PCB Technical - Analysis on the glass fiber effect of PCB circuit board materials at millimeter wave frequency​

Analysis on the glass fiber effect of PCB circuit board materials at millimeter wave frequency

Generally, in order to improve the strength of circuit board materials, the most common method is to add glass fiber/cloth to the printed circuit board(PCB) dielectric layer. Even the thinnest PCB, once glass fiber is added, its strength can be improved. But what is the price to be paid for this? What are the performance trade-offs? Glass has its own material characteristics. When it is combined with the medium and surface copper foil materials that make up the high-frequency circuit board material, what effect does it have on the electrical performance of the circuit? This blog will try to "insight" into the impact of glass fiber on high-frequency circuit boards, especially millimeter wave circuits. Because Millimeter wave circuit boards are becoming more and more important in emerging automotive radar systems (77GHz) and fifth-generation (5G) cellular wireless communication systems.

By mixing glass fiber with various resins that form circuit board materials, the strength and durability of the printed circuit board formed in this way will be greatly improved. When the circuit board requires high mechanical strength, one or more layers of glass cloth can be mixed into the dielectric substrate, and ceramic materials are mixed together as fillers to achieve high mechanical strength. Rogers’ RO4830™ laminates use this approach. However, glass fiber is usually a woven structure, which has a higher dielectric constant (Dk) than dielectric materials (and ceramic fillers). Materials with different Dk values usually cannot achieve perfect uniform distribution during the entire mixing process, which results in circuit board materials having different sizes and spacing Dk changes in a small area. At RF and microwave frequencies, this change in Dk may not be so important, but it will have a greater impact at millimeter wave frequencies with smaller wavelengths.

This effect of glass fiber on the circuit performance of the circuit board material is called the glass effect (GWE) or fiber effect (FWE). Glass fiber is the strengthening part of PCB materials, which really helps to make extremely thin and durable circuit board materials. Thinner materials have obvious advantages for applications with compact packaging requirements, and they are very suitable for higher frequency, small wavelength circuit applications, such as 28GHz or higher frequency millimeter wave circuits.

Ideally, printed circuit board materials will include glass fiber and copper foil to achieve consistent performance. Glass fiber is not only the focus of millimeter wave applications, but also affects high-speed digital circuits, affecting transmission delays and distortions between adjacent signals, and timing differences (leading to increased bit error rates). This blog will focus on how the glass fiber effect GWE affects 77 GHz and other millimeter wave circuits.

Identify changes

At millimeter wave frequencies, even small changes in the circuit board material Dk will cause changes in electrical properties, such as signal delay and transmission line phase differences. For thinner circuits, although glass fiber increases strength, it also increases Dk that is much higher than the surrounding dielectric materials. The Dk of the glass fiber is about 6.0, and the Dk of the dielectric material is about 2.1-2.6, and the overall Dk of about 3.0 can be obtained after mixing. The glass fiber/cloth used to form a high frequency PCB is usually not a perfect grid, and may be deformed due to transportation and handling before the circuit board material is manufactured.

In addition, circuit wiring on high-frequency PCB materials may also cause the glass fiber effect to have more or less performance impact on the entire circuit. The glass cloth is woven by glass fiber, and its pattern has the following characteristics: In the small area of the circuit board material, there will be interwoven and superimposed glass fibers in some places, but there are voids in some areas, and there is no glass fiber. The performance difference of the transmission line occurs in the areas where these glass fibers are interwoven. The areas with more glass fibers are called "knuckle-bundle areas", and the areas with less glass are called "bundle-open areas". The Dk value of the "knuckle crossing zone" will be higher than the "bundle opening zone" with less glass fiber. Because of the mixed nature of the circuit board materials, the transmission line may pass through the high glass fiber area, the glass-free area, or pass through the two areas at the same time in a "zigzag" shape, which will cause the same transmission line to pass through the place where the Dk is different. Big performance difference.

As the glass fiber effect becomes more and more important as the frequency increases or at higher digital speeds, the research and development personnel of circuit board materials try to minimize these effects through different types of glass fibers and patterns. The following different glass fiber types are commonly used in the circuit board materials of millimeter wave circuits, namely: 106-type opening balanced woven glass cloth, 1080-type opening unbalanced woven glass cloth and 1078 flat fiber-opening balanced woven glass cloth. The three kinds of glass fibers are relatively thin. The "balanced" weaving here refers to the ratio of the thickness and density of the glass warp yarns on the X axis to the weft yarns on the Y axis of the glass fiber. The open area between the glass fiber yarn bundles may have different geometric structures, but the thickness and density of the glass fiber yarn determines whether it is balanced. 1078 glass cloth has a flat fiber-opening woven structure and is evenly distributed in the material without fiber opening areas; while the material of 106 and 1080 glass cloth is different, there are openings between the woven glass fibers.

