Standardizing Glass Fabric Specifications for High Speed Printed Boards
Standardizing spread Low-Dk glass fabrics eliminates phase skew and stabilizes trace impedance across high-speed printed circuit board panels.
Trace Spatial Geometry
Physical Dimension
Arrangement of conductive paths in three-dimensional space is defined by the width, thickness and separation of the traces on a printed board. Precise trace spatial geometry is required to achieve a specific characteristic impedance for high-speed transmission lines. Variations in these dimensions, even on the scale of a few microns, can lead to signal reflections and timing errors.
The geometry is established during the etching and plating phases of fabrication and is verified through automated optical inspection.
Impedance Calculation
Distance between a signal trace and its reference plane is the primary factor in determining the capacitance of the transmission line. When the dielectric thickness varies, the impedance of the trace changes accordingly. This relationship is modeled using field solvers that account for the trapezoidal shape of the etched copper and the surrounding resin.
Designers specify tight tolerances for these physical features to ensure that the final board matches the simulation results. If the geometry is not controlled, the signal will lose energy as it travels, reducing the reach and reliability of the data link.
Tolerance Control
Modern manufacturing processes use laser direct imaging to maintain the accuracy of these features across the entire surface of the panel. This technology allows for the creation of fine lines that are necessary for high-density interconnects.