PCB Technical - Experience record of PCB circuit board drawing

1. Printed board design requirements

1. Correct

This is the most basic and most important requirement of PCB printed board design. It can accurately realize the connection relationship of the electrical schematic diagram and avoid the two simple and fatal errors of "short circuit" and "open circuit". This basic requirement is not easy to achieve in manual design and PCB designed with simple CAD software. Generally, products have to undergo more than two rounds of trial production and modification. The more powerful CAD software has inspection functions to ensure electrical connection. Correctness.

2. Reliable

This is a higher level requirement in PCB design. A correctly connected circuit board is not necessarily reliable. For example, unreasonable board selection, improper board thickness and installation and fixing, improper component layout and wiring, etc. may cause the PCB to fail to work reliably, early failure or even not to work correctly. Another example is that multi-layer boards are much easier to design than single and double panels, but they are not as reliable as single and double panels. From the perspective of reliability, the simpler the structure, the smaller the use surface, the fewer the number of board layers, and the higher the reliability.

3. Reasonable

This is a deeper and more difficult requirement in PCB design. A printed board component, from printed board manufacturing, inspection, assembly, debugging to complete machine assembly, debugging, until use and maintenance, is closely related to the reasonableness of the printed board, for example, the board shape is not well selected, and it is difficult to process. If the lead hole is too small, it is difficult to assemble, it is difficult to leave the pilot height, and it is difficult to maintain the board if the connection is not properly selected. Every difficulty may lead to increased costs and extended working hours. And every cause of difficulty stems from the designer's mistakes. There is no absolutely reasonable design, only the process of continuous rationalization. It requires the designer's sense of responsibility and rigorous style, as well as the experience of summing up and improving in practice.

4. Economy

This is a goal that is not difficult to achieve, nor easy to achieve, but must be achieved. Say "not difficult", choose the low price of the board, the size of the board as small as possible, use the direct-welded wire for the connection, use the cheapest surface coating, choose the lowest-priced processing plant, etc., and the PCB manufacturing price will drop. But don't forget that these cheap choices may cause poor workmanship and reliability, increase manufacturing costs and maintenance costs, and the overall economics may not be handled separately, so it is not easy. "Must" is the principle of market competition. Competition is ruthless. A product with advanced principles and high technology may die due to economic reasons.

Experience:

1. There must be a reasonable direction: such as input/output, AC/DC, strong/weak signal, high frequency/low frequency, high voltage/low voltage, etc., their directions should be linear (or separated), and they must not blend with each other. Its purpose is to prevent mutual interference. The best trend is in a straight line, but it is generally not easy to achieve. The most unfavorable trend is a circle. Fortunately, isolation can be set to improve. For DC, small signal, low voltage PCB design requirements can be lower. So "reasonable" is relative.

2. Choose a good grounding point: I don't know how many engineers and technicians have talked about the small grounding point, which shows its importance. Under normal circumstances, a common ground is required, such as: multiple ground wires of the forward amplifier should be merged and then connected to the main ground, and so on. In reality, it is difficult to achieve this completely due to various restrictions, but we should try our best to follow it. This question is quite flexible in practice. Everyone has their own set of solutions. It is easy to understand if it can be explained for a specific circuit board.

3. Reasonably arrange power supply filter/decoupling capacitors: Generally, only a number of power supply filter/decoupling capacitors are drawn in the schematic, but they are not pointed out where they should be connected. In fact, these capacitors are set up for switching devices (gate circuits) or other components that require filtering/decoupling. These capacitors should be placed as close as possible to these components. If they are too far away, they will have no effect. Interestingly, when the power supply filter/decoupling capacitors are arranged properly, the problem of the grounding point becomes less obvious.

4. The lines are exquisite: wide lines should never be thin if possible; high-voltage and high-frequency lines should be round and slippery, without sharp chamfers, and corners should not be used at right angles. The ground wire should be as wide as possible, and it is best to use a large area of copper, which can greatly improve the problem of grounding points.

5. Although some problems occur in post-production, they are brought about by PCB design. They are: too many vias, and the slightest carelessness of the copper sinking process will bury hidden dangers. Therefore, the design should minimize the wire hole. The density of parallel lines in the same direction is too large, and it is easy to join together when welding. Therefore, the line density should be determined according to the level of the welding process. The distance of the solder joints is too small, which is not conducive to manual welding, and the welding quality can only be solved by reducing the work efficiency. Otherwise, hidden dangers will remain. Therefore, the minimum distance of solder joints should be determined by comprehensive consideration of the quality and work efficiency of the welding personnel. The size of the pad or via is too small, or the size of the pad and the hole size are not properly matched. The former is unfavorable for manual drilling, and the latter is unfavorable for CNC drilling. It is easy to drill the pad into a "c" shape, but to drill off the pad. The wire is too thin, and the large area of the unwiring area is not provided with copper, which is easy to cause uneven corrosion. That is, when the unwiring area is corroded, the thin wire is likely to be over-corroded, or it may appear to be broken, or completely broken. Therefore, the effect of setting the PCB copper coating is not only to increase the ground wire area and anti-interference.