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Enhanced SMT pad design for signal transmission

Posted: 19 Jan 2015 ?? ?Print Version ?Bookmark and Share

Keywords:EM wave? signal reflection? attenuation? SMT pad? impedance?

In the high frequency domain, a signal or EM wave must propagate along the transmission path with uniform characteristic impedance. Whenever an impedance mismatch or discontinuity is encountered, a portion of the signal is reflected back to the transmitting end while remaining portion of the EM wave continues to travel to the receiving end. The severity of the signal reflection and attenuation depend on the magnitude of impedance discontinuity. When the magnitude of mismatched impedance increases, a larger portion of the signal is reflected and more attenuation or degradation of signal is observed at the receiver.

The phenomenon of mismatched impedance is encountered at SMT pads of AC coupling (a.k.a DC blocking) capacitors, board to board connectors, and cable to board connector (e.g., SMA).

In the case of SMT pad of AC coupling capacitor, as shown in figure 1, a signal that travels along the PCB traces with 100? differential impedance and 5 mils copper width, experiences an impedance discontinuity once it reaches the SMT pad with a wider copper (e.g., 30 mils width for 0603 package). This phenomenon is explained in Equation (2) and (1). The rise of copper's cross sectional area or width increases the strip capacitance, which in turn causes capacitive discontinuity (i.e., negative surge) to the characteristic impedance of the transmission channel.

In order to minimise the capacitive discontinuity, the reference plane area right under the SMT pads is cut out, and copper fill is constructed on the inner layer, as illustrated in Figure 2 and 3 respectively. This increases the distance between SMT pad and its reference or return path, which reduces the capacitive discontinuity. Micro stitching vias shall be inserted to provide electrical and physical connectivity between the original reference plane and the new reference copper on inner layer to have a proper signal return path to avoid EMI radiation issue.

However, the distance "d" shall not be increased too much until strip inductance overrides strip capacitance and causes inductive discontinuity, as explained in Equation (3).

Figure 1: Side view of PCB without plane cut-out.

Figure 2: Side view of PCB with plane cut-out.

Figure 3: Top view of PCB with plane cut-out.

C = strip capacitance (in pF)
L = strip inductance (in nH)
Zo = characteristic impedance (in ohm)
ε = dielectric permittivity
w = width of SMT pad
l = length of SMT pad
d = distance between SMT pad and reference plane underneath
t = thickness of SMT pad

The same concept is applicable to SMT pads of board to board (B2B) and cable to board (C2B) connectors, as shown in figure 4 and 5 respectively.

Figure 4: B2B connector.

Figure 5: C2B connector.

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