6 Layer PCB Impedance Control
In the production process of PCB circuit board, it has to go through such steps as copper sink, electroplating tin (or chemical plating, or hot spraying tin/HASL), soldering of connectors and the materials used in this link must have a low resistivity to ensure that the overall impedance of PCB is low enough to meet the product quality requirements, otherwise the PCB circuit board will not operate normally.
In addition, in the whole electronics industry, PCB circuit boards in tin-plating link is one of the most vulnerable, is a key link in the process of impact impedance, because in the PCB tin link, has been popular using chemical to attain the goal of tin plating technology, for the electronics industry, according to industry research, chemical tin plating layer is the most fatal weakness is easy to change color (both easy oxidation or deliquescence), poor solderability leads to difficult soldering, high impedance leads to poor electrical conductivity or instability of the whole PCB performance and easy to long tin whisker cause the PCB short circuit and burned or fire events....
Many products(the PCB board bottom or electronic products overall) performance is poor, and the main cause of the poor performance is due to impedance problems, because when unqualified chemical tin plating technology is used, the tin for PCB circuit board plating is not really pure tin(or pure metal elemental), but the tin compounds (i.e. no metal simple substance, but metal compounds, oxide or halide, more directly, the nonmetal material) or a mixture of tin compounds belong to the elemental Sikkim, but with the naked eye is hard to find....
Because again, PCB circuit board copper foil is the body of circuit line,tin plating layer is on the copper foil soldering pins, and electronic component is welding on tin coating through the solder paste (or solder wire), As a matter of fact, solder paste welding between the electronic component and tin coating in the melting state is metal tin (i.e. metal simple with good electrical conductivity). Therefore, it can be pointed out briefly that electronic components are connected with copper foil at the bottom of PCB board through tin plating, so the purity of tin coating and its impedance are the key.In addition, before the electronic component is inserted, when we directly use the instrument to detect the impedance, the two ends of the instrument probe (or the meter pen) are also connected with the current by touching the tin coating on the copper foil surface on the bottom of PCB plate first.Therefore, tin coating is the key to influence the impedance and PCB performance, and is also the key to be easily neglected.
Again, as we all know, except for metal elemental, the tin compounds is a poor conductor of electricity or even not conductive (this is the key for circuit line in distribution capacity or spreading capacity), so this kind of tin compounds or mixtures of conductive not conductive exist in the tin plating, the ready-made or future resistivity after electrolytic reaction by oxidation, be affected with damp is high and corresponding impedance is high(enough to influence the level or signal transmission of digital circuit) and the characteristic impedance is not consistent.Therefore, the performance of the circuit board and the whole machine will be affected.
So, for the present phenomenon of social production, coating material on the bottom of the PCB and performance is the main factor affecting the characteristic impedance PCB whole board and the most direct reason, but since it has with the variability of the electrolytic coating aging and be affected with damp, so the impedance of suffering influence become more hidden and changeability, the main reason for its hidden: first by the naked eye can't see(including its change), the second cannot be invariant measure, because it has change with time and environmental humidity and variability, so always is easy to be ignored. Or passing on the cause.
Therefore, after knowing the cause of high impedance, solving the plating problem is the key to the impedance problem.