Reverse step of PCB schematic and attention to detail

In the PCB board design, it is necessary to comprehensively consider the size of the PCB board, and select a suitable line width by a current. PCB copy board, also known in the industry as board copy board, board cloning, board copying, PCB cloning, PCB reverse design or PCB reverse development.

That is, under the premise that there are physical objects and circuit boards in the electronic product, the reverse analysis technology is used to reversely analyze the circuit board, and the original product PCB file, bill of materials (BOM) file, schematic file, and other technical documents and The PCB screen printing production file was restored 1:1.

Then use these technical documents and production documents for PCB board, component soldering, flying probe test, circuit board debugging, complete the complete copy of the original circuit board template.

With the continuous development and deepening of the copy board industry, today’s PCB copy board concept has been extended to a wider scope, no longer limited to simple board copying and cloning, but also involves the secondary development of products and new products. Research and development.

Then, according to the document map or the real thing, how to reverse the PCB schematic diagram, what is the reverse push process? What are the details of this attention?

Reverse step

  1. Record PCB related details

Get a PCB, first record the model, parameters, and position of all components on the paper, especially the direction of the diode, the three-stage tube, the direction of the IC gap. It is best to take a photo of two-component locations with a digital camera. A lot of PCB boards are getting more advanced. The above diodes are not noticed at all.

2. Scanned image

Remove all devices and remove the tin from the PAD hole. Clean the PCB with alcohol and put it into the scanner. When scanning, the scanner needs to slightly raise some scanned pixels to get a clearer image.

Then polish the top and bottom layers slightly with water gauze, polish it to the copper film, put it into the scanner, start PHOTOSHOP, and scan the two layers in color.

Note that the PCB must be placed horizontally and vertically in the scanner, otherwise the scanned image will not be used.

3. Adjust the corrected image

Adjust the contrast and brightness of the canvas so that the portion with the copper film and the portion without the copper film are strongly contrasted. Then turn the secondary image to black and white to check if the line is clear. If it is not clear, repeat this step. If it is clear, save the picture as black and white BMP format files TOP BMP and BOT BMP. If you find any problems with the graphics, you can use PHOTOSHOP to fix and correct them.

4. Verify the positional coincidence of PAD and VIA

Convert two BMP files to PROTEL format files, and transfer them to two layers in PROTEL. If the two layers of PAD and VIA are basically coincident, it indicates that the first few steps are very good. If there is a deviation, then Repeat the third step. Therefore, the PCB copy board is a very patient work, because a small problem will affect the quality and the degree of matching after copying.

5. Drawing Layers

Convert the BMP of the TOP layer to the TOP PCB. Note that the layer to be converted to the SILK layer is the yellow layer. Then you trace the line in the TOP layer and place the device according to the drawing in the second step. Remove the SILK layer after painting. Repeat until you have drawn all the layers.

6. TOP PCB and BOT PCB drawing

Transfer the TOP PCB and BOT PCB in PROTEL and combine them into one picture.

7. Laser printing TOP LAYER, BOTTOM LAYER

Print the TOP LAYER and BOTTOM LAYER on the transparencies with a laser printer (1:1 ratio), put the film on the PCB, and compare if there is any error. If it is correct, you are done.

8. Testing

The electronic technical performance of the test copy board is the same as the original board. If it is the same, it is really done.
Attention to detail 1. A reasonable division of functional areas

When the reverse design of the schematic of a good PCB board, the reasonable division of functional areas can help engineers reduce unnecessary troubles and improve the efficiency of drawing.

In general, components with the same function on a PCB will be arranged in a centralized manner, and the functional division area can have a convenient and accurate basis when deducing the schematic.

However, the division of this functional area is not arbitrary. It requires engineers to have a certain understanding of electronic circuit related knowledge.

1. find out the core components in a functional unit, and then find out the other components of the same functional unit according to the connection of the traces to form a functional partition.

The formation of functional partitions is the basis for the schematic drawing. In addition, in the process, don’t forget to use the component serial numbers on the board, they can help you to partition the function faster.

2. Find the reference piece

This reference piece can also be said to be the main component PCB network city at the beginning of the schematic drawing. After determining the reference part, drawing according to the pins of these reference parts can ensure the accuracy of the schematic diagram to a greater extent. Sex.

For the engineer, the determination of the reference part is not a very complicated matter. In general, the components that play a major role in the circuit can be selected as the reference parts. They are generally bulky and have many pins, which are convenient for drawing. Such as integrated circuits, transformers, transistors, etc., can be used as a suitable reference.

3. Correctly distinguish the lines and draw the wiring reasonably

For the distinction between ground, power, and signal lines, engineers also need to have relevant power knowledge, circuit connection knowledge, PCB wiring knowledge, and so on. The distinction between these lines can be analyzed from the connection of components, the width of the copper foil of the circuit, and the characteristics of the electronic product itself.

In the wiring drawing, in order to avoid the intersection and interweaving of the line, the grounding symbol can be used in a large amount on the grounding wire. Various lines can be clearly distinguished by using different lines of different colors, and special symbols can be used for various components, and even The unit circuits are drawn separately and finally combined.

4. Master the basic framework and draw on similar schematics

For the framework of some basic electronic circuits and the principle drawing method, engineers need to master, not only to directly draw the basic composition of some simple and classic unit circuits but also to form the overall framework of electronic circuits.

On the other hand, don’t neglect that the same type of electronic products has a certain similarity in the schematic diagram of the PCB network city. Engineers can make full use of similar circuit diagrams to carry out the reverse of new product schematic diagrams based on the accumulation of experience.

5. Check and optimize

After the schematic drawing is completed, it is necessary to pass the test and check the link to conclude the reverse design of the PCB schematic. The nominal value of the component sensitive to the PCB distribution parameters needs to be checked and optimized. According to the PCB file diagram, the schematic diagram is compared and analyzed to ensure that the schematic diagram and the file diagram are completely consistent.




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