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How to solve the impedance mismatch in the field of high frequency signal transmission

Release date:2021-12-28Author source:KinghelmViews:652

In the high frequency domain, the signal or electromagnetic wave must propagate along the transmission path with uniform characteristic impedance. In case of impedance mismatch or discontinuity, part of the signal is reflected back to the transmitting end, and the remaining electromagnetic wave will continue to be transmitted to the receiving end.

The degree of signal reflection and attenuation depends on the degree of impedance discontinuity. When the mismatch impedance amplitude increases, more signals will be reflected, and more signal attenuation or degradation will be observed at the receiver.
Impedance mismatch is often encountered at SMT pads of AC coupling (also known as DC isolation) capacitors, board to board connectors and cable to board connectors (such as SMA).   
In the case of the AC coupling capacitor SMT pad shown in Fig. 1, the signal propagating along the PCB route with 100 Ω differential impedance and 5MIL copper foil width will encounter impedance discontinuity when reaching the SMT pad with wider copper foil (such as 30mil width of 0603 package). This phenomenon can be explained by equations (1) and (2).
The increase of cross-sectional area or width of copper foil will increase the strip capacitance, which will bring capacitance discontinuity to the characteristic impedance of transmission channel, that is, negative surge.
In order to minimize the discontinuity of capacitance, it is necessary to cut out the reference plane area directly below the SMT pad and create a copper fill in the inner layer, as shown in figures 2 and 3 respectively.
This can increase the distance between the SMT pad and its reference plane or return path, so as to reduce the discontinuity of capacitance. At the same time, micro stitched vias shall be inserted to provide electrical and physical connections between the original reference plane and the inner new reference copper foil to establish the correct signal return path and avoid EMI radiation problems. 

However, the distance "d" should not be increased too much, otherwise the strip inductance will exceed the strip capacitance and cause inductance discontinuity. Where:
  • Strip capacitance (unit: PF);      
  • Strip inductance (unit: NH); 
  • Characteristic impedance (unit: Ω);  
  • Ε= Dielectric constant; 
  • Pad width;      
  • Pad length;      
  • The distance between the pad and the lower reference plane;   
  • Thickness of pad.      
The same concept can also be applied to SMT pads for board to board (B2B) and cable to board (C2B) connectors. 
The verification of the above concepts will be completed through TDR and insertion loss analysis. The analysis is completed by establishing the 3D model of SMT pad in empro software, and then importing it into keysight ads for TDR and insertion loss simulation.
1. The SMT pad effect of AC coupling capacitance is analyzed   
A 3D model of SMT with medium loss substrate is established in empro, in which a pair of microstrip differential traces are 2 inches long and 5MIL wide, adopt single ended mode, and are 3.5mil away from its reference plane. The pair of traces enter from one end of 30mil wide SMT pad and lead out from the other end.  

Figure 4 and figure 5 show the TDR and insertion loss diagrams obtained by simulation respectively.    
The impedance mismatch caused by the SMT design without cutting the reference plane is 12 Ω, and the insertion loss is - 6.5db at 20GHz. Once the reference plane area below the SMT pad is cut (where "d" is set to 10mil), the mismatch impedance can be reduced to 2 Ω and the insertion loss at 20GHz can be reduced to - 3dB.    
Further increasing "d" will cause the strip inductance to exceed the capacitance, resulting in inductance discontinuity and poor insertion loss (i.e. - 4.5db).
2、The SMT pad effect of B2B connector is analyzed  
A 3D model of SMT pad of B2B connector is established in empro, in which the pin spacing of the connector is 20MIL and the pin width is 6mil. The pad is connected to a pair of microstrip differential routing with length of 5 inches and width of 5MIL, which adopts single ended mode, and the routing is 3.5mil away from its reference plane.
The thickness of SMT pad is 40mil, including connector pins and solder, which is almost 40 times the thickness of microstrip PCB wiring.
The increase of copper thickness will lead to capacitance discontinuity and higher signal attenuation. This phenomenon can be seen from the TDR and insertion loss simulation diagrams shown in Fig. 6 and Fig. 7 respectively.    
The impedance mismatch can be minimized by cutting out the copper area with an appropriate spacing "d" (i.e. 7mil) directly below the SMT pad. 
3. Summary   
The analysis of this paper shows that cutting off the reference plane area directly below the SMT pad can reduce the impedance mismatch and increase the bandwidth of the transmission line.   
The distance between the SMT pad and the internal reference copper foil depends on the width of the SMT pad and the effective thickness of the SMT pad, including connector pins and solder. If conditions permit, 3D modeling and simulation shall be carried out before PCB production to ensure that the constructed transmission channel has good signal integrity.
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