When it comes to increasing the structural strength of PCB materials, woven glass is an obvious choice. It adds significantly to the mechanical stability of the laminate rendering it high structural strength. In fact, the use of woven glass in PCB fabrication dates back as early as to the 1960s when they added high tensile strength and stability to the substrate. In addition, glass fibers also bring the added advantages of high thermal as well as chemical resistance, insensitivity to moisture as well as the advantage of being an electrical insulator. In the early days, however, there was little concern about considering the effect of inhomogeneity in the resultant substrate. However, today it is also important to understand how the properties of components affect the behavior of the whole.
Very often concerns regarding the negative impact of woven glass, in terms of electrical performance, come to the fore. The “glass weave effect” is feared to impact the performance of high-frequency PCB.
Here is an overview of factors that impact the woven glass weave phenomenon:
It is a possibility that on account of the woven glass laminate, the Dielectric Constant or Dk as it is popularly known as, varies across areas. So, in areas where glass bundles are present, the Dielectric Constant is high, while in open areas between the bundles, the Dk is low. The variation can be as high as between 6 in areas of glass bundles and 3 in open areas. Since Dk is an important metric, the variation in Dk is a cause for concern as far as circuit designers are concerned. Dk variation can impact performance particularly at higher frequencies of 30 GHz and above, also referred to as millimeter-wave frequencies. When the electromagnetic wave faces a change in the Dielectric Constant or Dk, propagation can be impacted and resonances may occur. The impact can be seen even when the anomaly is as small as one-eighth of a wavelength. With woven glass stacked to form a laminate, it is less likely for glass bundles and openings to align. Consequently, it is less likely for discrete Dk variations. In cases where two or more layers of glass are used, therefore, the impact of the glass weave effect is decreased.
The bigger issue regarding the glass weave effect, plays out when circuits use a laminate which has a single layer of woven glass. The fear also is to do with the random impact of the glass weave effect and how it will play out in case of high volume production. On account of the glass weave effect there can be performance variations from one circuit to another. Often changes in the phase angles of high frequency waves is noticed which in turn impacts phase based modulation.
This is not to say that using multiple layers of woven glass fabric can overcome all the issues related to the glass weave effect. Often differential conductors on a single layer circuit can bring about a new set of issues. In case the Dielectric constant or Dk values of the wave propagation medium varies between the pair of lines, the phase angles are impacted. This in turn, results in a slowing down of the propagation of one signal wave over the other and results in a skew. Particularly in high speed circuits, this can affect performance.
However, fortunately there are ways to tide over this issue. Changes in Dk can be stemmed by using a laminate with a filled resin system. The use of the filler results in less variation in Dk. The combination of glass fabric, resin and filler, in fact averages out the variation in Dk values thereby ensuring that performance is not impacted.
Yet another way to ensure that performance isn’t negatively impacted, is to use flat glass fabric along with minimizing the relative glass content in the laminate. This also goes a long way in minimizing the variation in Dk. The use of spread glass is also beneficial as it offers the ability to wet the fibers which allows a chemical bond to be formed between the glass reinforcement and the resin. This in turn, is a significant improvement over using traditional tightly bound bundles of glass fibers which can lead to defects in the interface.
The way to overcome the glass weave effect totally, of course is to not use woven glass at all. There are ceramic filled laminates that ensure that the dielectric constant or Dk is maintained in a certain range across the circuit board. This consistency, in turn, works well for small wavelengths as also for coupled lines.
In future PCB substrates will use even better fibers and resins to produce more advanced and functional materials that will add to the performance.
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