Without impedance control, significant signal reflections and signal distortions will occur, resulting in design failure. Impedance control is required for common signals, such as PCI bus, PCI-E bus, USB, Ethernet, DDR memory, LVDS signals, etc. Impedance control eventually needs to be achieved by PCB design and a higher requirement for PCB board process is also put forward too. So we need to control the impedance of wiring as per the signal integrity requirement.
Definition of characteristic impedance
At a certain frequency, relative to a reference layer, the resistance of its high-frequency signals or electromagnetic waves in the process of transmission is called the characteristic impedance, which is the vector summation of electrical impedance, inductive reactance, capacitance resistance.
Basic elements that affect impedance
1) Thickness of trace (T1) 7%
2) Width of trace (W1) 4%
3) Height of laminate (H1) 35%
4) Dielectric constant of board material (Er1) 8%
5) Other factors can be involved, however they have much less affect on the impedance such solder resist, thickness of tin and lead, PCB humidity etc
Main factors affected impedance control (normally ±10%)
1) Generally, designed width of trace is already fixed, manufacturing procedure is controlled within ±15-20%
2) Generally, the thickness of trace is already fixed at designing period, manufacturing procedure of electroplate is controlled within ±0.8-1.2mil
3) The dielectric constant of the substrate is almost fixed, no much big changes.
4) The above-mentioned factors can be controlled stably, the main affecting factor of impedance control is only left height of laminate.
Main factors to the height of laminate
1) Thickness of inner layer copper
2) Residual rate of inner layer copper
3) Use which kind of prepreg
The classification of characteristic impedance
Common characteristic impedance is divided into: single ended (line) impedance, differential impedance, coplanar impedance, etc.
Single ended impedance: refers to the measured impedance of a single signal line.
Differential impedance: refers to the impedance measured between the two transmission lines with equivalent width and spacing in differential drive.
Coplanar impedance: refers to the impedance measured when the signal line is transmitting between its surrounding GND / VCC (the space between the signal line to GND / VCC on both sides is equal).
Testing the PCB
PCB testing is normally done on one test coupon integrated into the PCB panel, not on the PCB itself. Coupons are designed for specific time domain reflectometer.