PCB Technical - Understand the design principles of PCBA boards

The basic process can be divided into three steps: PCB schematic design, netlist generation, and printed circuit board design. Whether it is the layout of the PCB components on the board or the PCB traces, etc., there are specific requirements.

For example, the input and output wiring should be avoided as much as possible to avoid interference. The parallel routing of the two signal lines must be separated by the ground wire, and the wiring of two adjacent layers should be perpendicular to each other as much as possible. Parasitic coupling is likely to occur in parallel. The power and ground wires should be separated into two layers as far as possible to be perpendicular to each other. In terms of line width, a wide ground wire can be used as a loop for the digital circuit PCB to form a ground network (analog circuits cannot be used in this way), and a large area of copper is used.

The following article explains the principles and some details that need to be paid attention to in the PCB design of the microcontroller control board.

1. PCB component layout

In the layout of the components, the related components should be placed as close as possible. For example, the clock generator, crystal oscillator, and the clock input of the CPU are all prone to noise, so they should be placed closer.

For those devices that are prone to noise, PCB current circuits, high current circuit switching circuits, etc., keep them away from the logic control circuit and storage circuit (ROM, RAM) of the microcontroller as much as possible. If possible, these circuits can be made into circuits. Board, this is conducive to anti-interference and improve the reliability of circuit work.

2. Decoupling capacitor

Try to install decoupling capacitors next to key components, such as ROM, RAM, and other chips. In fact, printed circuit board traces, pin connections and wiring, etc. may contain large inductance effects. Large inductance may cause severe switching noise spikes on the Vcc trace.

The only way to prevent switching noise spikes on Vcc traces is to place a 0.1uF electronic decoupling capacitor between VCC and power ground. If surface mount components are used on the circuit board, chip capacitors can be used directly against the components and fixed on the Vcc pin.

It is best to use ceramic capacitors, because this type of capacitor has lower electrostatic loss (ESL) and high-frequency impedance, and the temperature and time of the dielectric stability of this type of capacitor are also very good. Try not to use tantalum capacitors, because their impedance is higher at high frequencies.

Pay attention to the following points when placing decoupling capacitors:

3. PCB ground wire design

In the PCB single-chip microcomputer control system, there are many types of ground wires, such as system ground, shield ground, logic ground, analog ground, etc. The reasonable layout of the ground wire will determine the anti-interference ability of the circuit board. When designing the ground wire and grounding point of the PCB, the following issues should be considered:

4. Other

In addition to the layout of the power line, the width of the trace should be as thick as possible according to the size of the current. In the PCB layout design, the routing direction of the power line and the ground line should be consistent with that of the data line. Finally, use ground wires to cover the bottom of the circuit board where there are no traces. These methods all help to enhance the anti-interference ability of the circuit.

The width of the data line should be as wide as possible to reduce impedance. The width of the data line is at least not less than 0.3mm (12mil), and it is more ideal if 0.46~0.5mm (18mil~20mil) is used.

Since a via on the circuit board will bring about 10pF of capacitance effect, this will introduce too much interference for high frequency PCB circuits, so when designing the PCB layout, the number of vias should be reduced as much as possible. Furthermore, too many vias will also reduce the mechanical strength of the circuit board.