2022-11-23
What are the causes of common defects in SMT welding?
In the SMT production process, we all hope that the quality of the substrate can be in a zero defect state from the installation process to the end of the welding process, but in fact, this is difficult to achieve. Because there are many SMT production processes, we cannot guarantee that each process will not have any error.
Therefore, we will encounter some welding defects during SMT production. These welding defects are usually caused by a variety of reasons. For each defect, we should analyze its root cause, so as to achieve the goal when eliminating these defects.
1. Bridging
Bridging usually occurs on integrated circuits with dense pins or between chip components with small spacing. This defect is the main defect in our inspection standard, which will seriously affect the electrical performance of the product, so it must be eliminated.
The main reasons for bridging are excessive solder paste or dislocation and edge collapse after solder paste printing.
2. Excessive solder paste
Excessive solder paste is caused by improper template thickness and opening size. Generally, 0.15mm thick formwork is used. The opening size is determined by the minimum pin or chip element spacing.
3. Misplacement of printing
The printed boards with the spacing between printing pins or chip elements less than 0.65mm shall adopt optical positioning, and the reference point shall be set on the diagonal of the printed board. If optical positioning is not adopted, positioning error will lead to misplacement of printing, resulting in bridging.
4. Edge collapse of solder paste
There are three phenomena that cause the solder paste to collapse:
(1) Printing collapse
The edges collapse when the solder paste is printed. This is closely related to the characteristics of solder paste, template and printing parameter settings: solder paste has low viscosity, poor shape retention, and is easy to collapse and bridge after printing; If the template hole wall is rough and uneven, the printing solder paste is also easy to collapse and bridge; Excessive scraper pressure will have a greater impact on the solder paste, the shape of the solder paste will be damaged, and the probability of edge collapse will be greatly increased.
Countermeasures: Select the solder paste with high viscosity; Adopt laser cutting template; Reduce the scraper pressure.
(2) Edge collapse during installation
When mounting SOP and QFP integrated circuits, the mounting pressure should be set appropriately. Excessive pressure will change the shape of solder paste and cause edge collapse.
Countermeasures: Adjust the mounting pressure and set the lowering position of the mounting nozzle including the thickness of the element itself.
(3) Edge collapse during welding heating
Edge collapse also occurs during welding heating. When the PCB assembly is heated rapidly, the solvent component in the solder paste will volatilize. If the volatilization speed is too fast, solder particles will be squeezed out of the welding area, forming edge collapse during heating.
Countermeasures: set appropriate welding temperature curve (temperature, time) and prevent mechanical vibration of conveyor belt.
Therefore, we will encounter some welding defects during SMT production. These welding defects are usually caused by a variety of reasons. For each defect, we should analyze its root cause, so as to achieve the goal when eliminating these defects.
1. Bridging
Bridging usually occurs on integrated circuits with dense pins or between chip components with small spacing. This defect is the main defect in our inspection standard, which will seriously affect the electrical performance of the product, so it must be eliminated.
The main reasons for bridging are excessive solder paste or dislocation and edge collapse after solder paste printing.
2. Excessive solder paste
Excessive solder paste is caused by improper template thickness and opening size. Generally, 0.15mm thick formwork is used. The opening size is determined by the minimum pin or chip element spacing.
3. Misplacement of printing
The printed boards with the spacing between printing pins or chip elements less than 0.65mm shall adopt optical positioning, and the reference point shall be set on the diagonal of the printed board. If optical positioning is not adopted, positioning error will lead to misplacement of printing, resulting in bridging.
4. Edge collapse of solder paste
There are three phenomena that cause the solder paste to collapse:
(1) Printing collapse
The edges collapse when the solder paste is printed. This is closely related to the characteristics of solder paste, template and printing parameter settings: solder paste has low viscosity, poor shape retention, and is easy to collapse and bridge after printing; If the template hole wall is rough and uneven, the printing solder paste is also easy to collapse and bridge; Excessive scraper pressure will have a greater impact on the solder paste, the shape of the solder paste will be damaged, and the probability of edge collapse will be greatly increased.
Countermeasures: Select the solder paste with high viscosity; Adopt laser cutting template; Reduce the scraper pressure.
(2) Edge collapse during installation
When mounting SOP and QFP integrated circuits, the mounting pressure should be set appropriately. Excessive pressure will change the shape of solder paste and cause edge collapse.
Countermeasures: Adjust the mounting pressure and set the lowering position of the mounting nozzle including the thickness of the element itself.
(3) Edge collapse during welding heating
Edge collapse also occurs during welding heating. When the PCB assembly is heated rapidly, the solvent component in the solder paste will volatilize. If the volatilization speed is too fast, solder particles will be squeezed out of the welding area, forming edge collapse during heating.
Countermeasures: set appropriate welding temperature curve (temperature, time) and prevent mechanical vibration of conveyor belt.
