PCB layout and design specification for PCB design

- Jan 14, 2020-

PCB layout and design specification for PCB design


We all know that "no rules, no circles", and the same in technology. What specifications should we pay attention to when designing PCB?


Layout design specification


A. the distance from the plate edge shall be greater than 5mm = 197mil


B. first place the components closely related to the structure, such as connectors, switches, power sockets, etc


C. priority should be given to the core components and larger components of the circuit function block, and then the surrounding circuit components should be placed with the core components as the center


D. place the components with high power in the position that is conducive to heat dissipation


E. components with high quality shall not be placed in the center of the board, and shall be placed close to the fixed edge in the case


F. components with high-frequency connection shall be as close as possible to reduce the distribution of high-frequency signal and electromagnetic interference


G. keep the input and output components as far away as possible


H. components with high voltage shall be placed in places not easily touched by hands during commissioning


I. the thermal element shall be far away from the heating element


J. the layout of adjustable elements shall be easy to adjust


K. consider the signal flow direction and arrange the layout reasonably to keep the signal flow direction as consistent as possible


L. the layout shall be uniform, neat and compact


M. for SMT components, the direction of pad shall be consistent as much as possible to facilitate assembly and welding and reduce the possibility of bridging


N. the decoupling capacitance shall be near the power input end


O. the height of components on the wave soldering surface is limited to 4mm


P. for PCB with double-sided components, larger and denser IC, plug-in components are placed on the top layer of the board, while the bottom layer can only place smaller components and fewer pins and loosely arranged chip components


Q. it is particularly important to add radiator to small size and high heat components. Under high-power components, copper can be laid to dissipate heat, and try not to put heat sensitive components around these components


R. high speed components shall be close to the connector as far as possible; digital circuit and analog circuit shall be separated as far as possible, preferably separated by ground, and then single point grounded


S. the distance from the positioning hole to the nearby pad shall not be less than 7.62mm (300mil), and the distance from the positioning hole to the edge of the surface mount device shall not be less than 5.08mm (200mil)


Wiring design specification


A. sharp angle and right angle shall be avoided for the line, and 45 degree routing shall be adopted


B. signal lines of adjacent layers are orthogonal


C. The high frequency signal shall be as short as possible


D. the input and output signals shall be kept away from the adjacent parallel lines as far as possible. It is better to add ground wire between lines to prevent feedback coupling


E. the direction of power line and ground wire of double-sided board should be consistent with the data flow direction to enhance the anti noise ability


F. digital ground and analog ground shall be separated. The line width of clock line and high frequency signal line shall be considered according to the requirements of characteristic impedance to achieve impedance matching


G. the wiring of the whole circuit board shall be evenly punched


H. separate power supply layer and stratum, power line and ground wire shall be as short and thick as possible, and the loop formed by power and ground shall be as small as possible


I. the wiring of clock shall be less perforated to avoid parallel wiring with other signal lines as much as possible, and shall be far away from general signal lines to avoid interference with signal lines; meanwhile, the power part on the Board shall be avoided to prevent mutual interference between power supply and clock; when there are multiple clocks of different frequencies on a circuit board, two clock lines of different frequencies shall not be parallel wiring; the clock line shall not be connected Near output interface to prevent high-frequency clock from coupling to the output cable and transmitting out; if there is a special clock generator chip on the board, it is not allowed to run the wire under it, and copper shall be laid under it, and special cutting shall be made if necessary


J. generally, the paired differential signal lines run in parallel, with as few holes as possible. When holes must be punched, the two lines should be punched together to achieve impedance matching


K. when the distance between the two welding points is very small, the welding points shall not be directly connected; the through hole from the pad shall be as far away from the pad as possible

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