Characteristic impedance is the ratio of voltage and current of a single wave propagating along the line. Characteristic impedance is very important to maintain the integrity of the signal. Especially for the high speed signals, it is very important. Any deviation in the characteristic impedance value cause signal reflection and hence signal integrity issues. Controlled impedance is the norm for the PCB designers for high frequency signals.
The following are the factors which affect the impedance value of PCB:
1. Thickness of the insulating material (Core/Prepeg) (H)
2. Width of the PCB trace (W)
3. Coating above the trace (Solder Mask)
4. Thickness of the PCB trace (T)
5. Dielectric Constant of the insulating material (Er)
6. Trace spacing for the differential signals
Impedance Calculation: Zo=[87/( Er +1.41)^1/2]*ln[5.98H/(0.8W+T)]
Before, proceeding with the PCB Layout, layout engineer has to talk with fabrication house and get the stack up details, so that layout engineer enter the stack up details into his layout and enter all the required constraints.
Relation between various PCB parameters:
1. Dielectric constant reduces as the frequency increases. This is due to the resin material properties used.
2. As the trace width is reduced, characteristic impedance is reduced.
3. Current carrying capability of trace depends on trace thickness and temperature rise.
4. As thickness of insulating material increases, characteristic impedance increases.
Tools used:
- Si9000 Transmission Line Field Solver from Polar Instruments
- Saturn PCB Toolkit
- Online Calculator - https://www.eeweb.com/tools/microstrip-impedance 4. online Calculator - http://www.hughescircuits.com/index.php/en/support/impedance-calculator
- Online Calculator - https://www.multi-circuit-boards.eu/en/pcb-design-aid/impedance-calculation.html
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