Printed Circuit Design & Fab - February 2008 - (Page 40)

LAMINATE MATERIALS Advanced GLASS REINFORCEMENT Technology for Improved Signal Integrity New glass fabrics promise to resolve challenges in high tech PCB design and fabrication. by RUSSELL DUDEK, PATRICIA GOLDMAN and JOHN KUHN Among the many challenges facing the electronics industry today are those related to the need for increased speed. These challenges include improved signal integrity and increased product reliability at multi-GHz clock rates with inherently smaller threshold voltages and picosecond rise/fall times. The implication of increased speed continuously challenges the industry. In recent years, numerous technical papers have proposed methods to deal with these challenges, coining the phrase “fiber weave effect” or FWE to describe one of the central issues associated with the influence of the glass reinforcement fiber on the electrical performance of the PCB1-7. While many solutions have been presented to deal with FWE, none have addressed the actual woven glass fabric in the laminate material. The questions posed are these: Can an innovative glass fabric technology improve end product performance? Can a raw material so far back in the supply chain have an impact on leading edge industry challenges? The answer to both questions is Yes. A new high performance glass fabric technology has been developed that demonstrates uniformity of glass weave resulting in a homogeneous reinforcement layer. Used with readily available resin technologies, this new glass fabric can provide improved physical properties (laser and mechanical drilling, dimensional stability and surface smoothness), superior electrical properties (CAF resistance), and enhanced performance properties (more uniform Dk and circuit impedance, reduced signal skew, and improved signal integrity). Many solutions have been proposed to address FWE in circuit boards, however, in all of these approaches there has been an implicit assumption that the glass fabric reinforcement layer itself cannot be improved upon. This assumption is not true. Consider a Style 1080 fabric, perhaps the most commonly used glass fabric in high performance multilayer PCBs. The standard construction of this style involves 60 yarns per inch in the warp or machine direction and 47 yarns per inch in the weft or cross-machine direction. The 1080 designation also specifies the type of yarn, as per IPC-4412A9. FIGURE 1 shows standard 1080 (a) and the high performance 1080 (b). Both have been made according to the IPC specification. Each has exactly the same quantity of glass with the same number of warp and weft yarns. The most obvious physical difference between these fabrics is the spread out ribbon-like yarns in A B FIGURE 2. Traditional yarn vs. TwistFreeTM, DirectFinishTM yarn. FEBRUARY 2008 FIGURE 1. 1080 glass fabric in a standard (a) vs. advanced (b) styles. 40 PRINTED CIRCUIT DESIGN & FAB

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Printed Circuit Design & Fab - February 2008

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