The silkscreen layer of the PCB board is the text layer. Its function is to facilitate the installation and maintenance of the circuit. The required logo patterns and text codes are printed on the top and bottom surfaces of the PCB board, such as the component label, the external component outline shape and manufacturer's logo, production date, etc. So what are the requirements for the silkscreen of PCB boards? Let us follow the Shenzhen PCB manufacturer-SysPCB to learn more about the requirements of silkscreen.
Detail's of silkscreen layer
The silk screen layer is the text layer, which belongs to the top layer and the bottom layer in the PCB and is generally used for annotation. This is to facilitate the installation and maintenance etc. of the circuit, it's a special layer for printing the required logo patterns and text codes on the top and bottom surfaces of the printed board.
Requirements of PCB silkscreen
1. All components, mounting holes and positioning holes have corresponding silkscreen labels to facilitate the installation of the finished board, the mounting holes on the PCB are printed H1, H2, Hn to identification.
2. The silkscreen characters follow the principle of left-to-right and bottom-to-up as much as possible. For polar components such as electrolytic capacitors and diodes, try to maintain the same direction in each functional unit.
3. On the components pad should no silkscreen, because the component label should not be covered after installation. (Except higher density and specify require no silk screen on the PCB).
4. The silkscreen spacing should be greater than 5mil.
5. The polarity of polar components is clearly shown on the silkscreen, the polarity direction mark is easy to recognize.
6. The identifier of the component on the PCB must be the same as the identifier in the BOM list.
7. The direction of the connector is clearly indicated on the silk screen.
8. There should be a bar code position mark on the PCB when the PCB board space allows, there should be a bar code silk screen frame on the PCB, and the position of the bar code should be considered for easy scanning.
9. The name, date, and version number of the PCB should be clear and eye-catching.
10. The manufacturer should have complete relevant information and anti-static mark on the PCB.
How to design PCB silk screen?
1. Location
Generally, do not use the different directions for resistors, capacitors, tubes, etc., as this will cause a lot of tiredness when looking at the silk screen during commissioning, maintenance, and welding (the board has to turn in several directions) . Therefore, it is recommended to place them in two directions, as shown in the figure below. In this way, it will be very easy to view the silk screen.
As shown in the figure below, the components are too dense. If the silk screen cannot be placed, you can write a silk screen on the nearby blank space and mark the arrow. It is best to draw a frame to make it easier to identify.
2. Try not to place silk screen on the via hole.
As shown in the figure below, the via hole is at the "8" character. After the board is finished, you will not be able to know whether it is R48 or R49.
3. Do not silk screen on the high-speed signal line (eg, clock line, etc.).
These lines are mainly aimed at high-speed signal lines on the top or bottom layer, because such signal lines can be regarded as microstrip lines. The speed (phase speed) of the signal running on the microstrip line is related to the medium. If the silk screen on the line, as shown in the figure below, the medium will become uneven, resulting in a change in the phase speed, and ultimately manifested as impedance discontinuity , affecting signal quality. Of course, there will be no such problem in the signal line of the inner layer.
4. The reading direction of silk screen should be consistent with the direction of use.
As shown in the figure below, the reading direction of the silk screen and the use direction of the chip should be the same, mainly when soldering, reducing the probability of reverse soldering. Others, such as electrolytic capacitors, can also not follow this recommendation, because you can indicate the positive and negative polarities.
5. The pin number should be clearly marked on the silk screen.
As shown in the figure below, the P3 connector is marked with 4 pin numbers to facilitate debugging/installation. In addition, it is best to mark the place where the pins are dense, such as: chip, FPC socket, etc. At the same time, it also meets the previous recommendation, the reading direction of P3 is consistent with the direction of use of the connector.
6. Silk screen with special packaging.
For special packages such as BGA and QFN, the size of the silk screen must be exactly the same as the size of the chip (as shown in the figure below), otherwise, it will be difficult to align, which will affect soldering.
7. Screen printing of mounting holes.
Here, the silk screen of the screw is added near the mounting hole, and the length and the total number of screws are also marked for easy installation.
8. Ambiguity of silk screen
The most commonly used RS232. Here, two arrows are added to indicate the flow of the indicator signal (as shown in the figure below), so that you can know how to wire at a glance. Not only RS232 can do this, but others, such as SPI, etc., with transceiver signal lines, can do so.
