The Influence and Solution of PCB Circuit Board Processing on Impedance Control
View：3093 Release Date：2023/6/8 19:14:23
1.Surface microstrip line and characteristic impedance The characteristic impedance value of the surface microstrip line is high and widely used in practice.Its outer layer is the signal line surface of the control impedance,which is separated from the adjacent reference plane by insulating materials.The calculation formula of the characteristic impedance is:
a.Microstrip
Z={87/[sqrt(Er+1.41)]}ln[5.98H/(0.8W+T)],where W is the line width,T is the copper sheet thickness of the wiring,H is the distance from the wiring to the reference plane,and Er is the dielectric constant of the PCB board material.This formula must be within 0.1<(W/H)<2.0 and 1<(Er)
b.Stripline
Z=[60/sqrt(Er)]ln{4H/[0.67π(0.8W+T)]},where H is the distance between two reference planes,and the routing is located in the middle of the two reference planes.This formula must be in W/H<0.35 and T/H
It can be seen from the formula that the main factors affecting the characteristic impedance are(1)the dielectric constant Er,(2)the dielectric thickness H,(3)the wire width W,and(4)the thickness T of the wire copper.Therefore,it can be seen that the characteristic impedance is closely related to the substrate material(copper clad plate),so the selection of substrate material is very important in PCB design.
2.Dielectric constant of materials and its influence
The dielectric constant of a material is determined by the manufacturer's measurement at a frequency of 1Mhz,and the same material produced by different manufacturers varies due to their different resin content.This study takes epoxy glass cloth as an example to study the relationship between dielectric constant and frequency variation.The dielectric constant decreases with increasing frequency,so in practical applications,the dielectric constant of the material should be determined based on the working frequency.Generally,the average value can meet the requirements.The signal transmission speed in the dielectric material will decrease as the dielectric constant increases.Therefore,in order to achieve high signal transmission speed,the dielectric constant of the material must be reduced,To achieve high transmission speed,it is necessary to use high characteristic resistance values,and high characteristic resistance values must use low dielectric constant materials.
3.The influence of wire width and thickness
The wire width is one of the main parameters that affect the change of characteristic impedance.The figure takes the surface microstrip line as an example to illustrate the relationship between the impedance value and the wire width.From the figure,it can be seen that when the wire width changes by 0.025mm,it will cause a corresponding change in impedance value of 5-6 ohms.However,in actual production,if the signal line surface used to control impedance is 18 ohmsμM copper foil,the allowable tolerance for wire width variation is±0.015mm,if the tolerance for controlling impedance variation is 35μThe allowable tolerance for wire width variation in m copper foil is 0.025mm.It can be seen that the allowable wire width variation in production can cause significant changes in impedance values.The width of the wire is determined by the designer based on various design requirements,which must meet the requirements of the wire