Impedance controlled PCB's are used more and more popular in our common life: Synchronous digital hierarchy (SDH), GSM, CDMA, PC and high power wireless phone etc.
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, desinged 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
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.