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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

2.1Characteristic impedance

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.

Characteristic impedance                              

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.

differential impedance

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

impedance calculation

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 :

 

L1(TOPLAYER)                             1.9mil

                                 1080+2116 7.3mil

       L2(GND)                                 1.2mil

                                 Core 44.5mil

       L3(VCC)                                 1.2mil

                                 1080+2116   7.3mil

L4(BOTTOMLAYER)                      1.9mil

 

Board thickness:65*0.0254=1.6mm

 

Er=4.2       H=7.3mil   T=1.9mil

 

Ln =2.71828

 

Input equation:50=[87/( 4.2 +1.41)1/2]*ln[5.98×7.3/(0.8W+1.9)]

 

Result:W=11.6

 

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