In simple terms, the impedance of a PCB track can be considered its AC "resistance" to ground, which is a function of DC resistance and AC inductance and capacitance. This means that although a track may exhibit a DC resistance to ground of many megohms its impedance will be much lower - typically 50 to 100 Ohms.
Once considered a black art it is now a common design issue of many printed circuit boards.
Chemitalic's product range also includes PCB's with controlled impedance and we offer TDR measurements to ensure that specified requirements are fulfilled.
Chemitalic does not offer design of PCB's for controlled impedance - but below is a number of links which you may find useful in understanding, calculating and verifying your designs for controlled impedance.
