Figure 11-4: 2D Field Solver Analysis Of 5 Mil Trace And 28 Mil Pad; Smt Pads - Xilinx Virtex-4 RocketIO User Manual

R
The results using these equations are not sensitive to rise time variation and are valid for
simulated TDR measurements provided that the leading and trailing transmission lines
are very close to 50Ω . However, for actual measurements, accuracy is very dependent on Z
being very close to 50Ω.

SMT Pads

For applications that require AC coupling between transmitter and receiver, SMT pads are
introduced in the channel to allow coupling capacitors to be mounted. Standard SMT pads
have excess capacitance due to plate capacitance to a nearby reference plane. In the
following example, a 5 mil trace with a Z
28 mils wide. In this example, there is 3 mils of FR4 above a solid ground plane. Cross-
sections of both the line and pad are shown in
Using a 2D field solver on these dimensions yields a Z
the 0402 pad is 16Ω because the pad has too much capacitance and too little inductance,
resulting in an impedance of less than 50Ω. Performance of this transition can be optimized
in one of two ways.
The first method makes the trace the same width as the pad and moves the ground plane
deeper into the stack-up to maintain the Z
require any special analysis, but there might be some error due to the fringing capacitance
of the SMT capacitor body. The trace density is greatly limited because traces are now
28 mils wide.
The second method, as illustrated by
pad, which removes much of the excess capacitance caused by the plate capacitance
between the pad and the ground plane. This technique allows for greater trace density than
the first method, but requires a 3D field solver analysis or measurement along with several
board iterations to get the desired performance.
Virtex-4 RocketIO MGT User Guide
UG076 (v4.1) November 2, 2008
Line
- 5.2 mils wide over 3 mil FR4 Dielectric
- L = 288 nH/m
- C = 116 pF/m
- Zo = 50Ω
Pad
- 28 mils wide over 3 mil FR4
- L = 98 nH/m
- C = 404 pF/m
- Zo = 16Ω

Figure 11-4: 2D Field Solver Analysis of 5 Mil Trace and 28 Mil Pad

- L = 241 nH/m
- C = 89 pF/m
- Zo = 52Ω
Figure 11-5: Transition Optimization
www.xilinx.com
of 50Ω transitions to an 0402 SMT pad that is
0
Figure 11-4.
5 Mil Trace
28 Mil Pad
of 50Ω for the 5 mil trace. The Z
0
of the transition at 50Ω. This method does not
0
Figure 11-5, clears the ground plane underneath the
28 Mil Pad
SMT Pads
0
UG072_c3_14_050206
for
0
UG072_c3_15_102505
247