SMT Assembly Process (2)

2 Crests

In the wave soldering process, the wave is the core. Preheated, flux-coated, contaminant-free metal can be sent to a soldering station on a conveyor belt, contacted with solder with a certain temperature, and then heated, so that the flux will chemically react, and the solder alloy will be interconnected by wave power. This is the most critical step. At present, the commonly used symmetrical crest is called the main crest, and the pump speed, crest height, wetting depth, conveying angle and conveying speed are set to provide all-round conditions for achieving good welding characteristics. The data should be properly adjusted and the solder run should be slowed down just after leaving the wave (outlet end) and then slowly stopped. The PCB eventually pushes the solder to the outlet as the wave travels. In the most hung situation, the surface tension of the solder and the wave operation of the optimized board allow zero relative motion between the component and the wave at the outlet end. This de-shell area is what achieves the removal of solder from the board. Sufficient inclination should be provided to avoid defects such as bridging, burrs, wire drawing and solder balls. Hot air flow is sometimes required at the crest outlet to ensure that possible bridging is excluded. After mounting surface mount components on the bottom of the board, sometimes turbulent chip crests are used before wave leveling is performed to compensate for air bubbles in the flux or “harsh crest” areas that form behind. The high vertical velocity of the turbulent wave crest helps to ensure solder-lead or pad contact. The vibrating section behind the flattened laminar wave can also be used to eliminate air bubbles, ensuring that the solder achieves a satisfactory contact assembly. The soldering station should basically do: high-purity solder (according to standard), wave peak temperature (230 ~ 250 ℃), total contact time with wave peak (3 ~ 5 seconds), printed board immersion in wave peak depth (50 ~ 80 %) to achieve parallel conveying tracks and flux content in the tin pot when the wave crest is parallel to the track.

3 Cooling After Wave Soldering

A cooling station is usually added at the tail of the wave soldering machine. This is to limit the tendency of copper-tin intermetallics to form solder joints, another reason is to speed up the cooling of the assembly and avoid board displacement when the solder is not fully solidified. Cool components quickly to limit exposure of sensitive components to high temperatures. However, consideration should be given to the hazard of thermal shock to components and solder joints by aggressive cooling systems. A well-controlled “soft and stable” forced air cooling system should not damage most components. There are two reasons to use this system: to be able to handle the plates quickly without holding them by hand, and to keep the components cooler than the cleaning solution. Of concern is the latter cause, which may be responsible for the blistering of some flux residues. Another phenomenon is that it sometimes reacts with some flux dross, thus making the residue “unwashable”. There is no formula that can meet these requirements in terms of ensuring that the data set at the welding station meets all the machines, all designs, all materials used, and process material conditions and requirements. Every step of the entire process must be understood. 4 Conclusions In a word, in order to obtain the best welding quality and meet the needs of users, it is necessary to control each process step before and during welding because each step of the entire assembly process of SMT is related and interacts with each other. Problems will affect the overall reliability and quality. The same is true for soldering operations, so all parameters, time/temperature, solder volume, flux composition, transfer speed, etc., should be strictly controlled. For defects in welding, the cause should be identified as soon as possible, analyzed, and corresponding measures should be taken to eliminate all kinds of defects that affect the quality in the bud. In this way, it can be ensured that the products produced meet the technical specifications.