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Analysis of the basic process and steps of PCB circuit board design

Posted: Tue Dec 10, 2019 7:16 pm
by liqian
Circuit board design steps

Generally speaking, the most basic process of designing a circuit board 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 netlist

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

(3). Design of printed circuit boards

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

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 parameters can also use system default values.

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

(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 netlist, which is used to prepare for subsequent circuit board design.

(7) File saving and printout
The last step is to file saving and printout.

The design principles of the microcontroller control board need to follow the following principles:

(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 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 much as possible. If possible, these circuits can be made into other circuits. Board, which is conducive to anti-interference and improves the reliability of circuit work.

(2) 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 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 this type of capacitor has low electrostatic losses (ESL) and high-frequency impedance. In addition, this type of capacitor has a 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:

Connect a 100uF electrolytic capacitor across the power supply input of the printed circuit board. 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 about 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 a lead.

(3) In the single-chip microcomputer control system, there are many types of ground wires, such as system ground, shield ground, logical ground, and analog ground. Whether the ground wire is laid out properly 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 the 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.

Pay attention to the choice of the ground point. When the frequency of the signal on 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 supply line, the width of the wiring should be as thick as possible. When wiring, the direction of the power supply line and the ground should be consistent with the data line. Covering the bottom layer of the circuit board with 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 mils), and it is more ideal if 0.46 to 0.5 mm (18 mils to 20 mils) is used.

Since 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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Re: Analysis of the basic process and steps of PCB circuit board design

Posted: Wed Dec 11, 2019 6:13 am
by Garth
I find a lot of problems with the post above. For starters, I would recommend going through Dr. Howard Johnson's articles indexed at ... eyword.htm . He is quite an industry guru, and the author of "High Speed Digital Design: A Handbook of Black Magic."