77 GHz difference

Research on different glass cloth types of circuit board materials has found that the transmission line circuits are located in different glass fiber "knuckle crossing areas" and "beam opening areas", and their performance will be significantly different. From the above three typical glass cloth types of circuit board materials, design the circuit for measurement. The material uses calendered copper to minimize the influence of the roughness of the copper foil, and select the circuits that go through the "knuckle crossing area" and the "beam opening area" to measure with the network analyzer. The measurement parameters include the group delay, propagation delay and phase angle response of each circuit, as well as the resulting performance differences, in order to gain insight into how different glass fibers and different glass weave structures produce different Dk values in the circuit.

This experiment uses a 4mil thickness of polytetrafluoroethylene (PTFE) material, no filler, calendered copper, and a combination of the above three different glass cloths. The 1078 type glass fiber circuit board material has a flat and balanced configuration, which minimizes the difference between the direction of the "knuckle crossing area" to the circuit and the direction of the "beam opening area". The test result shows that the phase difference of the circuit made of this type of 1078 glass fiber circuit board material is only 20 degrees at the frequency of 77 GHz.

How does the performance of the other two glass fibers compare? The same 4mil-thick PTFE non-filled rolled copper laminate material. The 106 type glass fiber used has an open weave and balanced structure. The phase angle of the "knuckle cross zone" and "beam open zone" directions at 77 GHz The average difference is 100 degrees. The 1080 type glass cloth used in the same circuit material has an open weave and unbalanced structure, and the average phase angle difference of the circuit at a frequency of 77 GHz is 149 degrees.

What is the difference in the Dk of the circuit board material caused by these differences caused by the glass fiber effect? The results of the same circuit above show that the difference between the circuit in the "knuckle cross-beam area" and the circuit in the "beam opening area" corresponds to a change in Dk of about 0.02 for the circuit using the 1078 glass cloth material. Using 106 type glass cloth material, the difference in Dk is relatively large, which is 0.09. The maximum Dk difference corresponding to the circuit using the 1080-type glass cloth material reaches 0.14.

For circuit laminates using a single layer of glass fiber, the glass fiber effect is more obvious than that of multilayer glass laminates, because the average stacking of multiple glass fibers will make the glass distribution more uniform. For millimeter wave circuits, the wavelength is very small, and the usual circuit is very thin, and the material is usually only reinforced by a layer of glass fiber. In this case, the circuit performance will be more affected by the glass fiber effect. Laminates with fillers (such as ceramics) have this additional material (the Dk is between the Dk of the glass and the Dk of the resin system), although it cannot completely solve the glass fiber effect, but it will make the circuit to a certain extent The Dk on the board material is more uniform to reduce the influence of the glass fiber effect under high frequency. For example, RO4830™ laminate produced by Rogers Corporation is this type of circuit material, with 1078 flat open fiber glass cloth and ceramic filler.

In addition, Rogers’ RO3003™ laminate does not contain glass cloth and is one of the commonly used circuit board materials for millimeter wave circuits. This is a ceramic-filled PCB material with a Dk of 3.00 and a Dk tolerance controlled within ±0.04. This Dk consistency is essential for differential pairs in millimeter wave circuits and high-speed digital circuits.

Remove glass fiber

One way to completely avoid the glass fiber effect is to use a circuit board material without glass fiber/cloth. Especially for automotive radar circuits such as the use of 77GHz millimeter waves, it is much better to use high-frequency circuit board materials without glass fiber than those with glass fiber reinforced circuit board materials. Rogers' latest RO3003G2™ circuit laminate is also a material that does not contain glass cloth. Tests have shown that it has very consistent performance between different circuit boards at millimeter wave frequencies, such as consistent microstrip transmission line impedance.

When it comes to impedance changes, other materials or circuit parameters, such as changes in conductor width, copper thickness, and substrate thickness, may also cause changes in the impedance of the transmission line. However, the newly released RO3003G2 high-frequency board material completely eliminates the glass fiber effect factor that affects circuit impedance or performance changes, which is critical for 77 GHz and higher frequencies.

Note: This blog is based on the original author’s webinar report "An Overview of Glass Weave Impact on Millimeter-Wave PCB Performance" (An Overview of Glass Weave Impact on Millimeter-Wave PCB Performance).

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