PCB Fabrication

Basic technology of laminated design in high-speed PCB

by:A-TECH      2021-03-17
With the improvement of the current system work rate, the switching time of high-speed PCB laminate design devices has become smaller and smaller, and the design complexity of PCB has gradually increased. The analysis of signal integrity is in addition to reflection, string burning and EMI. Etc., reasonable stacking design and stable and reliable power supply are also becoming more and more important.   One, the structure of the ply The structure of the ply is a very important factor that determines the emc performance of the system. A good layer structure has a good effect on suppressing radiation in PCB. In the current common high-speed circuit systems, multi-layer boards are mostly used instead of single-sided and double-sided boards. Pay attention to the following aspects when designing multiple panels. 1. A signal layer should be adjacent to a copper layer; 2. Power copper and ground copper should be tightly coupled; 3. Multiple ground copper layers can effectively reduce the impedance of the PCB board and reduce the common mode emi . 4. The high-speed signal in the system should be in the inner layer and between the two copper coatings, so that the two copper coatings can provide shielding for these high-speed signals and limit the radiation of these signals to the two copper-clad areas;    2. Board The parameters of the layer The parameters of the layer include the line width of the signal trace, the line thickness, the medium between the signal layer and the copper layer, and the thickness of the medium. The determination of the board layer parameters mainly takes into account the impedance control of the signal and the manufacturing process limitations of the pcb board. Of course, at frequencies above ghz, the skin effect of the transmission line and the loss of the medium need to be considered. For the commonly used medium fr-4, when ≥1ghz, the medium has a significant attenuation of the signal. The impedance of the signal line is mainly limited by a number of parameter variables, which can be simply described by the following formula. Among them: z. Is the impedance of the signal line; w: is the line width of the trace; h: the line height of the trace; h: the thickness of the medium; ε: the dielectric constant of the medium. Among these parameter variables, h has the greatest influence. Usually you can use polar cit25 software to calculate the impedance of the transmission line. Different transmission line types (microstrip line and strip line, etc.) also require some differences in the calculation parameters.   Third, the design of the power (ground) layer Before studying the design of the power layer, you should also know the problem of high-frequency signal return. The principle of high-frequency signal return is actually to return to the driving end of the signal along the path with the least impedance. Of course, the return of the signal is completely different when the waveform of the signal is switched. If the signal output is logic high, then the return of the signal must enter the power pin of the drive end. The opposite is true. There needs to be low inductance and high capacitance between the signal transmission line and the return path. The lower inductance is to reduce the magnetic flux passing through; the higher the capacitance is to better contain the electric field.
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