PCB design specifications and techniques based on Protel 99SE environment

Protel 99 SE is the most representative and common EDA design system software of Altium, which is powerful and popular among circuit designers. It is an EDA work platform composed of multiple practical tool software such as circuit schematic design, PCB board design, circuit simulation and PLD design, providing users with a full range of design solutions. From the initial project module planning to the final formation of production data, it can be realized according to the designer's own design method.

Although Protel 99SE software has such powerful functions. But it is impossible to design a PCB board with excellent performance and reasonable layout by relying on a powerful software alone. In the whole PCB board design process, it mainly depends on the designer's many years of experience in board design and some advanced design specifications and design skills. This article is based on the platform of Protel 99 SE software. According to some advanced design specifications and my many years of design experience, I will explain some design specifications and design skills that must be followed in the design process of PCB board, so that beginners can design an ideal PCB board faster and better, and improve the performance of the whole circuit.

According to the design process of PCB board diagram, this article will mainly explain four aspects: early design preparation, PCB board layout, wiring and design review.

1. Preliminary design preparation

Before designing the PCB board, the hardware project personnel must prepare the following materials: (1) Prepare the required component library. "If you want to do your work well, you must first sharpen your tools." To make a good board, in addition to designing a good schematic, you must also draw it well. Before designing the PCB, you must first prepare the component library of the schematic SCH and the component library of the PCB. The component library can use the library that comes with Protel, but it is generally difficult to find a suitable one. It is best to make your own component library based on the standard size information of the selected device. In principle, make the PCB component library first, and then make the SCH component library. The PCB component library has higher requirements, which directly affects the installation of the board. The requirements for the SCH component library are relatively loose, as long as you pay attention to defining the pin attributes and the corresponding relationship with the PCB components; (2) Based on the prepared schematic SCH component library, complete the design of the schematic, clarify the basic principles of the circuit and the wiring requirements of each part, and then go through technical review by relevant personnel to ensure the correctness, rationality and feasibility of the entire schematic plan; (3) The PCB structure diagram should indicate the overall dimensions, mounting hole size and positioning dimensions, connector positioning dimensions, prohibited wiring areas and other related mechanical dimensions.

2 PCB board layout

In the entire PCB board design process, component layout is extremely important. The quality of component layout fundamentally determines the design level, performance parameters and the difficulty of the next wiring of the PCB board. Before layout, the coordinate origin of the PCB circuit board must be determined first. Generally, the intersection of the extended lines on the left and bottom of the single board or the first pad in the lower left corner of the single board is used as the coordinate origin. According to many years of experience in designing board drawings and standard design specifications, the basic principles of layout operation are as follows:

(1) Set the size of the board frame according to the structural drawing, arrange the mounting holes, connectors and other devices that need to be positioned according to the structural elements, and give these devices a locked state. Then, mark the dimensions according to the requirements of the process design specifications, and set the prohibited wiring area according to the special requirements of the layout area and components.

(2) According to the layout principle of "big first, small later, difficult first, easy later", important unit circuits and core components should be laid out first. The design principles of the schematic diagram should be referred to in the layout, and the main components should be arranged according to the flow of signals and other rules;

(3) Optimize the layout according to the standards of uniform distribution, balanced center of gravity, and beautiful layout. Circuit parts with the same structure should adopt the "symmetrical" standard layout as much as possible;

(4) When laying out components, appropriate consideration should be given to placing devices using the same power supply together as much as possible to facilitate future power supply division. Circuits that perform the same function should be placed as close as possible, and the components should be adjusted to ensure the simplest wiring. At the same time, the relative positions of the functional blocks should be adjusted to make the wiring between the functional blocks as short as possible;

(5) For components with large mass, the installation position and installation strength should be considered. Temperature sensitive components other than temperature detection components should be placed away from heating components. Thermal convection measures should also be considered if necessary. High voltage and high current signals should be separated from weak signals with small current and low voltage. Analog signals should be separated from digital signals. High frequency signals should be separated from low frequency signals. The spacing between high frequency components should be sufficient.

(6) The same type of plug-in components should be placed in the same direction in the X or Y direction, and the same type of polar discrete components should also strive to be consistent in the X or Y direction to facilitate production and inspection;

(7) It is best to add a decoupling capacitor to each integrated circuit IC. The layout of the IC decoupling capacitor should be as close to the IC power pin as possible, and the loop formed by it, the power supply and the ground should be as short as possible;

(8) After the layout is completed, the correctness of the device package should be checked, and the signal correspondence between the single board, backplane and connector should be confirmed. Only after confirmation can wiring begin.

