PCB layout technology

1. Board design steps

(1). Design of circuit schematic

The circuit schematic design is mainly based on PROTEL099's schematic design system (Advanced Schematic) to draw a circuit schematic. In this process, we should make full use of the various schematic drawing tools and various editing functions provided by PROTEL99 to achieve our goal, that is, to obtain a correct and exquisite circuit schematic.

(2). Generate a network table

The netlist is a bridge between the schematic design (SCH) and the printed circuit board design (PCB), which is the soul of the board automatically. The netlist can be obtained from the schematic or extracted from the printed circuit board.

(3). Printed circuit board design

The design of the printed circuit board is mainly for the other important part of the PCB of PROTEL99. In this process, we use the powerful functions provided by PROTEL99 to realize the layout design of the board and complete the difficult work.

2. Drawing a simple circuit diagram

2.1 Schematic design process

The design of the schematic can be done as follows.

(1) Design drawing size After Protel 99/ Schematic, first plan the part drawing and design the drawing size. The size of the drawing is based on the size and complexity of the circuit diagram. Setting the appropriate drawing size is the first step in designing the schematic.

(2) Set Protel 99/Schematic design environment Set Protel 99/Schematic design environment, including setting grid size and type, cursor type, etc. Most parameters can also use system default values.

(3) Rotating parts According to the needs of the circuit diagram, the user takes out the parts from the parts library and places them on the drawing, and defines and sets the part number and part package of the placed parts.

(4) Schematic wiring With the various tools provided by Protel 99/Schematic, the components on the drawing are connected by electrically conductive wires and symbols to form a complete schematic.

(5) Adjusting the circuit The preliminary drawing of the circuit diagram is further adjusted and modified to make the schematic diagram more beautiful.

(6) Report output Various reports are generated by various reporting tools provided by Protel 99/Schematic. The most important report is the network table, which prepares for subsequent board design through the network table.

(7) File saving and printout The final steps are file saving and printout.

The principles that must be followed in the design principles of the MCU control board are as follows:

(1) In terms of component layout, the related components should be placed as close as possible. For example, clock generators, crystal oscillators, and CPU clock inputs are prone to noise, and should be placed closer to them when placed. For those devices that are prone to noise, small current circuits, high-current circuit switching circuits, etc., try to keep them away from the logic control circuit and memory circuit (ROM, RAM) of the microcontroller, and if possible, make these circuits into circuits. The board is good for anti-interference and improves the reliability of the circuit operation.

(2) Try to install decoupling capacitors on critical components such as ROM and RAM. In fact, printed circuit board traces, pin connections, and wiring can all have large inductance effects. Large inductors can cause severe switching noise spikes on the Vcc trace. The only way to prevent switching noise spikes on the Vcc trace is to place a 0.1uF electronic decoupling capacitor between VCC and the power ground. If a surface mount component is used on the board, a chip capacitor can be used to directly abut the component and be fixed on the Vcc pin. It is best to use ceramic capacitors because of their low electrostatic loss (ESL) and high frequency impedance, and the stability of the capacitor temperature and time is also good. Try not to use tantalum capacitors because of its high impedance at high frequencies. Pay attention to the following points when placing the decoupling capacitor:

Connect an electrolytic capacitor of about 100uF at the power input end of the printed circuit board. If the volume allows, the larger the capacitance, the better.

In principle, a 0.01uF ceramic capacitor needs to be placed next to each integrated circuit chip. If the space of the circuit board is too small to be placed, a 1~10 tantalum capacitor can be placed around every 10 chips.

For components with weak anti-interference ability, large current change during shutdown, and memory components such as RAM and ROM, a decoupling capacitor should be connected between the power supply line (Vcc) and the ground.

The lead of the capacitor should not be too long, especially the high-frequency bypass capacitor can not be leaded.

(3) In the MCU control system, there are many types of ground wires, such as system ground, shield ground, logic ground, analog ground, etc. Whether the ground wire is properly laid out will determine the anti-interference ability of the circuit board. When designing ground and ground points, you should consider the following questions:

The logic and the analog ground are to be separately routed, and they cannot be used together, and their respective ground lines are respectively connected to the corresponding power ground lines. At the time of design, the analog ground wire should be as thick as possible, and the grounding area of the lead end should be increased as much as possible. In general, the analog signal for input and output is preferably isolated from the microcontroller circuit by an optocoupler.

When designing the printed circuit board of the logic circuit, the ground line should form a closed loop form to improve the anti-interference ability of the circuit.

The ground wire should be as thick as possible. If the ground wire is very thin, the ground wire resistance will be large, causing the ground potential to change with the change of the current, causing the signal level to be unstable, resulting in a decrease in the anti-interference ability of the circuit. In the case where the wiring space allows, ensure that the width of the main ground wire is at least 2 to 3 mm or more, and the ground wire on the component leads should be about 1.5 mm.

Pay attention to the choice of grounding point. When the signal frequency on the circuit board is lower than 1MHz, since the electromagnetic induction between the wiring and the component has little influence, and the circulating current formed by the grounding circuit has a great influence on the interference, a grounding is adopted so that the loop is not formed. When the signal frequency on the circuit board is higher than 10 MHz, the impedance of the ground wire becomes large due to the obvious inductance effect of the wiring, and the circulation formed by the ground circuit is no longer a major problem. Therefore, multi-point grounding should be used to minimize the ground impedance.

In addition to the width of the current line, the layout of the power line should be as thick as possible. In the wiring, the wiring direction of the power line and the ground line should be consistent with the line of the data line. At the end of the wiring work, the ground line should be used. Covering the bottom layer of the board without traces, these methods help to enhance the circuit's immunity to interference.

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

Since a via of the board causes a capacitive effect of about 10 pF, this will introduce too much interference for the high frequency circuit, so the number of vias should be reduced as much as possible during routing. Furthermore, too many vias can cause a decrease in the mechanical strength of the board.