Principles for PCB Layout
Generally, basic PCB design process follows seven steps: pre-preparation- structural design- layout- optimization and silkscreen- net check, DRC check, and structural check- board fabrication. In this article, the third step- PCB layout will be outlined.
PCB layout is the major process when designing your printed circuit boards, which will directly affect the performance of the PCB board. In the PCB design process, PCB layout is generally divided into two realms: the first is to make lines running smoothly which is the most basic requirement. If fly wires are easily found, then this board is an unqualified board. Next is to meet electrical performance. This can evaluate whether a PCB board is qualified or not. Good layout can achieve the better electrical performance. PCB layout is mainly based on the following principles:
1. As a general rule, power cord and ground should be designed firstly to ensure the electrical performance of PCB board. Within the allowable condition, as far as possible to widen the power cord and ground, preferably ground is wider than the power cord. Usually the signal line width is: 0.2~0.3mm, the smallest width is up to 0.05~0.07mm, the power cord is generally 1.2 ~ 2.5mm.
2. Pre-design wires with more stringent requirement (such as high-frequency lines). Sine line of input and output should be avoided parallel to avoid reflection interference. If necessary, ground isolation should be added. Wirings on two adjacent layers should be parallel, otherwise parasitic coupling is prone to cause.
3. Ground the oscillator, and keep the clock line as short as possible.
Increase the area that under the clock oscillation circuit and the special high-speed logic circuit instead of using other signal lines.
4. As far as possible to design with 45℃ polyline instead of 90℃ polyline to reduce the high-frequency signal radiation.
5. Do not form a loop among any signal lines, if unavoidable, the loop should be as small as possible; the less through hole the better on the signal line.
6. The key lines are as short and thick as possible and are protected on both sides.
7. When transfer sensitive signals and noise field signals through the flat cable, use the way like ‘ground - signal – ground’.
8. Key signals should reserve test points for easy production and maintenance testing.