materials, demonstrating how materials must be carefully selected for millimeter-wave applications such as 77 GHz automotive radar sensors.
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Seeing Through the Glass Weave Effect
The glass weave effect in PCBs was covered in earlier ROG Blogs, in “Woven Glass Laminates in PCBs” (http://blog.rogerscorp.com/2017/05/26/woven-glass-laminates-pcbs/)(from May 26, 2017) and in “Millimeter Wave Frequencies: Getting a Grip on the Glass Weave Effect” (http://blog.rogerscorp.com/2017/11/29/millimeter-wave-frequencies-getting-grip-glass-weave-effect/)(from November 29, 2017). With the growing importance of millimeter-wave frequencies for automotive safety systems and wireless communications, this ROG Blog takes a detailed look at what the glass-weave effect can do to millimeter-wave circuits and what circuit designers must do to overcome those effects.