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Intel
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IXP45X and Intel
®
IXP46X Product Line of Network Processors
February 2007 HDD
Document Number: 305261; Revision: 004 61
General PCB Guide—Intel
®
IXP45X and Intel
®
IXP46X Product Line of Network Processors
• Poor routing density
• Uncontrolled signal trace impedance
• Lack of power/ground planes, resulting in unacceptable cross-talk
• Relatively high-impedance power distribution circuitry, resulting in
noise on the power and ground rails
• High-speed circuits require multi-layer printed circuit boards:
— Advantages:
• Controlled-impedance traces
• Low-impedance power distribution
— Disadvantages:
• Higher cost
•More weight
• Manufactured by fewer vendors
• Symmetry is essential to keep the board stack-up symmetric about the center
This minimizes warping
• For best impedance control, have:
— No more than two signal layers between every power/ground plane pair
— No more than one embedded micro-strip layer under the top/bottom layers
• For best noise control, route adjacent layers orthogonally. Avoid layer-to-layer
parallelism.
• Fabrication house must agree on design rules, including:
— Trace width, trace separation
— Drill/via sizes
• The distance between the signal layer and ground (or power) should be minimized
to reduce the loop area enclosed by the return current
— Use 0.7:1 ratio as a minimum.
For example: 5-mil traces, 7-mil prepreg thickness to adjacent power/ground.
Figure 20 gives an example for a six-layer and eight-layer board. For stripline (signals
between planes), the stackup should be such that the signal line is closer to one of the
planes by a factor of five or more. Then the trace impedance is controlled
predominantly by the distance to the nearest plane.
Figure 20 and Figure 21 illustrate the proposed stackup for the six- and eight-layer
boards.