SMT PCB assembly process

- Jul 26, 2018-

SMT assembly process is closely related to each process step before welding, including capital investment, PCB design, element weldability, assembly operation, flux selection, temperature/time control, solder and crystal structure.
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 flux technology, the temperature uniformity of solder in the whole solder pot is controlled, and a preheater is added.In the range of 230-240 ℃ set temperature of the solder pot is very common.In general, the components do not have uniform thermal quality and it is necessary to ensure that all soldering points reach sufficient temperature to form qualified soldering points.The important issue is to provide enough heat to increase the temperature of all leads and pads to ensure the fluidity of the solder and wet both sides of the solder joint.The lower the temperature of the solder will reduce the thermal shock to the element and the substrate, which will help to reduce the formation of the scum. Under the lower strength, the joint action of the solder coating operation and the flux compound can make the crest outlet have enough flux, so as to reduce the generation of burrs and solder balls.
The composition of solder in solder pot is closely related to time, that is, it changes with time, which leads to the formation of scum, which is the reason to remove the residue and other metal impurities from the welded components and the reason of tin loss in the welding process.The above factors can reduce the flux of solder.In procurement, the maximum tin content of the metal trace scum and solder shall be specified in each standard (such as IPC/ j-std-006).In the welding process, the requirement for solder purity is also stipulated in the ANSI/ j-std-001b standard.In addition to the restrictions on slag, 63% tin;The minimum tin content in 37% lead alloys is not less than 61.5%.The concentration of gold and organic swimming layer copper on the wave solder assembly is higher than in the past.This aggregation, coupled with significant tin losses, can result in a loss of fluidity in the solder and lead to welding problems.Rough and granular solder joints are often caused by slag in the solder.Poor, coarse, granular soldering spots may also be a sign of low tin content due to the accumulation of scum in the solder pot or the inherent residue of the components themselves, either locally specific soldering points or as a result of tin loss in the tin pot.This appearance may also be due to vibration or shock during solidification.
The appearance of the solder joint can directly reflect the process problem or material problem.It is important to check solder pot analysis in order to keep the solder "full" state and follow the process control scheme.It is usually unnecessary to "throw out" the soldering agent in the soldering tin pot because of the floating slag in the soldering tin pot. The soldering material in the tin pot is always full due to the requirement of adding soldering material to the tin pot in the conventional application.In the case of loss of tin, adding pure tin helps maintain the required concentration.In order to monitor the compounds in the tin pan, routine analysis should be performed.If tin is added, it should be sampled and analyzed to ensure the correct proportion of solder composition.Too much scum is another thorny problem.There is no doubt that there is always scum in the solder pot, especially when welding in the atmosphere.The use of "chip crest" is helpful for welding high-density components, as the surface of the solder exposed to the atmosphere is too large to oxidize the solder, resulting in more scum.In the solder pot, the surface of the solder is covered with slag layer, and the oxidation speed is slowed down.
In welding, more slag will be produced due to the turbulence and flow of wave peaks in the tin pot.The recommended general method is to skim off the scum. If skimming is done frequently, more scum will be produced and more solder will be consumed.The slag may also be mixed in the wave peak, causing the wave peak to be unstable or turbulent, thus requiring more maintenance of the liquid components in the solder pot.If the amount of solder in the tin pan is allowed to be reduced, the slag on the solder surface will enter the pump, which is likely to happen.Sometimes, granulated solder joints are mixed with scum.The original scum may have been caused by rough peaks and may have blocked the pump.The tin pan should be equipped with adjustable low capacity solder sensor and alarm device.
Two peaks
Wave peak is the core of wave peak welding technology.The preheated metal, coated with flux and free of dirt, can be sent to the welding workstation through the conveyor belt to contact the solder with a certain temperature, and then heated. In this way, the flux will react chemically.At present, the commonly used symmetric wave peak is called the main wave peak, and the pump speed, wave peak height, infiltration depth, transmission Angle and transmission speed are set to provide comprehensive conditions for achieving good welding characteristics.The data should be properly adjusted, and the solder should be run at a reduced speed after leaving the peak (the outlet end) and stop running slowly.The PCB will eventually push the solder to the outlet as the wave peaks.In the most hung case, the surface tension of the solder and the wave peak running of the optimized plate can realize zero relative motion between the wave peak of the component and the outlet end.This peeling area is used to remove the solder from the plate.Sufficient dip angles shall be provided to avoid defects such as bridge joints, burrs, wire drawing and soldering balls.In some cases, a hot air flow is required at the outlet of the crest to ensure that possible bridge connections are eliminated.When a surface mount element is placed at the bottom of the plate, it is sometimes used to compensate the flux or the bubbles in the "severe crest" area formed behind it, and before the wave crest is flattened, the turbulent chip crest is used.The high vertical speed of turbulent wave crest helps to ensure the contact between solder and lead wire or disc.The vibration part behind the leveling laminar flow crest can also be used to eliminate bubbles and ensure that the solder achieves a satisfactory contact component.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.
3 cooling after wave peak welding
A cooling station is usually set up at the end of the wave soldering machine.The other reason is to accelerate the cooling of the components and prevent the plate from shifting when the solder is not fully solidified.Rapid cooling of components to limit exposure to high temperatures.However, consideration should be given to the damage of corrosive cooling systems to the thermal shock of components and solder joints.A well-controlled "soft and stable" forced gas cooling system should not damage most components.There are two reasons for using this system: the ability to quickly process the plate without holding it by hand, and the ability to ensure that the component temperature is lower than that of the cleaning solution.People are concerned about the latter cause, which may be the cause of some flux residue blistering.Another phenomenon is that sometimes there will be a reaction with some flux scum, so that the residue "cannot be cleaned".No formula can meet these requirements in terms of ensuring that the data set up by the welding workstation meets all the machines, all the designs, all the materials used and the conditions and requirements of the process materials.You must understand every step of the process.Conclusion in conclusion, in order to obtain the best welding quality and meet the needs of users, every technological 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, amount of solder, flux composition and transmission speed should be strictly controlled.As for the defects in welding, the causes should be ascertained and analyzed as soon as possible, and corresponding measures should be taken to eliminate all kinds of defects affecting quality in the embryonic state.In this way, we can ensure that the products produced conform to the technical specifications.