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Analysis of circuit board layout design steps

Release Date:2020-03-25

1. The circuit board layout design steps

Generally speaking, the most basic process of layout circuit board design can be divided into three major steps.


(1) Design of circuit schematic

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


(2) Generate a netlist

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


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

2. Draw a simple circuit diagram

2.1 Schematic Design Process

The design of the schematic diagram can be completed according to the following process.


(1) Design drawing size: After Protel 99 / Schematic, we must first conceive the part drawing and design the drawing size. The drawing size is determined by the scale and complexity of the circuit diagram. Setting the appropriate drawing size is the first step in designing a good schematic.
(2) Set the Protel 99 / Schematic design environment: Set the Protel 99 / Schematic design environment, including setting grid size and type, cursor type, etc. Most of the parameters can also use the system default values.
(3) Rotating parts: According to the needs of the circuit diagram, the user takes the parts out of the parts library and places them on the drawing, and defines and sets the serial number of the placed parts and the package of the parts.
(4) Schematic wiring: Use various tools provided by Protel 99 / Schematic to connect the components on the drawing with wires and symbols with electrical meaning to form a complete schematic.
(5) Adjust the circuit: The preliminary drawn circuit diagram is further adjusted and modified 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 report is the network table, which is used to prepare for subsequent circuit board design.
(7) File saving and printout: The last step is file saving and printout.

2.2 The layout design of the microcontroller control board need to follow these principles:

(1) In the layout of components, the related components should be placed as close as possible. For example, clock generators, crystal oscillators, and CPU clock input terminals are prone to noise. They should be placed close when placed. For those devices that are prone to noise, small current circuits, high current circuit switch circuits, etc., they should be kept away from the logic control circuits and storage circuits (ROM, RAM) of the microcontroller as far as possible. If possible, these circuits can be made into separate circuits Board, which is conducive to anti-interference and improve the reliability of circuit work.


(2) Install decoupling capacitors next to key components, such as ROM and RAM chips. In fact, printed circuit board wiring, pin wiring, and wiring may all contain large inductance effects. Large inductances can cause severe switching noise spikes on Vcc traces. 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 circuit board, a chip capacitor can be directly pressed against the component and fixed on the Vcc pin. It is best to use ceramic capacitors because they have low electrostatic losses (ESL) and high-frequency impedance. In addition, the capacitors have good temperature and time dielectric stability. Try not to use tantalum capacitors, because its impedance is higher at high frequencies. Pay attention to the following points when placing the decoupling capacitors:


At the power input end of the printed circuit board, connect an electrolytic capacitor of about 100uF. If the volume allows, a larger capacitance is 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 fit, a 1 to 10 tantalum capacitor can be placed around every 10 chips.
For components with weak anti-interference ability and large current change during shutdown, and storage components such as RAM and ROM, a decoupling capacitor 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 should not have lead.


(3) In the single-chip microcomputer control system, there are many types of ground wires, such as systematic ground, shielded ground, logical ground, and analog ground. Whether the ground wire is properly laid out will determine the anti-interference ability of the circuit board. When designing the ground and ground points, the following issues should be considered:


Logical ground and analog ground must be routed separately and cannot be used together. Connect their respective ground wires to the corresponding power ground wires. When designing, the analog ground wire should be as thick as possible, and the ground area of the lead-out terminal should be enlarged as much as possible. Generally speaking, for input and output analog signals, it is best to isolate them from the microcontroller circuit by optocouplers.

When designing the printed circuit board of the logic circuit, its ground wire 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 resistance of the ground wire will be large, causing the ground potential to change with the change of the current, causing the signal level to be unstable and the anti-interference ability of the circuit to be reduced. When the wiring space allows, the width of the main ground wire must be at least 2 ~ 3mm, and the ground wire on the component pins should be about 1.5mm.


Layout design should pay attention to the choice of ground point. When the signal frequency of the circuit board is lower than 1MHz, the influence of electromagnetic induction between the wiring and the components is small, and the circulating current formed by the ground circuit has a large impact on the interference, so a little ground should be adopted so that it does not form a loop. When the signal frequency of the circuit board is higher than 10MHz, the impedance of the ground wire becomes very large due to the obvious inductance effect of the wiring. At this time, the circulating current formed by the ground circuit is no longer a major problem. Therefore, multi-point grounding should be used to minimize the impedance of the ground wire.


In addition to the layout of the power line, the wiring width should be as thick as possible according to the size of the current. When wiring, the power line and ground should be aligned with the data line. At the end of the wiring work, use the ground wire. Spreading the bottom layer 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 the impedance. The width of the data line is at least 0.3 mm (12 mil), and it is more ideal if 0.46 to 0.5 mm (18 mil to 20 mil) is used.
Because a via of the circuit board will bring about a capacitance effect of about 10 pF, which will introduce too much interference for high-frequency circuits, so the number of vias should be minimized when wiring. Furthermore, too many vias will also cause the mechanical strength of the circuit board to decrease.
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