PCB Technical - The key to PCB wiring skills of PCB technology

I. PCB design steps

Generally, the most basic process of designing a circuit board can be divided into three main steps. (1). Circuit schematic design The circuit schematic is mainly designed and drawn by the PROTEL099 schematic design system (Advanced 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 Netlist Netlist is the bridge between circuit schematic design (SCH) and printed circuit board design (PCB). PCB is the automatic soul of the circuit board.

The netlist can be obtained from the schematic circuit diagram or extracted from the printed circuit board.

(3). Printed circuit board design

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

two. Draw a simple circuit diagram 2.1 Schematic diagram design process

Schematic design can be carried out according to the following steps. (1) Design drawing size Protel 99 / Schematic, first of all, we must conceive a good part drawing and design a good drawing size.

The drawing size is based on the size and complexity of the circuit diagram. Setting the appropriate drawing size is the first step in designing a schematic diagram.

(2) Setting the Protel 99 / Schematic design environment Setting the Protel 99 / Schematic design environment, including setting the lattice size and type, cursor type, etc., most parameters can also use the system default values.

(3) According to the needs of the circuit diagram, the user rotates the part, removes the part in the part library and places it on the drawing, and places the part serial number, and the part is packaged to define and set the workpiece.

(4) Use various tools provided by Protel 99 / Schematic for schematic wiring. The components in the diagram are connected with electrically significant wires and symbols to form a complete schematic diagram.

(5) Adjust the circuit. The circuit diagram will be initially drawn for further adjustment and modification to make the schematic diagram more beautiful.

(6) Report output Various reports are generated through various report tools provided by Protel 99 / Schematic, the most important of which is the netlist, which prepares the subsequent circuit board design through the netlist.

(7) File saving and printing output The last step is to save the file and print it out. The design principles of the microcontroller control board should follow the following principles: (1) In the PCB component layout, the related components should be as close as possible. For example, clock generators, crystal oscillators, and CPU clock inputs are all prone to noise, and should be placed more closely. Get close to them.

For those devices that are prone to noise, low-current circuits, high-current circuit switching circuits, etc., keep them away from monolithic logic control circuits and storage circuits (ROM, RAM), if possible, these circuits can be made into another circuit board, Conducive to anti-interference and improve the reliability of circuit work. (2) As far as possible, install decoupling capacitors in key components, such as ROM, RAM and other chips. In fact, printed circuit board wiring, pin wiring and wiring, etc. may contain large inductance effects. Large inductors may cause severe switching noise spikes on the VCC line. The only way to prevent switching noise peaks on the VCC line is to place a 0.1uF electronic decoupling capacitor between VCC and the power supply. If you use surface mount components on the circuit board, you can fix them on the VCC pin and use chip capacitors that are directly attached to the components. Due to the low electrostatic loss (ESL) and high frequency impedance of the capacitor, as well as the medium stability of the capacitor temperature and time, it is best to use a ceramic capacitor. Try not to use tantalum capacitors, because they have higher impedance at high frequencies.

Pay attention to the following points when placing decoupling capacitors:

At the power input end of the printed circuit board, the electrolytic capacitor is connected to about 100uF, and it is better if the volume allows for a larger capacitance.

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

For components with weak anti-interference ability, the current changes greatly during the turn-off period. For storage components such as RAM and ROM, decoupling capacitors should be connected between the power line (VCC) and the ground line. The lead of the capacitor should not be too long, especially the high frequency bypass capacitor cannot be charged. (3) In the single-chip microcomputer control system, there are many types of ground wires, systems, shielding, logic, analog, etc. Whether the ground wire layout is reasonable or not will determine the anti-interference ability of the circuit board.

When designing the ground wire and pick-up location, the following issues should be considered: logic and analog are wired separately and cannot be combined, and their respective ground wires are connected to the corresponding power ground. In the 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.

Generally speaking, for the input and output of analog signals, it is best to isolate the circuit with the single-chip microcomputer through an optocoupler.

When designing the printed circuit version of the logic circuit, the ground wire should form a closed-loop format to improve the circuit's anti-interference ability. The ground wire should be as thick as possible. If the ground wire is very thin, the ground resistance will be large, which will cause the ground potential to change with the current, resulting in unstable signal level and a decrease in the anti-interference ability of the circuit.

When the PCB wiring space allows, in order to ensure that the width of the main ground wire is at least 2~3mm, the ground wire on the component pin should be about 1.5mm. Pay attention to choosing a pick-up location. When the signal frequency on the circuit board is less than 1MHz, because the electromagnetic influence between the wiring and the components is small, and the loop formed by the grounding circuit has a greater impact on the interference, it is necessary to use a point of grounding so that it does not form a loop. When the signal frequency on the circuit board is higher than 10MHz, the ground impedance becomes very large due to the obvious inductance effect of the wiring. At this time, the formation of the ground circuit is no longer a major problem.

Therefore, multi-point grounding should be used to minimize ground impedance.

In addition to the size of the current, the layout of the power line should be as wide as possible. The power line should also be used when wiring, the direction of the wire and the data line in the wiring. Finally, use the ground wire to connect the bottom of the circuit board without wiring. These methods are To help strengthen 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. Because the perforation of the circuit board will bring about 10pF capacitance effect, which will cause too much interference to the high-frequency circuit, so when wiring, the number of holes should be minimized. In any other case, too many holes will also reduce the mechanical strength of the circuit board.