SMT assembly process

- Oct 20, 2018-

1 solder

At present, the most commonly used solder for wave soldering is eutectic tin-lead alloy: 63% tin;Lead 37%, should always keep solder in solder pot temperature, its temperature should be higher than that of alloy liquid temperature 183 ℃, and the temperature uniformity.In the past, the temperature of 250 ℃ soldering pot is regarded as the "standard". 

With the innovation of solder technology, the uniformity of solder temperature in the whole solder pot is controlled, and the preheater is added.In the range of 230-240 ℃ set temperature of the solder pot is very common.Generally, components do not have uniform thermal mass and it is necessary to ensure that all solder joints are at sufficient temperature to form qualified solder joints.The important issue is to provide enough heat to raise the temperature of all leads and pads to ensure fluidity of solder and wet both sides of solder joints.Low solder temperature will reduce the thermal impact on the elements and substrate, help to reduce the formation of dross, in the lower intensity, the flux coating operation and the flux compound together, so that the wave crest outlet has enough flux, thereby reducing burrs and solder ball production. 

The solder composition in the solder pot is closely related to time, that is, it changes with time, thus resulting in the formation of scum, which is the reason to remove residue and other metal impurities from the welded components and the cause of tin loss in the welding process.These factors can reduce the fluidity of solder.The maximum limit of tin content of metallic micro scum and solders to be specified in the procurement shall be specified in each standard (as defined in IPC/ j-std-006).During welding, the requirement for solder purity is also specified in the ANSI/ j-std-001b standard.In addition to scum restrictions, for 63% tin;The prescribed tin content of 37% lead alloy shall not be lower than 61.5%.The concentration of copper in the gold and organic electrophoresis layer on the wave solder assembly is faster than in the past.This accumulation, coupled with significant tin loss, can make the solder fluidity and generate welding problems.The coarse, granular solder spots are often caused by the dross in the solder.Dark, coarse granular solder spots, due to the accumulated scum or inherent residue of the solder pot, may also be a sign of low tin content, either locally specific solder joints or as a result of tin loss in the solder pot.This appearance may also be caused by vibration or shock during solidification. 

The appearance of the solder can directly reflect process or material problems.It is important to check the solder pot analysis in order to keep the solder "full" and follow the process control plan.It is usually unnecessary to "dump" the solder in the solder pot due to the dross in the solder pot. The solder in the solder pot is always full due to the need to add solder to the solder pot in the normal application.In the case of loss of tin, adding pure tin helps to maintain the desired concentration.In order to monitor the chemical compounds in the tin pan, routine analysis should be performed.If solder is added, sample and analyze to ensure correct solder composition.Too much scum is another thorny problem.There is no doubt that there is always scum in the solder pan, especially when welding in the atmosphere.The use of "chip crests" is useful for welding high-density components, since the solder's surface exposed to the atmosphere is so large that it oxidizes the solder, resulting in more scum.With a scum covering the solder surface, the oxidation rate slows down. 

During welding, more scum is produced due to the turbulence and flow of the wave crest in the tin pan.The conventional method recommended is to skim the scum, which, if skimmed regularly, will produce more scum and require more solder.Scum may also be trapped in the crests, resulting in instability or turbulence in the crests, requiring more maintenance of the liquid component in the solder pot.If the solder pot is allowed to reduce the amount of solder, the dross on the solder surface will enter the pump, which is likely to occur.Sometimes, there is dross in granular solder joints.The first scum found may have been caused by rough crests and may have blocked the pump.The tin pan should be equipped with adjustable low capacity solder sensor and alarm device.