The SMT process assembly process is closely related to each process step prior to soldering, including capital investment, PCB design, component solderability, assembly operations, flux selection, temperature/time control, solder and crystal structure.
At present, the most commonly used solder for wave soldering is eutectic tin-lead alloy: tin 63%; lead 37%, the solder temperature in the solder pot should always be known, the temperature should be higher than the alloy liquid temperature 183 ° C, and the temperature is uniform. In the past, solder pot temperatures of 250 ° C were considered "standard".
With the innovation of flux technology, the SMT processing plant has controlled the uniformity of solder temperature in the entire solder pot and added a preheater. The trend is to use a solder pot with a lower temperature. It is common to set the solder pot temperature in the range of 230-240 °C. Usually, the components do not have a uniform thermal mass, and it is necessary to ensure that all solder joints reach a sufficient temperature to form a qualified solder joint. The important issue is to provide enough heat to increase the temperature of all leads and pads to ensure solder flow and wet both sides of the solder joint. Lower solder temperature reduces thermal shock to the component and substrate, helping to reduce scum formation. At lower strengths, the flux coating operation and flux compound work together to provide a peak exit. Sufficient flux to reduce burrs and solder balls.
The solder composition in the solder pot of SMT processing plants is closely related to time, that is, it changes with time, which leads to the formation of dross, which is the reason for removing residue and other metal impurities from the welded components. The cause of tin loss in the soldering process. These factors can reduce the fluidity of the solder. In procurement, the maximum limit of the tin content of the metal micro scum and solder to be specified is specified in each standard (as defined in IPC/J-STD-006). The solder purity requirements are also specified in the ANSI/J-STD-001B standard during the soldering process. In addition to the restrictions on scum, the minimum tin content in 63% tin and 37% lead alloy shall not be less than 61.5%. The concentration of copper in the gold and organic swimming layers on the wave soldering assembly is faster than in the past. This aggregation, combined with significant tin loss, can cause the solder to lose fluidity and create soldering problems. Rough, grainy solder joints are often due to scum in the solder. Due to the accumulation of scum in the solder pot or the inherent residue of the component itself, the faint, coarse grained solder joint may also be a sign of low tin content, not a local special solder joint, or the result of tin loss in the solder pot. This appearance may also be caused by vibration or shock during solidification.
The appearance of solder joints in SMT processing plants can directly reflect process or material problems. It is important to check the solder pot analysis in order to maintain the "full pot" state of the solder and in accordance with the process control scheme. It is usually unnecessary to "pour" the solder in the solder pot due to the scum in the solder pot. Since solder is added to the solder pot in conventional applications, the solder in the solder pot is always full. . In the case of loss of tin, the addition of pure tin helps maintain the desired concentration. In order to monitor the compounds in the tin pot, routine analysis should be performed. If tin is added, it should be sampled and analyzed to ensure that the solder composition ratio is correct. Too much scum is another tricky problem. There is no doubt that there is always scum in the solder pot, especially when soldering in the atmosphere. The use of "chip crests" is very helpful in soldering high-density components. Since the surface of the solder exposed to the atmosphere is too large to oxidize the solder, more scum is generated. The surface of the solder in the solder pot is covered with a scum layer and the oxidation rate is slowed down.
In the welding, more scum is generated due to the turbulence and flow of the crests in the tin pot. The recommended method is to remove the dross. If it is often boring, it will produce more scum and consume more solder. Scum may also be trapped in the crests, causing instability or turbulence in the crests, thus requiring more maintenance of the liquid components in the solder pot. If it is allowed to reduce the amount of solder in the solder pot, the scum on the solder surface will enter the pump, which is likely to occur. Sometimes, the granular solder joints will be mixed with dross. The scum that was originally discovered may be caused by rough peaks and may block the pump. The tin pot should be equipped with an adjustable low-capacity solder sensor and alarm.
The above is some knowledge of the assembly process of the SMT processing plant.