Several hidden dangers of PCB screen printing affecting installation and debugging

The processing of silk screen in PCB design is a link that is easily overlooked by engineers. Generally, everyone does not pay much attention to it and handles it at will, but random at this stage can easily lead to problems in the installation and debugging of board components in the future, or even complete destruction. Drop your entire design.



1. The device label is placed on the pad or via
In the placement of device number R1 in the figure below, “1” is placed on the pad of the device. This situation is very common. Almost every engineer has made this mistake when initially designing the PCB, because it is not easy to see the problem in the design software. When the board is obtained, it is found that the part number is marked by the pad or is too empty. Confused, it is impossible to tell.

2. The device label is placed under the package

For U1 in the figure below, maybe you or the manufacturer has no problem when installing the device for the first time, but if you need to debug or replace the device, you will be very depressed and can’t find where U1 is. U2 is very clear and is the correct way to place it.

3. The device label does not clearly correspond to the corresponding device

For R1 and R2 in the following figure, if you don’t check the design PCB source file, can you tell which resistance is R1 and which is R2? How to install and debug it? Therefore, the device label must be placed so that the reader knows its attribution at a glance, and there is no ambiguity.

4. Device label font is too small

Due to the limitation of board space and component density, we often have to use smaller fonts to label the device, but in any case, we must ensure that the device label is “readable”, otherwise the meaning of the device label will be lost. In addition, different PCB processing plants have different processes. Even with the same font size, the effects of different processing plants are very different. Sometimes, especially when making formal products, in order to ensure the effect of the product, you must find the processing accuracy. High manufacturers to process.

The same font size, different fonts have different printing effects. For example, the default font of Altium Designer, even if the font size is large, it is difficult to read on the PCB board. If you change to one of the “True Type” fonts, Even if the font size is two sizes smaller, it can be read very clearly.

5. Adjacent devices have ambiguous device labels
Look at the two resistors in the figure below. The package library of the device has no outline. With these 4 pads, you cannot judge which two pads belong to a resistor, let alone which is R1 and which is R2. NS. The placement of the resistors may be horizontal or vertical. Wrong soldering will cause circuit errors, or even short circuits, and other more serious consequences.

6. The placement direction of the device label is random
The direction of the device label on the PCB should be in one direction as much as possible, and at most two directions. Random placement will make your installation and debugging very difficult, because you need to work hard to find the device you need to find. The component labels on the left in the figure below are placed correctly, and the one on the right is very bad.

7. There is no Pin1 number mark on the IC device
IC (Integrated Circuit) device package has a clear start pin mark near Pin 1, such as a “dot” or “star” to ensure the correct orientation when the IC is installed. If it is installed backwards, the device may be damaged and the board may be scrapped. It should be noted that this mark cannot be placed under the IC to be covered, otherwise it will be very troublesome to debug the circuit. As shown in the figure below, it is difficult for U1 to judge which direction to place, while U2 is easier to judge, because the first pin is square and the other pins are round.

8. There is no polarity mark for polarized devices
Many two-leg devices, such as LEDs, electrolytic capacitors, etc., have polarity (direction). If they are installed in the wrong direction, the circuit will not work or even the device will be damaged. If the direction of the LED is wrong, it will definitely not light up, and the LED device will be damaged due to voltage breakdown, and the electrolytic capacitor may explode. Therefore, when constructing the package library of these devices, the polarity must be clearly marked, and the polarity marking symbol cannot be placed under the outline of the device, otherwise the polarity symbol will be blocked after the device is installed, causing difficulty in debugging. C1 in the figure below is wrong, because once the capacitor is installed on the board, it is impossible to judge whether its polarity is correct, and the way of C2 is correct.

9. No heat release
Using heat release on the component pins can make soldering easier. You may not want to use thermal relief to reduce electrical resistance and thermal resistance, but not using thermal relief can make soldering very difficult, especially when the device pads are connected to large traces or copper fills. If proper heat release is not used, large traces and copper fillers as heat sinks may cause difficulty in heating the pads. In the figure below, the source pin of Q1 has no heat release, and the MOSFET may be difficult to solder and desolder. The source pin of Q2 has a heat release function, and the MOSFET is easy to solder and desolder. PCB designers can change the amount of heat release to control the resistance and thermal resistance of the connection. For example, PCB designers can place traces on the Q2 source pin to increase the amount of copper connecting the source to the ground node.