The idea of glass weave effect has been around for many years, and it is a topic that is somewhat controversial. In theory, the glass weave effect is a concern that the structure of the glass fabric can have a negative influence on high-frequency or high-speed digital circuit performance.
Most woven glass fabric used to improve mechanical properties of laminates has areas of glass bundles and open areas between the bundles. The glass bundles’ dielectric constant (Dk) is typically about 6 and the areas between the bundles can have a Dk of around 3, depending on the resin system used to make the laminate. One concern for the glass weave effect is that if a critical circuit conductor is perfectly aligned to the pattern of bundles and open areas, the conductor will experience different Dk in small isolated areas. It is possible at very high frequencies or extremely high speed digital rates that these isolated Dk differences could have an influence on the circuit performance.
One example of the glass weave effect: A microstrip transmission line circuit (two copper-layer circuit) uses a laminate with a Dk of 3.0. At 77 GHz, the ¼ wavelength will be about 0.024” and the ⅛ wavelength approximately 0.012”. Theoretically, it is known that when an electromagnetic wave is propagating in a medium and encounters an anomaly that is ¼ wavelength or larger, the wave can be disturbed and possible resonances can occur. Additionally, real-world experience has shown that anomalies in the medium which are ⅛ wavelength can cause wave propagation issues. If a laminate has a glass style with openings or bundle sizes which are ⅛ wavelength or larger, the discrete Dk anomalies of the glass bundles and open areas could cause circuit performance issues. Many different glass styles are used in the industry, and several glass styles have dimensions of 0.012” (⅛ wavelength at 77 GHz) or larger.
If the laminate is using two or more layers of woven-glass fiber, this may lessen the concern for glass weave effect in this example. When two layers of glass fabric are used to make the laminate, the odds of the bundles or open areas aligning is very unlikely. This means the discrete Dk anomalies due to the glass bundle and open areas will be greatly lessened and the impact on the wave could be minimal or insignificant.
To read this entire column, which appeared in the April 2017 issue of The PCB Design Magazine, click here.