Minimizing The Effect Of Fiber Weave; Overview Of Fiber Weave; Breakout Geometries For Various I/O Interfaces - Intel Quark SoC X1000 Design Manual

Stack-Up and PCB Considerations—Intel
Table 4 lists examples of single-ended and differential impedances specified for
different interfaces. The microstrip single-ended impedance tolerance is ±15%. The
stripline and dual-stripline single-ended impedance and differential impedance
tolerance is ±10%. The microstrip differential-impedance tolerance is ±15%.
Table 3.

Breakout Geometries for Various I/O Interfaces

I/O Interfaces Component
USB2/ PCIe/ SOC
Platform Clocks
DDR3 SOC
2
Default SOC
SOC
Notes:
1. Equivalent pairs mean both pairs have equal swing and signal propagation direction. Non-equivalent pairs mean both pairs
have different swing or propagation direction.
2. Default refers to legacy signals which are not listed in the table, i.e., low speed non-critical signals.
2.6

Minimizing the Effect of Fiber Weave

2.6.1

Overview of Fiber Weave

Fiber weave in the PCB material impacts the routing of high-speed traces. The PCB
material is made up of a composite of fiber and resin. The strands of fiber run
perpendicular to each other. Depending on the orientation of the weave relative to the
trace, there could be either resin or a fiber bundle beneath the trace. Due to the
differing dielectric constants of these two materials, a phase skew could be introduced
among signals that comprise a differential pair. This phase skew manifests itself as an
AC common mode noise at the receiver, affecting both voltage and timing margin at the
receiver. The layout recommendations in this section will minimize the effect of the PCB
fiber weave on the routing of differential signals. The choice of a particular mitigation
technique is dependent on the configuration and layout of the platform and, hence, this
choice is left to the platform designer. Typical printed circuit boards, due to their basic
construction consisting of woven fiberglass fabric (Er~6) strengthened and bound
together with epoxy resin (Er~3.5), present a non-homogeneous medium for signal
propagation of differential pairs. As shown in
with the weave strands running horizontal and vertical relative to the board edges. For
differential pairs with trace width and spacing comparable to the dimensions of the
fiberglass cloth, the PCB's non-uniform dielectric can give rise to substantial
propagation differences between the conductors of the pair, see
June 2014
Order Number: 330258-002US
®
Quark™ SoC X1000
Trace
Stack-Up Units
Width
MS
mils 4
MS mils 4
MS mils 4
mils 3.5
MS
Minimum
Minimum
Pair-to-
Trace
Equivalent
Spacing
1
Pair Spacing
4 8
4 NA
4 NA
4 NA
Figure 8, there are several weave types,
Figure
Intel
Minimum Pair-to-
Non-Equivalent
1
Pair Spacing
8
NA
NA
NA
9.
®
Quark™ SoC X1000
PDG
23