Reverse step of PCB schematic and attention to detail

Reverse step of PCB schematic and attention to detail

Schematic to PCBA

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


That is, under the premise that there are physical objects and circuit boards, 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 silk screen production file was restored 1:1. Then use these technical documents and production documents for PCB board, circuit board soldering, flying probe test, PCBA 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 range, no longer limited to simple board copying and cloning, but also involves 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?


PCB schematic reverse step process:


  1. 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 higher and higher. The above diodes are not noticed at all.


  1. 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 separately by color.

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


  1. 3. Adjust the corrected image

Adjust the contrast and darkness 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 that there is a problem with the graphics, you can use PHOTOSHOP to fix and correct it.


  1. 4. Verify the positional coincidence of PAD and VIA

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


  1. 5. Drawing layers

Convert the TOP layer BMP to TOP PCB, pay attention to the SILK layer, which is the yellow layer, then you can 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 into PROTEL and combine them into one picture.


  1. 7. Laser printing TOP LAYER, BOTTOM LAYER

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


  1. 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.



PCB schematic reverse push attention to details:


  1. 1. 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.

Generally speaking, the components with the same function on one PCB will be arranged in a centralized manner, and the functional division area can have a convenient and accurate basis when the schematic is reversed.

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

First, find out the core components in a functional unit, and then according to the trace connection, you can find out the other components of the same functional unit to form a functional partition.

The formation of functional partitions is the basis for 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.


  1. 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.


  1. 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 line crossing and interspersing, the grounding line can be used in a large number of grounding symbols. Various lines can be clearly distinguished by using different lines of different colors, and special signs can be used for various components, and even The unit circuits are drawn separately and finally combined.


  1. 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 have certain similarity in the schematic diagram of 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.


  1. 5. Check and optimize


After the schematic drawing is completed, it is necessary to pass the test and check 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.