The SMT assembly process is closely related to each process step before welding, including capital investment, PCB design, component solderability, assembly operation, flux selection, temperature/time control, solder and crystal structure, etc.
solder
At present, the most commonly used solder for wave soldering is eutectic tin lead alloy: tin 63%; Lead is 37%, and the temperature of the solder in the solder pot should be controlled at all times. The temperature should be 183 ℃ higher than the temperature of the alloy liquid, and the temperature should be uniform. In the past, a soldering pot temperature of 250 ℃ was considered “standard”.
With the innovation of flux technology, the uniformity of solder temperature in the entire solder pot has been controlled, and a preheater has been added. The development trend is to use lower temperature solder pots. It is common to set the temperature of the soldering pot within the range of 230-240 ℃. Usually, components do not have uniform thermal mass, and it is necessary to ensure that all solder joints reach sufficient temperature to form qualified solder joints. The important issue is to provide sufficient heat to increase the temperature of all leads and pads, thereby ensuring the fluidity of the solder and wetting both sides of the solder joint. A lower temperature of solder will reduce the Thermal shock to components and substrates, helping to reduce the formation of dross. Under a lower strength, the joint action of flux coating operation and flux compounds can make the wave crest outlet have enough flux, so that the generation of burrs and solder balls can be reduced.
The composition of the solder in the solder pot is closely related to time, that is, it changes over time, which leads to the formation of scum. This is why residues and other metal impurities need to be removed from the welded components and the reason for tin loss in the welding process. The above factors can reduce the fluidity of solder. In procurement, the maximum limit for the tin content of trace metal dross and solder should be specified in various standards, such as IPC/J-STD -006. During the welding process, the requirements for the purity of the solder are also specified in the ANSI/J-STD-001B standard. In addition to the limitation on dross, for 63% tin; The minimum tin content specified in 37% lead alloy shall not be less than 61.5%. The concentration of gold and organic layer copper on the wave soldering component accumulates faster than in the past. This aggregation, combined with significant tin loss, can cause the solder to lose fluidity and cause welding problems. The rough and granular appearance of solder joints is often caused by dross in the solder. Due to the accumulation of dross in the solder pot or the inherent residues of the components themselves, dull and rough granular solder joints may also be a sign of low tin content, either as local special solder joints or as a result of tin loss in the solder pot. This appearance may also be caused by vibration or impact during the solidification process.
The appearance of the solder joint can directly reflect the process or material issues. It is important to maintain the “full pot” state of the solder and analyze the solder pot according to the process control plan. Due to the presence of dross in the solder pot, it is usually unnecessary to “pour” the solder from the solder pot. In conventional applications, it is required to add solder to the pot, so that the solder in the pot is always full. Adding pure tin helps to maintain the required concentration in the case of tin loss. To monitor the compounds in the tin pot, routine analysis should be performed. If tin is added, sampling analysis should be conducted to ensure the correct proportion of solder composition. Excessive scum is another thorny problem. Undoubtedly, there is always scum in the solder pot, especially when welding in the atmosphere. The use of ‘chip peaks’ is very helpful for welding high-density components, as the surface of the solder exposed to the atmosphere is too large, which oxidizes the solder and produces more scum. The surface of the solder in the solder pot is covered by a layer of scum, which slows down the oxidation rate.
During welding, more scum is generated due to the turbulence and flow of the wave peaks in the tin pot. The recommended routine method is to skim off the dross. If skimmed frequently, it will produce more dross and consume more solder. Dross may also be mixed in the wave crest, causing instability or turbulence of the wave crest, therefore requiring more maintenance of the liquid composition in the solder pot. If it is allowed to reduce the amount of solder in the tin pot, the scum on the surface of the solder will enter the pump, which is likely to occur. Sometimes, granular solder joints may be mixed with dross. The initial discovery of scum may have been caused by rough wave peaks and may have blocked the pump. The tin pot should be equipped with adjustable low capacity solder sensors and alarm devices.
peaks
In the wave soldering process, the wave crest is the core. Preheated, flux coated, and dirt free metal can be transported to the welding workstation through a conveyor belt to come into contact with solder at a certain temperature, and then heated. This way, the solder will undergo a chemical reaction, and the solder alloy will form interconnections through peak power, which is the most critical step. At present, the commonly used symmetrical wave peak is called the main wave peak, which sets the pump speed, wave peak height, infiltration depth, transmission angle, and transmission speed to provide comprehensive conditions for achieving good welding characteristics. Appropriate adjustments should be made to the data, and the solder should be slowed down and slowly stopped after leaving the peak (outlet end). The PCB will eventually push the solder to the outlet with the peak operation. In the most hanging case, the surface tension of the solder and the optimized peak operation of the plate can achieve zero relative motion between the peaks of the component and the outlet end. This shelling area achieves the removal of solder from the board. Adequate inclination should be provided to avoid defects such as bridging, burrs, wire drawing, and solder balls. Sometimes, the peak outlet needs to have hot air flow to ensure the elimination of possible bridging. After installing surface mount components at the bottom of the board, sometimes compensating for the solder flux or the formation of bubbles in the “harsh peak” area at the back, and before smoothing the peak, turbulent chip peaks are used. The high vertical velocity of the turbulent wave crest helps to ensure contact between the solder and the lead or pad. The vibration part behind the flattened laminar wave peak can also be used to eliminate bubbles, ensuring satisfactory contact between the solder and the component. The welding workstation should basically achieve: high-purity solder (according to the standard), peak temperature (230-250 ℃), total time of contact with the peak (3-5 seconds), depth of immersion of the printed board into the peak (50-80%), parallel transmission tracks, and flux content in the tin pot in a state where the peak is parallel to the track.
Cooling after wave soldering
Usually, a cooling workstation is added at the tail of the wave soldering machine. To limit the trend of copper tin intermetallic compounds forming solder joints, another reason is to accelerate the cooling of components and avoid board displacement when the solder is not fully cured. Quickly cool the components to limit the exposure of sensitive components to high temperatures. However, the harm of corrosive cooling system to Thermal shock of components and solder joints shall 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 manual clamping, and can ensure that the component temperature is lower than the temperature of the cleaning solution. People are concerned about the latter reason, which may be the cause of foaming of certain flux residues. Another phenomenon is that sometimes there is a reaction with certain flux dross, which makes the residue “unable to be cleaned”. No formula can meet these requirements in terms of ensuring that the data set up at the welding workstation meets all machines, designs, materials used, and process material conditions and requirements. It is necessary to understand every step of the entire process. In summary, in order to achieve the best welding quality and meet the needs of users, it is necessary to control every process step before and during welding, because each step of the entire assembly process of SMT is interrelated and interacted with each other, and any problems in any step will affect the overall reliability and quality. The same applies to welding operations, so all parameters, time/temperature, solder quantity, flux composition, and transfer speed should be strictly controlled. Defects generated during welding should be identified and analyzed as soon as possible, and corresponding measures should be taken to eliminate various defects that affect quality in their embryonic state. Only in this way can we ensure that the products produced comply with technical specifications.
