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Typically, the most common method to improve the strength of circuit board materials is to add glass fiber/cloth to the dielectric layer of the printed circuit board (PCB). Even for the thinnest PCBs, once glass fibers are added its strength can be improved. But at what cost? What are the performance trade-offs? Glass has its own material properties, so how does it affect the electrical performance of the circuit when combined with the dielectric and surface copper cladding materials that make up the HF circuit board material? This blog will try to gain insight into the effect of glass fiber on HF circuit boards, especially on millimeter-wave circuit boards. This is because millimeter-wave circuit boards are becoming increasingly important in emerging automotive radar systems (77 GHz) and fifth generation (5G) cellular wireless communication systems.
By mixing and weaving glass fibers with a variety of resins that form the pcb material, the strength and durability of the printed circuit boards formed in this way will be greatly enhanced. When high mechanical strength is required, it can be achieved by mixing one or more layers of glass cloth into a dielectric substrate and ceramic materials as fillers. Rogers RO4830 laminates utilize this approach. However, glass fibers are usually woven structures, which have a higher dielectric constant (Dk) than dielectric materials (and ceramic fillers). Materials with different Dk values are usually not perfectly uniformly distributed throughout the mixing process, which results in varying sizes and intermittent Dk variations in a small area of the circuit board material. At RF and microwave frequencies, this Dk variation may not be so important, but at millimeter-wave frequencies, where the wavelengths are smaller, it can have a greater effect.
This effect of glass fibers on the circuit performance of PCB materials is known as the glass effect (GWE) or fiber effect (FWE).Glass fibers are the reinforcing part of the PCB material and really help to create extremely thin and durable circuit board materials. Thinner materials have clear advantages for applications with compact packaging requirements, and they are well suited for higher frequency, small wavelength circuit applications such as millimeter-wave circuit boards at 28 GHz or higher.
Ideally, printed circuit board materials would include a combination of glass fibers and copper foils to achieve consistent performance. Glass fibers are not only a concern for millimeter-wave applications, but also affect high-speed digital circuits, affecting transmission delays and distortions between adjacent signals, as well as timing differences (resulting in increased bit error rates). This blog will focus on how the glass fiber effect GWE affects 77 GHz and other millimeter wave circuits.
Recognizing Changes
At millimeter-wave frequencies, even small changes in the Dk of a circuit board material can lead to changes in electrical properties, such as signal delay and differences in the phase of the transmission line. For thinner circuits, while glass fiber adds strength, it also adds a much higher Dk than the surrounding dielectric material. glass fiber has a Dk of about 6.0, while the dielectric material has a Dk of about 2.1-2.6, which blends to a total Dk of about 3.0. glass fiber/cloth used to form high-frequency PCBs is usually not perfectly gridded and may deform due to transport and handling prior to PCB material manufacture. The fiberglass/cloth used to form high frequency PCBs is usually not a perfect lattice and may be deformed by transportation and handling before the PCB material is manufactured.
In addition, the circuit routing on the high frequency PCB material may cause the glass fiber effect to have more or less of a performance impact on the overall circuit. Glass cloth is made by weaving glass fibers in a pattern that has the following characteristics: in smaller areas of the circuit board material, some areas will have interwoven overlays of glass fibers, but some areas will have gaps where there are no glass fibers. Differences in the performance of the transmission line is to occur in these different areas of glass fiber intertwining. Areas with more glass fibers are called knuckle-bundleareas and areas with less glass are called bundle-openareas. Knuckle-bundle areas have higher Dk values than bundle-open areas with less glass fiber. Because of this mixing of board materials, it is possible for a transmission line to pass through a high fiberglass area, a non-glass area, or both areas at the same time in a sawtooth pattern, which can result in considerable performance differences in the Dk values of the areas where the same transmission line passes through.
As the glass fiber effect becomes more and more significant as the frequency increases or at higher digital speeds, circuit board material developers try to minimize these effects with different types of glass fibers and patterns. The following types of glass fibers are commonly used in millimeter-wave circuits as circuit board materials: Type 106 open-end balanced woven glass fabric, Type 1080 open-end unbalanced woven glass fabric, and Type 1078 flat open-end balanced woven glass fabric. The three kinds of glass fiber are relatively thin, here the balanced knitting refers to the glass fiber on the X axis of the glass warp yarn and Y axis of the weft yarn thickness density ratio. The fiberglass yarn bundles and the open areas between them may have different geometries, but the thickness of the fiberglass yarns determines whether they are balanced or not. 1078 glass cloth has a flat open fiber braid structure and is evenly distributed in the material, with no fiber open areas, whereas 106 and 1080 glass cloths are different materials, with openings in the glass fibers between each other braids.