3 Wiring

A good layout is important, but a good layout only lays a good foundation for the design of the PCB board diagram. Wiring is the most important process in the entire PCB design. This will directly affect the performance of the PCB board. In the design process of PCB, wiring is generally divided into three levels: the first is wiring, which is the most basic requirement for PCB design. If the lines are not wired through and there are flying wires everywhere, it will be an unqualified board, which can be said to be not yet entry-level. The second is the satisfaction of electrical performance. This is the standard for measuring whether a printed circuit board is qualified. This is after the wiring is through, carefully adjust the wiring so that it can achieve the best electrical performance. Then there is beauty. If your wiring is through, there is nothing that affects the electrical performance, but it looks messy at a glance, plus colorful and colorful, then no matter how good your electrical performance is, it is still a piece of garbage in the eyes of others. This brings great inconvenience to testing and maintenance. The wiring should be neat and uniform, and it cannot be criss-crossed and disorderly. These must be achieved while ensuring the performance of the electrical appliances and meeting other individual requirements, otherwise it will be a waste of time. The wiring is mainly carried out according to the following principles:

(1) In general, the power and ground wires should be routed first to ensure the electrical performance of the circuit board. As far as conditions permit, try to widen the width of the power and ground wires. It is best if the ground wire is wider than the power wire. Another thing is to abide by the "20H rule". Since the electric fields of the power layer and the ground layer are changing, electromagnetic interference will be radiated outward at the edge of the board, which is called edge effect. In this way, we shrink the power layer so that the electric field is only conducted within the range of the ground layer. In units of one H (the thickness of the dielectric between the power supply and the ground), if it is shrunk by 20H, 70% of the electric field can be confined to the edge of the ground layer.

(2) Sharp angles and right angles should be avoided in PCB design. 45° zigzag lines should be used whenever possible. 90° zigzag lines should not be used to reduce the radiation of high-frequency signals. Double arc lines should be used for lines with high requirements.

(3) Device decoupling rules: Add necessary decoupling capacitors on the PCB board to filter the interference signals on the power supply and stabilize the power supply. In multi-layer boards, the location of decoupling capacitors is generally not very demanding. However, for double-layer boards, the layout of decoupling capacitors and the power wiring method directly affect the stability of the entire system. Generally, the current should pass through the filter capacitor before being used by the device. At the same time, the impact of the power noise generated during the period on downstream devices must be fully considered. Generally, a power supply mode with a bus structure design is adopted;

(4) To prevent mutual interference between modules with different operating frequencies, the wiring length of the high-frequency part is generally shortened as much as possible. At the same time, the analog and digital circuits should be arranged on both sides of the PCB board respectively, using different layer wiring, and isolated in the middle by the ground layer.

(5) The oscillator housing should be grounded, and the clock line should be as short as possible and should not be led everywhere. The ground area under the clock oscillator circuit and the special high-speed logic circuit should be increased, and other signal lines should not be run, so that the surrounding electric field approaches zero;

(6) Pre-wire the lines with strict requirements (such as high-frequency lines). Avoid adjacent parallel lines between the input and output ends to avoid reflection interference. If necessary, ground isolation should be added. The wiring of two adjacent layers should be perpendicular to each other to avoid parasitic coupling and crosstalk between layers caused by parallel routing.

(7) When designing the wiring, the wiring length should be as short as possible to reduce the interference caused by the long wiring. In particular, for some important signal lines, such as clock lines, the oscillator must be placed very close to the device. Generally, do not have a wiring with one end floating in the air to avoid the "antenna effect" and reduce unnecessary interference radiation and reception, otherwise it will bring unpredictable results;

(8) No signal line should form a loop. If it is unavoidable, the loop should be as small as possible, so that the external radiation is reduced and the external interference is also reduced. The signal line should have as few vias as possible, the key line should be as short and thick as possible, and a protective ground should be added on both sides to minimize the signal loop area.

Finally, we need to optimize the wiring and silk screen. As the saying goes, "there is no best, only better". No matter how hard you think about the design, after you finish drawing, you will still feel that many places can be modified. The general design experience is: the time for optimizing the wiring is twice the time for the initial wiring. After you feel that there is nothing to modify, you can lay copper (Pla-ce->polygon Plane). Copper is generally laid for ground wires (note the separation of analog ground and digital ground), and it may also be necessary to lay power for multi-layer boards. For silk screen, be careful not to be blocked by the device or removed by vias and pads. At the same time, when designing, look directly at the component side, and the bottom layer of words should be mirrored to avoid confusing the layers.

4 Design Review

After the PCB board design is completed, the design will be reviewed according to the corresponding quality review requirements. The review content is generally divided into the following aspects:

(1) Check whether the mechanical dimensions of the positioning holes and positioning parts are consistent with the structural diagram. (2) Check whether the high-frequency, high-speed, clock and other signal lines that are susceptible to interference have the smallest loop area, are they far away from interference sources, have any redundant vias and windings, and whether they cross the ground layer division area. (3) Check whether there are signal lines passing under the crystal, transformer, optocoupler, and power module. Try to avoid running wires under them, especially laying grounded copper foil under the crystal. (4) Check whether the serial numbers of the components are arranged in order, and whether the silk screen markings cover the pads, vias, etc. (5) Check whether the wiring is 100% completed, whether there are any wire ends, and whether there are any isolated copper foils.