The common impedance term for PCB is "characteristic impedance".
For example, take the PCB board to upload the characteristic impedance of the transmission line: usually if the same PCB interconnect line characteristic impedance everywhere to maintain consistency, so the PCB board to upload the transmission line has become a high quality. What kind of circuit board is called the controlled impedance of the circuit board? The controlled impedance circuit board is the characteristic impedance of all transmission lines on the PCB board to conform to the uniform target specification, generally refers to the value of the characteristic impedance of all transmission lines between 25 and 70.

What are the main reasons for the high impedance of the PCB circuit board? Or PCB circuit board itself can not see the quality of the problem but with the longer the problem (including impedance), or the main reason for the performance is not stable?
Please look at the following professional academic analysis:
Impedance is actually refers to the resistance and reactance parameters, because the PCB circuit board (bottom) to consider the plug mounting electronic components, after insertion and consider conductive performance and signal transmission performance, so the inevitable requirement of impedance is lower better, the resistivity is lower than per square cm 1 x 10 negative 6 times square below.
On the other hand, PCB circuit board in the production process to experience heavy copper and electrical tin (or chemical plating, or thermal spray tin), connectors, soldering process and the part of the material must have a guarantee that the bottom of the resistivity, in order to ensure the overall impedance of the circuit board to low even to meet the requirements of the quality of the products, or the circuit board will can not operate normally.
Also in the electronics industry as a whole, PCB circuit board in tin link is the most problematic, is a key link in the impedance of, because the PCB tin link, it is now popular use of electroless tin plating technology to realize the tin to, but we as the electronics industry benefit people, observation and contact of electrical or electronic processing industry for more than 10 years, throughout the country can do without many of the chemical tinning (for PCB or electronic tin field) of enterprises, because of electroless tin plating process in China is a rising star and all the enterprises of the technical level uneven missing...

For the electronics industry, according to a nationwide survey line, chemical tin coating is the most fatal weakness is changeful color (which are easy to oxidation and deliquescence), brazing difference lead to difficult to weld, impedance high lead to poor conductive property or instability of the whole board performance, easy to be Changxi lead to PCB circuit short circuit even burned or fire events...
It is reported that the first study of electroless tin when is in the 1990s at the beginning of Kunming University of science and technology, is at the end of the 1990s, Guangzhou with modest chemical (enterprise). Until now, 10 years, were recognized the two institutions is doing the best. Among them, according to our access to many enterprises screening survey, experimental observation and long-term endurance tests confirmed with modest chemical tin plating layer is pure tin of low resistivity layer, conductive and brazing quality can guarantee to a higher level, it is no wonder that they dare to foreign guarantee deposit in without the need of any closed and prevent discoloration protective agent, can maintain a year no color, no blistering, peeling, permanent long tin whisker.

Later, when the whole social production industry to a certain extent, many later participants are often copying each other, in fact, a considerable part of the enterprise itself does not develop or creative ability, so that products and electronic products caused many users (circuit board or electronic products) poor performance caused by the main reason poor performance is because of the impedance problem, because when the unqualified chemical tin plating technology in the process of using, the PCB circuit board plating up tin are not really pure tin (or pure metal compounds, but the single quality) tin (i.e. no metal oxide or metal compounds, but. Halide, speak more directly to the non metal material) or a mixture of compounds with tin metal, but with the naked eye is difficult to find Of.......
Because of the main line, PCB circuit board is copper foil, the copper foil is solder tin layer, and the electronic component is through the solder paste (or solder wire) welding in the tin layer above, in fact in the melting state of solder paste welding between electronic components and metal tin tin coating is conductive (i.e. good, so can metal) briefly pointed out that the electronic component is connected with the PCB plate at the end of the copper foil by tin plating, tin plating and so the purity of the impedance is the key; again, but not before inserting electronic components, we directly use the instrument to detect the impedance, in fact, the instrument probe (or as the pen) is through the first contact ends of tin coating copper surface PCB board and PCB board and the bottom of the bottom of the copper foil to the current connection. So tin coating is the key, is the key to affect the impedance of the PCB and the key to the performance of the entire board, but also easy to be ignored key.

And, as is known to all, in addition to the elemental metal, the compounds are poor conductors of electricity or even non conducting (also, which is caused by wire distribution capacity or spreading capacity of key), so tin coating in the like conducting and non conducting tin compounds or mixtures when, the off the shelf resistivity or future oxidation, damp the electrolytic reaction after the resistivity and the corresponding impedance is very high (foot has affected the digital circuit level or a signal transmission) and the characteristic impedance is not consistent. So it can affect the performance of the circuit board and the whole machine.
So, the current phenomenon of social production, PCB board on the bottom of the coating material and performance is the main reason for the influence of PCB whole board characteristic impedance and the most direct reason, but because it has with the coating aging and damp electrolytic changes, so suffering effects produced by the impedance becomes more recessive and variability, the main cause of the hidden is not be visible (including the change), the second can not be be constant measured, because of its change and change with the change of time and the humidity of the environment, it is always easy to be ignored. Or pass on the cause of the error.
So, after knowing the cause of the high impedance, solving the problem of coating is the key to the problem of impedance.