With the location of the components, it is a rule to route the components according to their location. The traces are short, and the occupied channel and area are small, so the through rate will be higher. The wires on the input and output sides of the PCB should be kept as parallel as possible. It is better to place ground wires between the two wires. In order to avoid circuit feedback convergence. If the printed board is a multi-layer board, the signal line routing direction of each layer must be different from that of the adjacent board layer. For some important signal lines, it is necessary to reach agreement with the circuit designers. Special differential signal lines should be routed in pairs and try their best to make them parallel and close to each other with little difference in length. All components on the PCB are minimized and shortened. Leads and connections between the components are minimized. The minimum width of the conductors in the PCB is mainly determined by the adhesion strength between the conductor and the insulating substrate and the value of the current flowing through them. When the thickness of the copper foil is 0.05mm and the width is 1-1.5mm, the temperature through 2A will not exceed 3 degrees. Wire width 1.5mm can meet the requirements, for integrated circuits, especially digital circuits, usually choose 0.02-0.03mm. Of course, as far as possible, we use as much as possible the wide line, especially the power line and the ground line on the PCB, and the minimum spacing of the wires is mainly determined by the line-to-line insulation resistance and breakdown voltage in the worst case. For some integrated circuits (ICs), the pitch can be less than 5-8mm in terms of process considerations. The bent portion of the printed conductor is generally the smallest in the arc, avoiding the use of traces less than 90 degrees. The right angle and the included angle will affect the electrical performance in the high frequency circuit. In short, the wiring of the printed circuit board should be uniform, the density should be proper, and the consistency should be good. Avoid the use of large-area copper foil in the circuit as much as possible, otherwise, in the course of the use of excessive heat for a long time, the phenomenon of copper foil expansion and detachment is prone to occur. If a large-area copper foil must be used, grid-like wires may be used. The port of the wire is the pad. The pad center hole is larger than the device lead diameter. Solder pads are too large to form an inadvertent solder during soldering. The pad diameter D is generally not less than (d + 1.2) mm, where d is the hole diameter, and the minimum diameter of the pad for some high-density components is desirable (d + 1.0) Mm, After the pad design is completed, the device's outline frame should be drawn around the pad of the PCB, and the text and characters should be marked. The height of the general text or frame should be about 0.9mm, and the line width should be about 0.2mm. And do not press lines and other characters on the pad. If it is a double deck, the underlying characters should be mirrored.

Second, in order to make the designed product work better and more effectively, the PCB has to consider its anti-interference ability in the design, and has a close relationship with the specific circuit.

The design of the power line and ground line in the circuit board is particularly important. According to the size of the current flowing through the different circuit boards, the width of the power line is increased as much as possible to reduce the loop resistance, and the power line and the ground line as well as the data The transmission direction is the same. Contributes to the circuit's enhanced noise immunity. There are logic circuits and linear circuits on the PCB, so that they can be separated as much as possible. The low-frequency circuit can be connected with a single point, and the actual wiring can be connected in series and then connected to the ground. The high-frequency circuit uses a multi-point series connection. The ground wire should be short and thick. For the high-frequency components around the grid can be a large area of ​​foil, the ground wire should be as thick as possible, if the ground wire is very thin wire, ground potential with current changes, so that the anti-noise performance is reduced. Therefore, the grounding wire should be thickened so that it can reach the allowable current on the circuit board. If the design allows the width of the ground wire can be more than 2-3mm diameter, in the digital circuit, the laying of the ground circuit into the loop can mostly improve the noise resistance. In the design of the PCB, it is common practice to configure an appropriate decoupling capacitor at a critical portion of the printed board. Connect a 10-100uF electrolytic capacitor across the power supply input. Generally, a 0.01PF ceramic capacitor should be placed near the 20-30 pin. The power pin of the 20-30 pin integrated circuit chip is generally required. A 0.01 PF disk capacitor should be placed nearby. For larger chips, there will be several power pins. It is best to add a decoupling capacitor near them. For a chip with more than 200 feet, it is on all four sides. Add at least two retraction capacitors. If the gap is not sufficient, a 1-10 PF tantalum capacitor may be arranged on 4-8 chips. For a component with weak anti-interference ability and large power supply variation, the device should be directly connected between the power line and the ground line of the device. , Whichever type of access capacitor lead is not too long.

3. After the circuit board's components and circuits have been designed, the design of the circuit board must be considered. The purpose is to eliminate all kinds of undesirable factors before the start of production, and at the same time, to take into consideration the manufacturability of the circuit board in order to produce high-quality products. And batch production.

The above mentioned aspects of the circuit board have been involved in the positioning and wiring of components. The process design of the circuit board is mainly to organically assemble the circuit board and the component that we designed through SMT production line, thus realize the good electrical connection and reach the position layout of our design product. Pad design, wiring, anti-jamming, etc. We also need to consider whether the board we designed is convenient for production. Can we use the modern assembly technology - SMT technology to assemble, and at the same time, we must achieve the design in the production so as not to produce defective products. height. Specifically there are the following aspects:

1: Different SMT production lines have their own different production conditions, but the size of the PCB, pcb single board size is not less than 200 * 150mm. If the long edge is too small to be used for imposition, and the ratio of length to width is 3:2 or 4:3, the board size of the circuit board is larger than 200 x 150 mm, and the mechanical strength of the circuit board should be considered.

2: When the circuit board size is too small, it is difficult for the whole SMT production process, and it is not easy to mass production. The best method is to use the form of puzzles, that is, according to the size of the single board, the 2 boards, 4 boards, 6 boards, etc. Combine them together to form an entire board that is suitable for mass production. The entire board size is suitable for the size of the sticker.

3: In order to adapt to the placement of the production line, the board should leave 3-5mm without any component, the board has 3-8mm process edge, and the process edge and the PCB connection have three forms: A no edge, There are separation troughs, B has edges, there are separation troughs, C has edges, no separation troughs. With punching technology to build a country. According to the shape of the PCB board, there are ways to apply different types of puzzles. The process of the PCB edge according to different types of positioning methods are different, and some have to provide positioning holes in the edge of the process, the diameter of the hole in the 4-5 cm, relatively speaking, compared to the positioning accuracy of the edge, so there is positioning The hole positioning model must have positioning holes for PCB processing, and the hole design should be standard to avoid inconvenience to production.

4: In order to better position and achieve higher placement accuracy, it is necessary to set a reference point for the PCB. Whether there is a reference point and good or bad design directly affects the mass production of the SMT production line. The shape of the datum points can be square, round, triangular, and so on. And the diameter is in the range of about 1-2mm, and it is within the range of 3-5mm around the reference point, without any components and leads. At the same time, the reference point should be smooth and flat, without any pollution. The design of the datum point should not be too close to the edge of the plate, and should have a distance of 3-5mm.

5: From the overall production process, the shape of the board is preferably in the form of a distance, especially for wave soldering. Use rectangles for easy delivery. If there is a missing slot on the PCB board, use a process edge to fill in the missing slot. For a single SMT board, there should be a missing slot. However, the lack of slots should not be too large to be less than 1/3 of the length of the side.

In short, the production of defective products is possible in every aspect. However, this aspect of the PCB design should be considered from various aspects, so that it is a good realization of the purpose of designing the product, but also suitable for the production of SMT production line In mass production, we try our best to design high-quality PCB boards to minimize the occurrence of defective products.