Printed circuit boards (PCB) usually in the glass epoxy substrate bonded to a layer of copper foil, copper foil thickness usually has 18 μm, 35 μm, 55 μ m and 70 μ M4 species. The most commonly used copper foil thickness is 35 μm. The copper foil used in the domestic thickness is generally 35~50 μ m, and there is more than this thin 10 μm, 18 μ m, and is thicker than this like 70 μ m. The thickness of the composite copper foil on the substrate of l~3mm is about 35 μ m; the thickness of the composite copper foil on the substrate is less than the lmm thickness of about 18 μ m,5mm on the thick substrate of the compound copper foil thickness of about 55 μm. If the PCB thickness of copper foil is 35 μ m, the printed line width 1mm, then each 10mm long, its resistance value of 5m ω around, its inductance is 4nH. When the PCB on the digital IC chip work DI/DT for 6mA/ns, the working current is 30mA, each 10mm long printed wiring contains the resistance value and inductance value to estimate the noise voltage generated by each part of the circuit is 0.15mV and 24mV respectively.