Protel-based PCB board design

writer: G March 05, 2019

1. Quickly determine the PCB shape

Designing the PCB first determines the shape of the board, usually by prohibiting the wiring layer from drawing electrical wiring. Unless otherwise specified, the general board shape is rectangular, and the aspect ratio is generally 3:2 or 4:3. You can draw two horizontal lines and two vertical lines before painting, and then use the Set Origin tool in the Place Toolbar to set the end point of a line segment to the origin, that is, the coordinates are (0,0). Double-click each line segment, and change the coordinate values of its starting point and ending point accordingly, so that the four line segments meet end to end to form a closed rectangular frame, and the appearance of the circuit board is determined. If you need to adjust the size of the board during the drawing process, just modify the corresponding coordinate value of each line segment. Considering the cost, the length of the copper wire, and the noise resistance, the smaller the board size, the better. However, if the board size is too small, the heat dissipation is poor, and adjacent wires are likely to cause interference. However, when the size of the circuit board is larger than 200 mm × 150 mm, the mechanical strength of the circuit board should be considered, and a fixing hole is appropriately added to serve as a support.

2. Component layout

Before starting the layout, you must first load the components through the network table. In this process, you will often encounter errors that the network table cannot be fully loaded. It can be classified into two categories: one is that the component cannot be found, and the solution is to confirm the schematic. The package form of the component has been defined, and it is confirmed that the corresponding PCB component library has been added. If the component is still not found, it is necessary to make a component package; the other is the missing pin, the most common one is the diode and the triode. The foot is lost. This is because the pins in the schematic are generally the letters A, K, E, B, and C, and the pins of the PCB component are the numbers 1, 2, and 3. The solution is to change the definition of the schematic, or Change the definition of the PCB components to make them consistent. Experienced designers generally build a library of their own PCB components based on the package shape of the actual components, which is easy to use and error-prone.

There are some basic rules that must be followed when laying out the layout:

(1) Special considerations for special components

The high-frequency components should be as close as possible, the shorter the connection, the better; the distance between components with high potential difference should be as large as possible; the components with large weight should be fixed by brackets; the components with heat should be kept away from the thermal components and installed. The corresponding heat sink or placed outside the board; the layout of adjustable components such as potentiometer, adjustable inductor, variable capacitor, micro switch, etc. should consider the structural requirements of the whole machine, which is convenient for adjustment. In short, some special components should be considered from the characteristics of the components themselves, the structure of the chassis, the convenience of maintenance and debugging, etc. in order to ensure a stable and easy to use PCB board.

(2) According to the circuit function layout

If there is no special requirement, arrange the components as much as possible according to the component arrangement of the schematic diagram. Usually, the signal is input from the left side, the right side is output, and the input is from the upper side and the lower side. According to the circuit flow, the position of each functional circuit unit is arranged to make the signal flow more smooth and maintain the same direction. In addition, the digital circuit portion must be laid out separately from the analog circuit portion to reduce interference.

(3) Text labeling of silk screen layer

In order to facilitate the installation and maintenance of the circuit, it is generally necessary to print the required logo pattern and character code on the upper and lower surfaces of the printed board, such as component label and nominal value, component outline shape, manufacturer logo, etc., many beginners I often skip the design of the silk screen layer, or just pay attention to the neat and beautiful appearance of the text symbol. After the PCB board is actually produced, the characters on the board are not blocked by the component or invaded the soldering area and erased. Hit on adjacent components, causing inconvenience in assembly and maintenance. The correct silk screen layer character layout principle should be ambiguous, sewed, and elegant.

3. Routing

This is a very important part of PCB design. PCB layout has single-sided wiring, double-sided wiring and multi-layer wiring. There are two ways to route: automatic routing and interactive routing. Pay attention to the following issues during the wiring process:

(1) Line length

The copper wire should be as short as possible, especially in high frequency circuits. The bend of the copper wire should be rounded or beveled. The right angle or sharp corner will affect the electrical performance in the case of high frequency circuits and high wiring density. In addition, when double-sided wiring, the wires on both sides should be perpendicular, oblique or curved to avoid parallel to each other to reduce parasitic capacitance.

(2) line width

The width of the copper wire should be based on the requirements of electrical characteristics and easy to produce. The minimum value depends on the current flowing through it, but it should not be less than 0.2mm. If the plate area is large enough, the copper wire width is best. Do not lower than 0. 3mm. The relationship between the ground wire and the power cable is: ground wire > power wire > signal wire, usually the power cable width is 1.2-2.5mm, and the signal wire width is 0.2-0.3mm.

(3) Line spacing

The spacing between adjacent copper wires should meet electrical safety requirements, and for ease of production, the spacing should be as wide as possible. The minimum spacing can withstand at least the peak value of the applied voltage. Generally, the copper wire distance between potential differences of 2000V should be greater than 2mm. In the case of low wiring density, the spacing should be as large as possible. Usually the line spacing is preferably not less than 0.3 mm.

(4) Shielding and grounding

The common ground wire for the copper wire should be placed as close as possible to the edge of the board. The copper foil should be kept as much as possible on the board to make the grounding wire, which can enhance the shielding ability.

4. Conclusion

The design of the PCB board diagram is a complicated and simple process. For the same circuit or the same instrument, even if the components and parameters are identical, the result will be different due to the difference in component layout design and electrical wiring direction. Therefore, it is necessary to combine the three aspects of how to correctly design the layout of the printed circuit board components and the correct selection of the wiring direction and the process structure of the overall instrument. The reasonable process structure can eliminate the humming interference caused by improper wiring. It is convenient for installation, commissioning and overhaul in production.