The SMT assembly process is closely related to each process before welding, including capital input, PCB design, component weldability, assembly operation, flux selection, temperature / time control, solder and crystal structure, etc.
At present, the most commonly used solder for wave soldering is eutectic tin lead alloy: tin 63% and lead 37%. The temperature of solder in the soldering pot should be mastered at all times. The temperature should be higher than the temperature of the alloy at 183 C, and the temperature is uniform. In the past, the temperature of soldering pot at 250 degrees was regarded as “standard“.
With the innovation of flux technology, the uniformity of the solder temperature in the whole solder pot is controlled and the preheater is added. The development trend is to use the solder pot with low temperature. It is common to set solder pot temperature at 230-240 degrees Celsius. Usually, the components do not have uniform thermal quality. It is necessary to ensure that all solder joints reach enough temperature to form qualified solder joints. The important problem is to provide enough heat to raise the temperature of all leads and pads, thus ensuring fluidity of solder and wetting both sides of solder joints. The low temperature of the solder will reduce the thermal shock to the component and the substrate, and help to reduce the formation of the slag. Under the lower strength, the flux coating operation and the flux compound can make the peak of the wave with sufficient flux, thus reducing the burr and welding ball. The solder composition in the solder pot is closely related to the time, which changes with time, which leads to the formation of the scum, which is the reason for the removal of residual and other metal impurities from the welded components and the loss of tin in the welding process.
These factors can reduce the fluidity of the solder. The maximum limit of tin content in metal scum and solder should be specified in the procurement. In the welding process, the requirements for the purity of solder are also specified in the ANSI/J-STD-001B standard. In addition to the limitation of scum, the minimum content of tin in 63% tin and 37% lead alloys should be no less than 61.5%. The concentration of gold and organic swimming layer on the wave soldering assembly is faster than in the past. This aggregation, combined with obvious tin loss, can cause solder loss of fluidity and produce welding problems. Rough and granular solder joints are often caused by scum in solder. Because of the concentrated pomum in the solder pot or the innate and coarse grainy welding spot of the component itself, it may also be a sign of low tin content, not a local special solder, or the result of tin loss in the tin pot. This appearance may also be caused by vibration or shock during solidification.
The appearance of solder joints can directly reflect technological problems or material problems. In order to maintain the “full pot” state of the solder and to analyze the solder pot according to the process control plan, it is very important to analyze the solder pot. The flux in the soldering pan is usually not necessary because of the dross in the soldering pan, which is usually unnecessary, because the solder in the tin pot is required in the conventional application to make the solder in the tin pan always full. In the case of loss of tin, the addition of pure tin helps to maintain the desired concentration. In order to monitor the compounds in tin pot, routine analysis should be carried out. If tin is added, sampling and analysis should be taken to ensure the correct proportions of solder components. Too much scum is a tricky problem. There is no doubt that there are always scum in the tin pot, especially when welding in the atmosphere. The use of “chip wave peak” is very helpful for high density welded components. Because the surface of the material exposed to the atmosphere is too large to oxidize the solder, more dross will be produced. The solder is covered with a scum layer and the oxidation rate slows down. In welding, more turbulence will occur due to the turbulence and flow of the crest in the tin pot. The recommended method is to skim the scum. If skipping regularly, more scum will be generated and more solder will be consumed. Scum may also be involved in the wave crest, causing the instability or turbulence of the crest. Therefore, it is necessary to give more maintenance to the liquid components in the tin pot. If the amount of solder in the tin pot is allowed to reduce, the scum on the solder will enter the pump, which is likely to happen. Sometimes, granular solder joints are mixed with scum. The scum found initially may be caused by rough crest and may block the pump. Tin pots shall be equipped with adjustable low capacity solder sensors and alarm devices.
Crest is the core of wave crest in wave soldering process. The most critical step is to bring the preheated, coated, and non – dirt metal through the conveyor belt to the welding workstation, contact the solder with a certain temperature, and then heat it, so that the flux will produce chemical reactions, and the solder alloy forms interconnect through the wave peak power. At present, the commonly used symmetrical peak is called the main wave peak, which sets the pump speed, wave peak height, depth of infiltration, transmission angle and transmission speed, so as to provide a full range of conditions to achieve good welding characteristics. The data should be adjusted properly, and the solder should be slowed down and slowly stopped after leaving the crest. PCB with the wave crest running, the solder will be pushed to the outlet. In the most reliable case, the surface tension of solder and the optimal crest operation of the plate can achieve zero relative motion between the components and the crest of the outlet. This shelling area is achieved by removing solder from the plate. Sufficient inclination angle should be provided to avoid defects such as bridge, burr, wire drawing and solder ball. Sometimes, the outlet of the crest needs hot air flow to ensure that possible bridging is eliminated. When the surface mount element is installed at the bottom of the plate, sometimes, the bubbles in the “harsh wave peak” region formed in the back of the flux, and the wave peak is used before the wave peak is leveled. The high vertical velocity of the turbulent crest helps to ensure the contact between the solder and the lead or pad. The vibration part behind the leveling laminar peak can also be used to eliminate bubbles and ensure satisfactory contact components of the solder. The welding workstation should basically achieve: high purity solder (according to standard), wave peak temperature (230~250 degrees C), total time of contact peak (3~5 seconds), depth (50 ~ 80%) of PCB immersed in wave peak (50 ~ 80%), to achieve parallel transmission track and flux content in tin pot in parallel state of wave peak and orbit. Cooling after wave soldering is usually equipped with cooling workstations at the end of the wave soldering machine. It is a trend to restrict the formation of solder joints between copper and tin intermetallic compounds, and the other is to accelerate the cooling of the components, and to avoid the plate displacement when the solder is not fully solidified. The rapid cooling assembly is designed to limit the exposure of sensitive components to high temperatures. However, the danger of corrosive cooling system to thermal shock of components and solder joints should be taken into account. A well controlled “soft, stable” and forced gas cooling system should not damage most components. There are two reasons for using this system: it can process the board quickly without hand holding, and ensures that the temperature of the component is lower than that of the cleaning solution. People are concerned about the latter reason, which may be responsible for the foaming of some flux residues. Another phenomenon is that sometimes there is a reaction with some flux dross, so that residues can not be washed away. No formula can meet the requirements of all machines, all designs, all materials and technical material conditions and requirements that are guaranteed to meet the data set by the welding workstation. We 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 the users, every step in the pre welding and welding process must be controlled, because each step of the assembly process of SMT is interrelated and interacted with each other, and any step has the dependability and quality in the whole. Welding operation is also the case, so all parameters, time / temperature, solder volume, flux composition and transmission speed should be strictly controlled. For the defects produced in the welding, the causes should be identified early, and the corresponding measures should be taken to eliminate all the defects that affect the quality in the bud. In this way, the products produced can be guaranteed.