PCB Technical - Summary of dry goods PCB drawing experience

1. Printed circuit board design requirements

1. Correct

This is the most basic and most important requirement for printed circuit 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 circuit boards designed with simple CAD software. General products have to undergo more than two rounds of trial production and modification. The more powerful CAD software has inspection functions to ensure electrical The correctness of the connection.

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 circuit board assembly, from the manufacture, inspection, assembly, debugging of the printed circuit board to the assembly and debugging of the whole machine, until the use and maintenance, are all closely related to the reasonableness of the printed board, for example, the board shape is not well selected and the processing is difficult, 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 connection improperly and so on. 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 low price for the board, keep the board size as small as possible, use direct-welded wires for connection, use the cheapest surface coating, choose the lowest-priced processing plant, etc., and the printed board 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.

PCB 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 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.

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 role of setting copper is not only to increase the area of the ground wire and anti-interference.

2, Protel print settings

The print setting of SCH is relatively simple. Fill in all 0 in the Top Bottom Left Right of Margins and click Refresh, so that the page can be occupied in the largest range and the printed SCH image will be larger.

PCB settings: Open File>Setup Printer... to set up before printing.

In the Printer Setup menu that pops up, you must first select the printer: the first ones are the default printers, the next two are the printers we have installed, one of the two suffixes is Final, one is Composite, and the first one means printer Only print one layer at a time, the latter one is to print all selected layers at a time, choose by yourself according to your needs! Next step: Click the Options button below to set the properties. Suppose you select final and then enter Options to set up. Generally, you don't need to change the options after entering. Scale is the printing ratio, and the default is 1:1. If you want to print a full page, check the small box, oh! The Show Hole on the right is very important. Select it to print the holes on the circuit board. Okay, click Setup to set the paper size to complete the printer options. It's not over yet! Trouble! Go back to the dialog box for selecting printer properties, select Layers, and set the printing layer. After entering, you can see it! Is it very familiar? Choose according to your needs.

3. Commonly used PCB library files

1. The component library contained in the component database in the \library\pcb\connectors directory contains most of the PCB packages of the connector components

1) D type connectors.ddb, a package containing parallel port and serial port interface components

2) headers.ddb: package containing various plug components

2. The component library contained in the database in the \library\pcb\generic footprints directory contains most of the PCBpackages of common components.