77GHz Differences
A study of different glass cloth type circuit board materials found that the performance of transmission line circuits located in different glass fiber knuckle crossbundle and bundle opening areas can have significant differences. Measurements were made on circuits designed from the three typical glass cloth type circuit board materials described above. To minimize the effect of copper foil roughness by using calendered copper as the material, circuits were selected for Network Analyzer measurements through the joint intersection area and bundle opening area, respectively. Measurement parameters include group delay,propagation delay, and phase angle response for each circuit,as well as the resulting performance differences, to gain insight into how different glass fibers and different glass braiding configurations produce different Dk values in the circuits.
For this experiment, a 4-mil-thick polytetrafluoroethylene (PTFE) material, a filler-free, rolled copper, and a combination of the three different glass fabrics mentioned above were used. The circuit board material, Type 1078 glass fabric, has a flat, balanced configuration that minimizes the difference between the orientation of the knuckle cross-bundle area to the circuit and the orientation of the bundle opening area. Tests have shown that circuits made with this type 1078 glass fiber board material have a phase difference of only 20 degrees at 77 GHz.
How do the other two glass fibers compare? The exact same 4 mil thick PTFE unfilled calendered copper laminate material, in the use of 106 type glass fibers with open weave, balanced structure, 77GHz when the average difference in phase angle between the direction of the knuckle cross-bundle area and the bundle open area is 100 degrees. In the same circuit materials used in the 1080-type glass cloth, which has an open weave, unbalanced structure, the circuit at 77GHz frequency phase angle average difference of 149 degrees.
What is the change in Dk of the circuit board material due to these differences brought about by the glass fiber effect? The results for the same circuits as above show that for the circuit using the 1078 type glass cloth material, the difference between the circuits in the knuckle crossing area and the circuits in the beam opening area corresponds to a change in Dk of about 0.02. With the 106-type glass cloth material, the difference in Dk is larger at 0.09, while the maximum Dk difference for the circuit using the 1080-type glass cloth material reaches 0.14.
For the circuit laminates using a single layer of glass fiber, the glass fiber effect is more pronounced than that of the multi-layer glass laminates because the even stacking of multiple layers of glass fibers results in a more uniform distribution of the glass. For millimeter-wave circuits, where the wavelengths are small and the circuits are usually very thin, and the material is usually reinforced with only one layer of glass fibers, the performance of the circuits in this case will be more affected by the glass fiber effect. Laminates with fillers (e.g. ceramics) with this additional material (whose Dk is between that of the glass Dk and that of the resin system), although not completely resolving the glass fiber effect, will to a certain extent make the Dk of the circuit board material more homogeneous to minimize the effect of the glass fiber effect at high frequencies. For example, Rogers' RO4830 laminate is this type of circuit material, with 1078 flat open fiber glass cloth and ceramic filler.
In addition, Rogers RO3003 laminate, which does not contain glass cloth, is one of the popular circuit board material choices for millimeter wave circuits. This is a ceramic-filled PCB material with a Dk of 3.00 and a Dk tolerance of 0.04. This Dk consistency is critical for millimeter-wave circuits as well as differential pairs in high-speed digital circuits.
Glass Fiber Removal
One way to avoid the glass fiber effect altogether is to use circuit board materials without glass fibers/cloth. Especially for automotive radar circuits, such as those using 77 GHz millimeter waves, it is much better to use high frequency circuit board materials without glass fibers than to include board materials reinforced with glass fibers. Rogers' newly released RO3003G2 circuit laminates, which also contain no glass cloth, have been tested and shown to have very consistent performance, such as consistent microstrip line impedance, across boards at millimeter-wave frequencies.
Speaking of impedance variations,other material or circuit parameters,such as variations in conductor width, copper thickness and substrate thickness, can also cause variations in transmission line impedance. However, the newly released RO3003G2 high-frequency material completely eliminates the glass fiber effect factor that contributes to changes in circuit impedance or performance, which is critical at frequencies of 77 GHz and higher.
Note: This blog is based on the original author's webinar report An Overview of Glass Weave Impact on Millimeter-Wave PCB Performance.