..1: Draw the schematic diagram.
..2: Create component library.
..3: Establish the network connection relationship between the schematic diagram and the components on the printed board.
..4: Routing and placement.
..5: Create printed board production usage data and placement production usage data.
.. After determining the position and shape of the components on the PCB, consider the layout of the PCB.
1. With the position of the component, the wiring is carried out according to the position of the component. It is a principle that the wiring on the printed board is as short as possible. The traces are short, and the channel and area occupied are small, so the pass-through rate will be higher. The wires of the input terminal and the output terminal on the PCB board should try to avoid being adjacent to each other in parallel, and it is better to place a ground wire between the two wires. To avoid circuit feedback coupling. If the printed board is a multi-layer board, the routing direction of the signal line of each layer is different from that of the adjacent board layer. For some important signal lines, you should reach an agreement with the line designer, especially the differential signal lines, they should be routed in pairs, try to make them parallel and close, and the lengths are not much different. All components on the PCB should minimize and shorten the leads and connections between components. The minimum width of the wires in the PCB is mainly determined by the adhesion strength between the wires and the insulating layer substrate and the current value flowing through them. When the thickness of the copper foil is 0.05mm and the width is 1-1.5mm, the temperature will not be higher than 3 degrees when a current of 2A is passed. When the wire width is 1.5mm, it can meet the requirements. For integrated circuits, especially digital circuits, 0.02-0.03mm is usually selected. Of course, as long as it is allowed, we use wide wires as much as possible, especially the power wires and ground wires on the PCB. The minimum distance between the wires is mainly determined by the insulation resistance and breakdown voltage between the wires in the worst case.
For some integrated circuits (IC), the pitch can be made smaller than 5-8mm from the perspective of technology. The bend of the printed wire is generally the smallest arc, and the use of less than 90-degree bends should be avoided. The right angle and the included angle will affect the electrical performance in the high-frequency circuit. In short, the wiring of the printed board should be uniform, dense and consistent. Try to avoid the use of large-area copper foil in the circuit, otherwise, when the heat is generated for a long time during use, the copper foil will expand and fall off easily. If a large-area copper foil must be used, grid-shaped wires can be used. The terminal of the wire is the pad. The center hole of the pad is larger than the diameter of the device lead. If the pad is too large, it is easy to form a virtual weld during welding. The outer diameter D of the pad is generally not less than (d+1.2) mm, where d is the aperture. For some components with relatively high density, the minimum diameter of the pad is desirable (d+1.0) mm, after the design of the pad is completed, the outline frame of the device should be drawn around the pad of the printed board, and the text and characters should be marked at the same time. Generally, the height of the text or frame should be around 0.9mm, and the line width should be around 0.2mm. And lines such as marked text and characters should not be pressed on the pad. If it is a double-layer board, the bottom character should mirror the label.
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 it 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 different circuit boards, the width of the power line should be increased as much as possible to reduce the loop resistance. At the same time, the direction of the power line and the ground line and the data The direction of transmission remains the same. Contribute to the enhancement of the anti-noise ability of the circuit. There are both logic circuits and linear circuits on the PCB, so that they are separated as much as possible. The low-frequency circuit can be connected in parallel with a single point. The actual wiring can be connected in series and then connected in parallel. The ground wire should be short and thick. Large-area ground foil can be used around high-frequency components. The ground wire should be as thick as possible. If the ground wire is very thin, the ground potential will change with the current, which will reduce the anti-noise performance. Therefore, the ground wire should be thickened so that it can reach the allowable current on the circuit board.If the design allows the diameter of the ground wire to be more than 2-3mm, in digital circuits, the ground wire can be arranged in a loop to improve the anti-noise ability. In PCB design, appropriate decoupling capacitors are generally configured in key parts of the printed board. A 10-100uF electrolytic capacitor is connected across the line at the power input end. Generally, a 0.01PF magnetic chip capacitor should be arranged near the power pin of the integrated circuit chip with 20-30 pins. For larger chips, the power lead There will be several pins, and it is better to add a decoupling capacitor near them. For a chip with more than 200 pins, add at least two decoupling capacitors on its four sides. If the gap is insufficient, a 1-10PF tantalum capacitor can also be arranged on 4-8 chips. For components with weak anti-interference ability and large power-off changes, a decoupling capacitor should be directly connected between the power line and the ground line of the component. , No matter what kind of lead connected to the capacitor above, it is not easy to be too long.
3. After the component and circuit design of the circuit board is completed, its process design should be considered next, in order to eliminate all kinds of bad factors before the start of production, and at the same time, take into account the manufacturability of the circuit board in order to produce high-quality products. and mass production.
.. When talking about the positioning and wiring of components, some aspects of the process of the circuit board have been involved. The process design of the circuit board is mainly to organically assemble the circuit board and components we designed through the SMT production line, so as to achieve good electrical connection and achieve the position layout of our designed products. Pad design, wiring and anti-interference, etc. also need to consider whether the board we designed is easy to produce, whether it can be assembled with modern assembly technology-SMT technology, and at the same time, it is necessary to meet the conditions of not allowing defective products to be produced during production. high. Specifically, there are the following aspects:
1: Different SMT production lines have different production conditions, but in terms of the size of the PCB, the single board size of the PCB is not less than 200*150mm. If the long side is too small, imposition can be used, and the ratio of length to width is 3:2 or 4:3. When the size of the circuit board is greater than 200×150mm, the mechanical strength of the circuit board should be considered.
2: When the size of the circuit board is too small, it is difficult for the entire SMT line production process, and it is not easy to produce in batches. The best way is to use the board form, which is to combine 2, 4, 6 and other single boards according to the size of the board. Combined together to form a whole board suitable for mass production, the size of the whole board should be suitable for the size of the stickable range.
3: In order to adapt to the placement of the production line, the veneer should leave a range of 3-5mm without any components, and the panel should leave a 3-8mm process edge. There are three types of connection between the process edge and the PCB: A without overlapping, There is a separation tank, B has a side and a separation tank, and C has a side and no separation tank. Equipped with punching process equipment. According to the shape of the PCB board, there are different forms of jigsaw boards, such as Youtu. The process side of the PCB has different positioning methods according to different models, and some have positioning holes on the process side. The diameter of the hole is 4-5 cm. Relatively speaking, the positioning accuracy is higher than that of the side, so there are The model with hole positioning must be provided with positioning holes during PCB processing, and the hole design must be standard to avoid inconvenience to production.
4: In order to better position and achieve higher mounting accuracy, it is necessary to set a reference point for the PCB. Whether there is a reference point and whether the setting is good or not will directly affect the mass production of the SMT production line. The shape of the reference point can be square, circular, triangular, etc. And the diameter should be within the range of 1-2mm, and the surrounding of the reference point should be within the range of 3-5mm, without any components and leads. At the same time, the reference point should be smooth and flat without any pollution. The design of the reference point should not be too close to the edge of the board, there must be a distance of 3-5mm.
5: From the perspective of the overall production process, the shape of the board is preferably pitch-shaped, especially for wave soldering. Rectangular for easy delivery. If there is a missing groove on the PCB board, the missing groove should be filled in the form of a process edge, and a single SMT board is allowed to have a missing groove. But the missing groove is not easy to be too large and should be less than 1/3 of the length of the side
Post time: May-06-2023