The assembly process is closely related to each process step before soldering, including capital investment, PCB design, component solderability, 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; 37% lead. The temperature of the solder in the solder pot should be controlled at all times. In the past, a solder pot temperature of 250°C was considered the “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, and the development trend is to use a solder pot with a lower temperature. It is common to set the solder pot temperature in the range of 230-240°C. Typically, components do not have uniform thermal mass, and it is necessary to ensure that all solder joints reach a sufficient temperature in order to form acceptable solder joints. The important issue is to provide enough heat to raise the temperature of all leads and pads to ensure that the solder flows and wets both sides of the solder joint. The lower temperature of the solder will reduce the thermal shock to the components and the substrate, which will help to reduce the formation of dross. Under the lower strength, the flux coating operation and the flux compound can make the wave outlet have a Sufficient flux, which reduces burrs and solder balls.

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 dross, which is the reason for removing residues and other metal impurities from the soldered components and during the soldering process. Cause of tin loss. These factors above can reduce the fluidity of the solder. In procurement, the maximum limit of metal trace dross and tin content of solder to be specified is in each standard (such as IPC/J-STD-006 has clear regulations). During the soldering process, the requirements for the purity of the solder are also specified in the ANSI/J-STD-001B standard. In addition to the restriction on scum, the minimum tin content in 63% tin; 37% lead alloy shall not be less than 61.5%. Gold and organic layer copper concentrations build up faster than in the past on wave soldered assemblies. This accumulation, combined with significant tin loss, can cause the solder to lose fluidity and cause soldering problems. Rough, grainy solder joints are often caused by dross in the solder. Dull, rough grainy solder joints due to accumulated scum in the solder pot or inherent residue from the component itself can also be a symptom of low tin content, either localized specialty solder joints or the result of tin loss in the solder pot. This appearance may also be caused by vibration or shock during the setting process.

The appearance of solder joints can directly reflect process problems or material problems. It is important to check the solder pot analysis in order to maintain a “full pot” of solder and follow the process control plan. “Dumping” the flux in the solder pot due to scum in the solder pot is usually unnecessary, since the solder pot is always filled to the brim with conventional applications . In the case of depletion of tin, the addition of pure tin helps to maintain the desired concentration. In order to monitor the compounds in the tin pot, routine analysis should be performed. If tin is added, it should be sampled and analyzed to ensure that the solder composition is correct. Excessive scum is another vexing problem. There is no doubt that there is always dross in the solder pot, especially when soldering in the atmosphere. Use “chip crests” which are helpful when soldering high-density assemblies, as the solder surface exposed to the atmosphere is too large, which oxidizes the solder and therefore creates more dross. The surface of the solder in the solder pot is covered with a scum layer, and the oxidation rate is slowed down.

In soldering, more dross is produced due to the turbulence and flow of the crests in the tin pot. The usual recommended method is to skim off the scum, and if skimming is done more often, more scum will be created and more solder will be used. Dross may also become entrained in the wave crest, causing instability or turbulence in the wave crest, thus requiring more maintenance on the liquid content of the solder pot. If allowed to reduce the amount of solder in the tin pot, scum on the surface of the solder will get into the pump, which is likely to happen. Occasionally, grainy solder joints will contain scum. The initial scum found may be the result of rough peaks and may clog the pump. The tin pot should be equipped with an adjustable low volume solder sensor and alarm.

2 crests

In the wave soldering process, the wave crest is the core. The preheated, flux-coated, and dirt-free metal can be sent to the welding workstation through a conveyor belt, contacted with solder at a certain temperature, and then heated, so that the flux will undergo a chemical reaction, and the solder alloy will form interconnections through wave power. is the most critical step. At present, the commonly used symmetrical peak is called the main peak. Set the pump speed, peak height, penetration depth, transmission angle and transmission speed to provide all-round conditions for achieving good welding characteristics. Appropriate adjustments should be made to the data, and the solder should be slowed down and stopped slowly after leaving the peak (exit end). The PCB runs with the crest and eventually pushes the solder to the outlet. In the worst case, the surface tension of the solder and the optimized board crest run allow zero relative motion between the component and the crest at the outlet end. This shelling area is what enables the removal of solder from the board. Sufficient inclination shall be provided without defects such as bridging, burrs, wire drawing and solder balls. Sometimes it is necessary to have a hot air flow at the outlet of the crest to ensure that possible bridging is ruled out. Turbulent chip waves are sometimes used to compensate for flux or air bubbles in the “harsh wave” area formed later after the surface mount components are mounted on the bottom of the board, before wave leveling is performed. The high vertical velocity of the turbulent crest helps ensure solder contact with the lead or pad. The vibrating section behind the flattened laminar flow peak is also used to eliminate air bubbles, ensuring that the solder makes satisfactory contact with the assembly. The welding workstation should basically do: high-purity solder (according to the standard), peak temperature (230-250°C), total time of contact with the peak (3-5 seconds), depth of the printed board immersed in the peak (50-80 %), to achieve parallel conveying tracks and the flux content in the tin pot under the condition that the crests are parallel to the tracks.

3 Cooling after wave soldering

A cooling station is usually added at the end of the wave soldering machine. The purpose 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 shifting of the board when the solder is not fully solidified. Rapid cooling of components to limit exposure of sensitive components to high temperatures. However, the hazards of thermal shock to components and solder joints from aggressive cooling systems should be considered. A well-controlled “gentle steady”, forced gas cooling system should not damage most components. This system is used for two reasons: it enables quick handling of boards without hands-on clamping, and it keeps components cooler than the cleaning solution. The latter cause is of concern, as it may be responsible for some flux residue blistering. Another phenomenon is that it sometimes reacts with certain flux scums, thus making the residue “uncleanable”. There is no fixed formula that can meet these requirements in terms of ensuring that the data set by the welding workstation meets all machines, all designs, all materials and process material conditions and requirements used. Every step in the process must be understood.

4 Conclusion

In short, in order to obtain the best welding quality and meet the needs of users, each process step before and during welding must be controlled, because every step of the entire assembly process of SMT is interrelated and interacts, and any problem in any step will cause serious problems. In-shadow to overall reliability and quality. The same is true for soldering operations, so all parameters, time/temperature, solder volume, flux composition and transfer speed, etc. should be strictly controlled. For defects in welding, the cause should be found out early, analyzed, and corresponding measures should be taken to eliminate all kinds of defects that affect the quality in the bud. In this way, the products produced can be guaranteed to meet the technical specifications.

Send us an inquiry , we will get back to you soon ,thank you