5. The design of PCB--design considerations and assembly rules
. PCB design considerations
Juvtmall ( a company supply PCB Prototyping, PCBA service and sell kinds of components, modules and so on)
(1) avoid setting important signal lines on the edge of PCB, such as clock and reset signal.
(2) the distance between the ground wire of the enclosure and the signal line is at least 4mm; keep the length and width of the ground wire to be less than 5:1 to reduce the inductance effect.
(3) lock the device and line that identified location with LOCK function, so that they will not be misused.
(4) the width of the conductor should not be less than 0.2mm (8mil). In high-density and high-precision printed circuit, the width and spacing of the wires are generally preferable to 12mil.
(5) Routing in the IC pin of DIP package, 10-10 and 12-12 principles can be applied, as between the two pins by 2 lines, diameter of bonding pad can be set to 50 mil, width and spacing of line is 10 mil, when only 1 line, diameter can be set to 64 mil, width and spacing is 12 mil.
(6) when the diameter of the bonding pad is 1.5mm, the length is not less than 1.5mm, and the width is 1.5mm and the long round welding disk in order to increase the tensile strength of the welding plate.
(7) when the tracing is thin, the connection between the welding disc and the tracing shall be designed as a “water droplet”, so the welding disc will not be easy to peel, and the thread and the welding disc will not be disconnected easily.
(8) when bonded copper in large area, the copper should with open window heat emission hole, and the opening window should be designed into a network.
(9) to minimize the connection between high frequency components and reduce their distribution parameters and electromagnetic interference between them. Vulnerable components cannot get too close to each other, and input and output elements should be kept away from each other.
In the design, from the assembly angle of the PCB board, the following parameters should be considered:
(1) the diameter of the hole shall be determined according to the maximum material conditions (MMC) and the minimum material conditions (LMC). The diameter of the hole of an unsupported component should be selected in such a way that the hole’s MMC subtract the pins MMC, the difference value between 0.15 and 0.5mm.
In addition, the inner diameter difference value between the diagonals of pin and the unsupported holes will not exceed 0.5 mm, and not less than 0.15mm.
(2) reasonably place smaller components so that they won't be covered by larger components.
(3) the thickness of the solder resist shall not be greater than 0.05mm.
(4) the printing mark of silk screen cannot meet any welding plate.
(5) the upper part of the circuit board should be the same as the lower half to achieve structural symmetry. Because an asymmetric circuit board may bend.