A good reflux curve should be a temperature curve that can achieve good welding of various surface mount components on the PCB board to be welded, and the solder joint not only has good appearance quality but also good internal quality. In order to achieve a good lead-free reflow temperature curve, there is a certain relationship with all the production processes of lead-free reflow. Below, Chengyuan Automation will talk about the reasons for poor cold welding or wetting of lead-free reflow spots.
In lead-free reflow welding process, there is an essential difference between the dull luster of lead-free reflow solder joints and the dull phenomenon caused by incomplete melting of solder paste. When the board coated with solder paste passes through the furnace of high temperature gas, if the peak temperature of the solder paste can not be reached or the reflux time is not enough, the activity of the flux will not be released, and the oxides and other substances on the surface of the solder pad and the component pin can not be purified, resulting in poor wetting during lead-free reflow welding.
The more serious situation is that due to the insufficient set temperature, the welding temperature of the solder paste on the surface of the circuit board can not reach the temperature that must be achieved for the metal solder in the solder paste to undergo phase change, which leads to the cold welding phenomenon at the lead-free reflow welding spot. Or because the temperature is not enough, some residual flux inside the solder paste can not be volatilized, and it precipitates inside the solder joint when it is cooled, resulting in a dull luster of the solder joint. On the other hand, due to the poor properties of the solder paste itself, even if other relevant conditions can meet the requirements of the temperature curve of lead-free reflow welding, the mechanical properties and appearance of the solder joint after welding cannot meet the requirements of the welding process.
Post time: Jan-03-2024