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

HS laMinatES PcB Dielectric Materials for hiGh-sPeeD aPPlicaTioNs The glass weave in a laminate can affect propagation velocity and loss profile, leading to signal loss and skew in differential pairs. by RAVINDRA GALI As data rates continue to increase, PCB designers are moving toward serial buses with differential signaling. High-speed serial interfaces such as PCI-Express, 10 Gigabit Ethernet XAUI, OC768 and CEI use differential signaling for transmitting and receiving data. FGPA devices with embedded high-speed transceiver blocks provide an easy, cost-effective approach for implementing these high-speed applications, as illustrated in FiGurE 1. Differential signaling uses two output drivers to drive two independent transmission lines. One driver carries one bit, the other its complement. The difference of the two signals, measured between the two traces, carries the desired information. A differential pair is a pair of transmission lines with some amount of coupling between the two legs of the pair. Ideally, routing a differential pair on a PCB should meet two criteria. Electrically, the two traces should be identical. This means each trace of the differential pair should have the same cross sectional dimensions and must be surrounded by the same type or types of dielectric materials. The spacing between the two traces should also be the same for the entire length of the trace. Skew or time delay between the two traces of the differential pair should be zero. Thus, the edge of the difference signal is sharp and well defined. Any skew between the two traces results in a common mode noise and also leads to a degradation of the differential signal. Dielectric Material Selection Proper dielectric material selection enables the PCB designer to minimize the dielectric loss because at frequencies above 1 GHz, dielectric loss is dominant compared to conductor loss. The dielectric loss is dependent on the loss tangent/dissipation factor for a given dielectric material. The lower the loss tangent, the better it behaves at higher frequencies and from a signal loss standpoint. The dielectric constant and the loss tangent information for the various dielectric materials typically used in the PCB design are shown in taBlE 1. A test board with different dielectric materials and glass weaves was specifically designed to study the various aspects of the high-speed channel design. The test board was designed as an 8-layer board with four routing layers (L1, L3, L6, L8) and referencing different dielectric materials. Layers FiGurE 1. FPGa floor plan. 40 FiGurE 2. a plot showing differential insertion loss. DECEMBER 2008 printEd CirCuit dESign & fAB

Table of Contents Feed for the Digital Edition of Printed Circuit Design & Fab - December 2008

Printed Circuit Design & Fab - December 2008 Contents Our Line Market Watch Around the World Happenings ROI Global Sourcing EMC for the Real World Interconnect Strategies On the Forefront Final Finish Forum Test and Inspection Electronic System Design Data Management 101 Designers Take on Technology Challenges in 2008 PCB Signal Integrity, Power Integrity and EMC Challenges What’s in a Name? Ad Index PCB Dielectric Materials for High-Speed Applications Off the Shelf Marketplace BGA Bulletin

Printed Circuit Design & Fab - December 2008

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