In the SMT production process, we all hope that from the installation process to the end of the soldering process, the quality of the substrate can be in a state of zero defects, but in fact, it is very difficult to achieve. Because there are many SMT production processes, we can not guarantee that each process will not have a little error. Therefore, we will encounter some solder defects in the SMT production process. These solder defects are usually caused by multiple reasons. For each defect, we should analyze its root cause in order to achieve our goal in eliminating these defects.
Bridging usually occurs on pin-dense integrated circuits or between chip components with small spacing. This defect is the main defect in our inspection criteria and will seriously affect the electrical performance of the product, so it must be eliminated.
The main cause of bridging is due to solder paste overload or misalignment and collapse of the solder paste after printing.
Solder paste overload
Solder paste overload is caused by improper thickness and opening size of the template. Generally use 0.15mm thick stencil. The size of the opening is determined by the minimum pin or sheet component spacing.
The printed board with the printed pin pitch or sheet component pitch less than 0.65mm should be optically positioned, and the reference point should be set on the diagonal of the printed board. If optical positioning is not used, the positioning error will lead to printing misalignment and cause bridging.
Solder paste collapse edge
The following three phenomena cause the collapse of the solder paste
1. Printing collapse
The edge of the solder paste collapses when it is printed. This is closely related to the characteristics of the solder paste, template and printing parameters settings: low viscosity of solder paste, poor conformability, easy to collapse and bridging after printing; if the template hole wall is rough and uneven, the printed solder paste is also easy to collapse, bridging; scraper pressure will have a greater impact on the solder paste, the shape of the paste will be damaged, the probability of edge collapse will greatly increase.
Countermeasure: choose the solder paste with higher viscosity; adopt laser cutting template; reduce scraper pressure.
2. Collapse edge when mounting
When mount machine in mount SOP, QFP class integrated circuit, its mount pressure should be set properly. The pressure is too big to make the solder paste shape change and collapse the edge.
Countermeasure: adjust the mounting pressure and set the falling position of the mounting nozzle including the thickness of the component itself.
3. The collapsed edge when the solder is heated
Edge collapse can also occur during soldering heating. When the printed board assembly is heated rapidly, the solvent component in the solder paste will evaporate. If the volatilization rate is too fast, the solder particles will be squeezed out of the solder area, forming edge collapse during heating.
Countermeasure: Set the proper soldering temperature profile (temperature, time) and prevent mechanical vibration of the conveyor belt.