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The Significance of Impedance for PCB

Impedance - in fact, refers to the resistance and reactance parameters, because the PCB circuit (board bottom) to consider the installation of electronic components patch, after the plug consider the conductivity and signal transmission performance issues, it is inevitable that the lower the impedance the better, Resistivity to less than 1 × 10-6 per square centimeter below.
On the other hand, PCB circuit board in the production process to go through copper, electroplating tin (or electroless plating, or hot spray tin), solder joints and other links and materials used in the link must ensure that the resistivity of the bottom, to ensure the overall impedance of the board is low enough to meet product quality requirements or the board will not function properly.
In addition, the electronic industry as a whole, PCB circuit board in the tin is the most prone to problems, is a key part of the impedance, because the circuit board tin parts, is now the popular use of electroless tin plating technology to achieve the purpose of tin.
For the electronics industry, according to an in-industry survey, the most lethal weakness of electroless tin plating is its easy discoloration (easy oxidation or deliquescence), poor solderability resulting in poor soldering, high impedance leading to poor conductivity or poor overall plate performance ...
PCB circuit board is the main circuit of copper foil, solder joints in the copper is the tin layer, and the electronic components is through the solder paste (or solder wire) soldered on the tin layer above, in fact, solder paste in the molten state Soldered between the electronic components and the tin plating is metallic tin (ie, a metal of good electrical conductivity), so it can be briefly stated that the electronic components are connected to the copper foil on the bottom of the PCB through the tin plating. 
Therefore, the Purity and its impedance is the key; but, before plugging the electronic components, we directly use the instrument to detect the impedance, in fact, the instrument probe (or call the pen) at both ends is through the first contact with the bottom surface of the copper foil tin plating and then with the bottom of the copper PCB to connect the current. So the tin coating is the key, is the key to influence the impedance and affect the performance of PCB board the key, but also easy to overlook the key.
It is a well-known fact that except for the simplex of metals, the compounds are all poor conductors of electricity or even non-conductive (again, this is also the key to the distribution capacity or distribution capacity in the circuit) so that the tin- of the tin compound or mixture, the resistivity and its corresponding impedance after the existing or subsequent oxidation, moisture electrolysis reaction occurs is quite high (which has affected the level of digital circuits or signal transmission) and the characteristic impedance is not consistent either. So it will affect the performance of the circuit board and its whole machine.
Therefore, for the current phenomenon of social production, the material and properties of the plating on the bottom of the PCB are the most important and the most direct cause that affect the characteristic impedance of the PCB. However, due to the aging of the plating and the electrolytic corrosion variability, so the impact of their resistance becomes more hidden and variability, the main reason for its covert is: the first can not be seen by the naked eye (including its changes), the second can not be measured regularly because it has varies with time and changes in ambient humidity, so it is always easy to overlook.

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