SMT assembly process

- Aug 18, 2018-

SMT assembly process is closely related to each process step before welding, including capital investment, PCB design, component weldability, assembly operation, flux selection, temperature/time control, solder and crystal structure, etc. 

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. 

Two peaks

In the wave soldering process, the wave peak is the core.The preheated, flux-coated, non-soiled metal can be sent to the welding station through the conveyor belt to contact the solder with a certain temperature, and then heated, so that the flux will have a chemical reaction, solder alloys through wave peak power interconnection, which is the most critical step.At present, the commonly used symmetric wave crest is called the main wave crest, and the pump speed, wave peak height, infiltration depth, transmission Angle and transmission speed are set to provide all-round conditions for achieving good welding characteristics.The data should be properly adjusted, and the solder should be slowed down after leaving the crest and slowly stopped.The PCB will eventually push the solder to the outlet as the wave crests run.Under the most suspended condition, the solder's surface tension and the crest of the optimized plate run, achieving zero relative motion between the crest of the component and the exit end.This dehulling area is achieved by removing the solder from the plate.Sufficient dip Angle shall be provided without defects such as bridge, burr, wire drawing and soldering ball.Sometimes, the crest outlet needs to have a hot air current to ensure that the possible bridging is excluded.After mounting the top surface at the bottom of the plate to attach the element, sometimes to compensate for the flux or bubbles forming in the "harsh crests" area behind the plate, while the turbulent chip crests are used before the crests are levelled.The high vertical velocity of a turbulent crest helps to ensure that the solder is in contact with the lead or pad.The vibration part behind the levelled laminar wave crest can also be used to eliminate air bubbles and ensure the solder to achieve satisfactory contact components.Welding workstation should be basically: high purity solder (according to the standard), peak temperature (230 ~ 250 ℃), contact crest the total time (3 ~ 5 seconds), depth of PCB in wave (50 ~ 80%), parallel transfer orbit and wave parallel to orbit in the state of flux content in tin pan. 

Cooling after welding of 3 wave crests

A cooling station is usually installed at the end of the wave crest welding machine.The aim is to limit the tendency of copper-tin compounds to form solder joints, and another reason is to accelerate the cooling of the components, avoiding plate displacement while the solder is not fully solidified.Quick cooling module to limit sensitive element exposure to high temperature.However, the hazards of the thermal shock of the erosive cooling system to the elements and solder joints should be considered.A well-controlled "soft and stable" forced gas cooling system should not damage most components.There are two reasons for using this system: it can quickly process the board without holding it by hand, and it can guarantee that the temperature of the component is lower than the temperature of the cleaning solution.People are concerned about the latter cause, which may cause some flux residue to bubble.Another phenomenon is sometimes the appearance of a reaction with some of the flux dross, so that the residue "can not be cleaned out."No formula can meet these requirements in terms of ensuring that the data set up in the welding station meets all machines, all designs, all materials used and process material conditions and requirements.You must understand every step of the whole process.4 conclusion in conclusion, in order to obtain the best welding quality and meet the needs of users, every process step before and during welding must be controlled, because every step of SMT's entire assembly process is interrelated and interacted with each other, and any problem in any step will affect the overall reliability and quality.The same is true for welding operations, so all parameters, time/temperature, solder volume, flux composition, transmission speed, and so on should be strictly controlled.The defects in welding shall be identified as early as possible, analyzed, and corresponding measures shall be taken to eliminate the defects affecting quality in the bud state.In this way, we can ensure that the products produced conform to the technical specifications.