May 20, 2023 
Reasons
1. The reflow temperature is too low or the residence time at the reflow soldering temperature is too short, resulting in insufficient heat during reflow and incomplete melting of the metal powder.
2. In the cooling stage, the strong cooling air, or the movement of the uneven conveyor belt disturbs the solder joints, and presents uneven shapes on the surface of the solder joints, especially at a temperature slightly lower than the melting point, when the solder is very soft.
3. Surface contamination on and around pads or leads can inhibit flux capability, resulting in incomplete reflow. Sometimes unmelted solder powder can be observed on the surface of the solder joint. At the same time, insufficient flux capacity will also result in incomplete removal of metal oxides and subsequent incomplete condensation.
4. The quality of solder metal powder is not good; most of them are formed by the encapsulation of highly oxidized powder particles.
Solutions for cold welding of SMT processing
1. According to the PCBA board’s size and the board’s thickness, combined with the heat coefficient of the components, set the appropriate reflow temperature and soldering time.
2. Pay special attention to the stability of the transmission of reflow soldering equipment.
3. Use highly active solder paste or appropriately increase the amount of solder paste flux; the incoming inspection system should be strictly controlled, and the storage environment of components and PCB should be paid attention to.
4. Do not use inferior solder paste, formulate a management system for the use of solder paste to ensure the quality of solder paste.
The above content is shared by SZTech-SMT for you about the causes and solutions of cold welding in SMT processing. I hope it will be helpful to you. To learn more about SMT chip processing, welcome to visit www.sztech-smt.com
