How to Calculate the PCB Trace Impedance

Introduction

Impedance is a combination of resistance and reactance. We all know that resistance is formed because of resistor and the reactance formed due to the reactive elements inductor and capacitor. Resistance is a DC characteristic meanwhile impedance is AC characteristic which means that the frequency is additionally take part in impedance. Impedance is an opposition of current flow in a circuit. Impedance mismatch will cause more reflection that can affect EMI. Impedance was determined in letter ‘Z’ and its unit is ohm (Ω)

TRACE IMPEDANCE

Impedance is monitored for both single ended and differential trace. Trace impedance is between 25 and 125 ohm. The differential impedance is always smaller than twice the single ended impedance. Impedance is not constant for all the signals, it will differ for high speed differential signals and single ended signals. Designers should work closely with the manufacturer to control and match the impedance. The signals allow certain impedance tolerance. While designing, try to keep the calculated impedance value to the exact impedance value. To control the impedance should follow the below factors:

  • Use proper trace width and thickness
  • Spacing between the trace and special care for high speed differential signals.
  • Try to use less via for single signals
  • Choose proper prepreg and core thickness while building stack up and consider material’s dielectric constant
  • Distance between the reference plane and trace
  • Avoid trace stubs and via stubs

IMPEDANCE MATCHING

Match the impedance to keep the signal more stable from the source to the load. By matching the impedance can improve the EMI performance. Should follow some of the rules to avoid impedance mismatch.Avoid trace stubs, The long trace stubs can act as antennas and increase problems with EMC standards. It can also produce signal reflections that affects the signal integrity

Figure 1 : Routing With Stub

STUB

Figure 2 : Routing Without Stub

WITHOUT_STUB

If the high speed signals routed over void in a reference ground plane, the return signal cannot follow the signal trace. The return signal choose another path to reach the source. The space between the trace and return signal act as a loop antenna. If there is a void are split in reference plane, then route the signals around the void

Figure 3 :Routing on voids

Figure 4 : Routing corrected

A wide trace has a lower impedance compare to the thin trace. Similarly the larger pad has lower impedance than the trace which is connected to the pad. The sudden change in impedance is called impedance discontinuity. This impedance discontinuity can cause reflections. To avoid this problem a plane obstruct should be placed under the larger pad to match the impedance

Figure 5 : Plane cut under larger pads

Another way to match the impedance is add termination resistor parallel to the receiver that equals the trace impedance. Resistor connected to a sink and ground the resistor that reflect some signal and create an impedance.The impedance is varied for the traces routed on the outer and inner layers that named as two types are

  • Microstrip
  • Stripline

MICROSTRIP

Microstrip is indicates the trace routed on the outer layer of PCBs and it had single reference plane that separated by a dielectric material. The finished PCBs that coated with solder mask is called embedded microstrip. Solder mask is also a dielectric so, while calculate the impedance must consider solder mask. It has a better signal characteristics than the strip line. Fabrication process for micro strip also less expensive

Figure 6 : Microstrip

Figure 7 : Embedded Microstrip

The differential pair traces that routed on outer layer is called edge coupled microstrip. The finished PCBs with solder mask is called embedded edge coupled microstrip. The length of these traces should be same from the transmitter to receiver

Figure 8 :Edge Coupled Microstrip

EC_MICROSTRIP

IMPEDANCE CALCULATION FOR MICROSTRIP

To calculate the impedance of microstrip need to know the width of the trace and thickness and dielectric thickness that shown in below image

Figure 9 : Microstrip example

Formula:

Z   = impedance

W = width of the microstrip

Ԑr = dielectric constant

T  = Thickness of trace

H  = height of dielectric material

STRIPLINE

Stripline indicates the trace routed on inner layer with two reference plane layer. stripline is surronded with dielectric material. Stripline was more complex to manufacture because it requires multiple layer to suppport the embedded trace between two reference planes

Figure 10 : Stripline Example

Symmetric stripline is a balanced stripline that it is exactly in the middle of two reference plane layer. The dielectric height should be adjusted for the trace being embedded between two planes

Assymetric stripline is unbalanced stripline that the trace is close to one of the reference plane and away from another reference plane. In that case, the closer ground plane is taken as reference for the stripline

Figure 11 : Symmetric stripline

symetric-stripline

Figure 12 : Assymetric stripline

ASYMETRIC-stripline

Same like microstrip the differential pair that routed on inner layer are called edge coupled stripline. The differential pair were coupled in same layer side by side. The another type to route differential pair in inner layer is broadside coupling. In broadside, the differential pair was coupled on different layers that seperated by dielectric material. In both cases, the coupled striplines are consider to have equal length

Figure 13 : Edge coupled
Stripline

EDGE-coupled stripline

Figure 14 : Broadside coupled Stripline

BS-coupled stripline

IMPEDANCE CALCULATION FOR STRIPLINE

To calculate the impedance of Stripline we need to know the width of the trace and thickness and dielectric thickness that shown in below image

Figure 10 : Stripline

STRIP_IMP

Formula:

Z   = impedance

W = width of the microstrip

Ԑr = dielectric constant

T  = Thickness of trace

H  = height of dielectric material

Leave a Reply