How to calculate impedance value before PCB fabrication
It’s very important how to calculate impedance value before PCB fabrication.
1.What is impedance?
Impedance is the total resistance of alternating current produced at known frequency.it’s resistance sum of signal layer and reference plane.
2. Impedance Type
Characteristic impedance is the ratio of voltage and current of a single wave propagating along the line.Signal produces reflection in the impedance discontinuity if characteristic impedance of transmission path changes when signal is in the transmission process.
2.2 Differential impedance
Trace carries the opposite polarity of two same signal waveform transmission by individual,subtraction the two differential signal at receiver. Differential impedance is the impedance between the two lines.
2.3 Even mode impedance
Even mode impedance is the impedance of one line when the pair is driven commonly.
3. The factors affect impedance
W—Trace width/space. The impedance decrease when trace width increase, The impedance increases when the trace space.
H1—Insulation thickness. The impedance increase when insulation thickness increase
T—Copper thickness.The impedance decrease when copper thickness increase
C1—Solder mask thickness.The impedance decrease when solder mask thickness increase
Er—Dielectric constant. The impedance decrease when dielectric constant increase
Undercut—W1-W2. The impedance increase when undercut increase.
4. Impedance calculate equation
Zo=[87/( Er +1.41)1/2]*ln[5.98H/(0.8W+T)]
For example: Calculate impedance 50 ohm for 4 layer board :
Er=4.2 H=7.3mil T=1.9mil
Input equation：50=[87/( 4.2 +1.41)1/2]*ln[5.98×7.3/(0.8W+1.9)]