Printed Circuit Design & Fab - February 2008 - (Page 41)

FIGURE 4. Improved signal integrity and clock rate. FIGURE 3. Colors overlayed on photos to illustrate layer coverage. the right-hand photo. To determine the properties of these fabrics, we must first understand the underlying technologies. In the manufacture of traditional fiberglass, the yarns are coated with a vegetable-based starch-oil mixture to facilitate weaving. This coating is then removed in a lengthy heat clean step, after which the yarn bundle receives a silane coating. Depending on the tightness of the weave and/or any ash residue left from the heat cleaning, the individual yarns may or may not be fully wetted by the resin matrix. In addition, the heat-cleaning step reduces the glass fabric strength by approximately one third. Therefore, elimination of this damaging step will create a stronger, more dimensionally stable composite. Direct application of a final resin-compatible finish during the fiber-forming process provides a better interface between the glass fiber reinforcement and the resin matrix8. This resincompatible finish is applied to the pristine surface of individual glass fibers immediately as they are formed, remaining on the yarn and glass cloth throughout the manufacturing process. Another significant difference is seen in FIGURE 2, where the contrasts between a twisted fiber bundle and the more advanced untwisted yarn are highlighted. Traditionally, the glass fiber bundle is twisted to give strength and mechanical integrity to the yarn and make it easier to weave into fabric. Twisted yarns are somewhat rope-like, making thicker cross-points in the glass fabric (often referred to as knuckles). Twisted glass fiber is also known to contribute to stresses within the laminate. Untwisted yarn is more ribbon-like, lies flatter and spreads out easily. This yarn construction yields a more consistent fabric, where glass fibers are more uniformly distributed, and weave knuckles and open areas are minimized. Refer again to Figure 1, which shows two different fabrics with the same amount of glass, but with the glass fibers distributed differently. The traditional glass reinforcement (Figure 1a) is thicker and unevenly distributed, leading to inconsistent substrate properties. There are essentially three differing glass TABLE 1. Comparison of 1080 glass products. TRADITIONAL 1080 Fabric Thickness % Coverage (2 / 1 / 0 layers) Dielectric Constant (Dk) Dissipation Factor (Df) 2.1 mil (0.053 mm) 22/55/23 6.6 – 6.9 0.006 (avg.) NOVASPEED 1080TM thicknesses in the fabric: areas with two layers of yarns (at the knuckles), areas with just one layer, and the interstices between the yarns where there is no glass. In Figure 1a these areas are very distinct from each other, while in Figure 1b the fibers have been spread out to fill the interstices. FIGURE 3 further illustrates this concept. For the best mechanical, electrical and performance properties, total glass uniformity is key. This is a direct result of maximizing the amount of two-layer coverage. In Figure 3 we have quantified the amount of zero, one and two-layer coverage using red, yellow and green areas (respectively) overlayed onto the photos from Figure 1. The percentages shown are based on measurements from the photographic images. Considering that these two fabrics contain the same amount of glass, it can be presumed that the fabric in Figure 1b is thinner than 1a, and this is indeed the case as confirmed by measuring total fabric thickness9. In fact, the two-layer areas of glass in the traditional fabric are approximately 50% thicker than the two-layer areas in the advanced glass on the right (2.1 mils vs. 1.4 mils). In addition to a more uniform glass configuration, this new glass fabric utilizes a lower dielectric constant (Dk) glass composition to reduce the Dk difference between glass and resin. This Dk difference is at the root cause of FWE. In traditional 1080 fabric using E-glass, the Dk difference between the glass and high performance resin is approximately 4 units. With the new fabric this Dk difference is reduced by half. When the lower Dk is combined with smoother fabric, the FWE can be significantly reduced. In FIGURE 4, the photos from Figure 1 have been overlayed with 3-mil lines to approximate scale for illustration purposes. Both fabrics have the same amount of glass but it is distributed differently. Estimates of single end impedance and effective Dk variation for traditional 1080 fabric are based on data presented in a paper by Brist et al at IPC Expo 20042. Among the many proposed solutions to address FWE is the use of a heavier fabric style that has a balanced construction and tighter weave that can reduce the open spaces7. TABLE 2. Comparison of various laminates for Dk and Df. MATERIAL Glass Reinforcement Laminate A Laminate B Laminate C DK 4.5 to 5.0 2.97 3.12 2.98 DF .005 .0079 .0080 .0039 1.4 mil (0.036 mm) 77/22/1 4.5 – 5.0 0.005 (avg.) FEBRUARY 2008 PRINTED CIRCUIT DESIGN & FAB 41

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Printed Circuit Design & Fab - February 2008

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