The silk screen processing in PCB design is a link that is easily overlooked by engineers. Generally, everyone pays little attention to it and handles it at will, but the randomness at this stage can easily lead to the installation and debugging of future board components, and even completely destroyed. Drop your entire design. I borrowed an article from “All about circuits” to enumerate several hidden dangers in silk screen processing in PCB design.
The device number is placed on the pad or via
As shown in the figure below, the device number R1 is placed and the “1” is placed on the pad of the device. This situation is very common, and almost every engineer made this mistake when initially designing the PCB, because it is not easy to see the problem in the design software.
When I got the board, I found that the device number was misplaced by the pad or the air, and I couldn’t tell. R2 is the correct way to place.
The device number is placed under the package
U1 in the picture below, maybe you or the manufacturer has no problem when installing the device for the first time, but if you need to debug and need to replace the device, it will be very depressed, and you can’t find where U1 is.
U2 is very clear and is the right way to put it.
The device label does not clearly correspond to the corresponding device.
As shown in the figure below, R1, R2, if you do not check the design PCB source file, you can determine which resistor is R1, which is R2? How to install and debug?
Therefore, the placement of the device label must be such that the reader knows its attribution at first glance, and does not cause ambiguity.
Device label font is too small
Due to the limitation of board space and component density, many times we have to use smaller font size to make the device label, but in any case we must ensure that the device label is “readable”, otherwise the meaning of the device label is lost. In addition, the process of different PCB processing factories is different. Even if the same type of font is used, the effects processed by different processing factories are very different. Sometimes, especially when making a formal product, in order to ensure the effect of the product, it is necessary to find the machining accuracy. High manufacturers to process.
The same font size, different fonts print the same effect, such as Altium Designer default font, even if the font size is large, it is difficult to read on the PCB, if you change to a “True Type” font, Even if the font size is two, it can be read very clearly.
Adjacent devices have ambiguous device numbers
The two resistors in the figure below, the package library of the device has no outline. With these 4 pads, you can’t judge which two pads belong to one resistor, let alone which R1 is and which R2 is. .
The placement of the resistors may be horizontal or vertical, and incorrect soldering may result in circuit errors or even more serious consequences such as short circuits.
The device labels are placed randomly.
The orientation of the device labels on the PCB should be as far as possible in one direction, up to two directions. Random placement can make installation and debugging difficult, because you need to work hard to find the device you need to find.
The component numbers on the left side of the above figure are correct, and the right side is very bad.
No Pin1 numbering on the IC device
The IC (integrated circuit) device package has a clear starting pin mark near Pin 1 , such as a “dot” or “star” to ensure that the IC is installed in the correct orientation. If the device is reversed, the device may be damaged and the board may be discarded. It should be noted that this mark cannot be placed under the IC, otherwise it is very troublesome when debugging the circuit.
As shown above, it is difficult for U1 to determine which direction to place. U2 is easier to judge because the first pin is square and the other pins are round.
Polarized devices have no polarity marking
Many two-legged devices, such as LEDs and electrolytic capacitors, have polarities (directions). If the orientation is incorrect when installed, the circuit will not work or the device will be damaged. The direction of the LED is definitely not bright, and the LED device may be damaged due to voltage breakdown. The electrolytic capacitor may explode and shoot. Therefore, when constructing the package library of these devices, it is necessary to mark the polarity, and the polarity of the mark symbol can not be placed under the device outline, otherwise the polarity symbol will be blocked after the device is installed, which makes debugging difficult.
C1 in the figure below is wrong, because once the capacitor is mounted on the board, it is impossible to judge whether the polarity is correct or not, and the way of C2 is correct.
Do not use heat release
The use of heat release on the electronic component pins makes pcb soldering easier. You may not want to use heat release to reduce resistance and thermal resistance, but not using heat release can make soldering very difficult, especially when the device pads are connected to large traces or copper fill. Large traces and copper fills as heat sinks can cause pad heating difficulties if proper heat release is not used. In the figure below, the source pin of Q1 has no heat release and the MOSFET may be difficult to solder and desold. The source pin of Q2 has a heat release function, and the MOSFET is easy to solder and desolder.
The PCB designer can change the amount of heat released to control the resistance and thermal resistance of the connection. For example, a PCB designer can place traces on the Q2 source pins to increase the amount of copper that connects the source to the ground node.