SMT Assembly Process (1)

The SMT assembly process is closely related to each process step before soldering, including capital investment, PCB design, component solderability, assembly operations, 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; 37% lead. The temperature of the solder in the solder pot should be kept under control at all times, and the temperature should be 183°C higher than the temperature of the alloy liquid, and the temperature should be uniform. In the past, a solder pot temperature of 250°C was considered the “standard”.

With the innovation of flux technology, the uniformity of the solder temperature in the whole solder pot has been controlled, and a preheater has been added, and the development trend is to use a lower temperature solder pot. It is common to set the solder pot temperature in the range of 230-240°C. Often, assemblies do not have uniform thermal mass, and it is necessary to ensure that all solder joints reach sufficient temperatures to form acceptable solder joints. The important issue is to provide enough heat to raise the temperature of all leads and pads so that the solder flows and wets both sides of the solder joint. The lower temperature of the solder reduces thermal shock to components and substrates, helps reduce dross formation, and at lower strengths, the combined action of the flux coating operation and the flux compound allows the wave exit to have a sufficient flux so that burrs and solder balls are reduced.

The composition of the solder in the solder pot is closely related to time, i.e. changes with time, which leads to the formation of dross, which is why it is necessary to remove residues and other metal impurities from the soldered components and in the soldering process. Causes of tin loss. These factors can reduce the flowability of the solder. In procurement, the maximum limit of tin content of metal trace dross and solder to be specified, in various standards, (such as IPC/J-STD-006 have clear regulations). During the soldering process, the requirements for solder purity are also specified in the ANSI/J-STD-001B standard. In addition to the restrictions on scum, the minimum tin content in 63% tin; 37% lead alloys shall not be less than 61.5%. Gold and organic layer copper concentrations build up on wave soldered components faster than in the past. This buildup, combined with significant tin loss, can cause the solder to lose flow and create soldering problems. Rough, granular solder joints are often caused by dross in the solder. Dull, coarse grained solder joints due to build-up of dross in the solder pot or inherent residues of the component itself can also be a sign of low tin content, either localized specialty solder joints or the result of tin loss in the tin pot. This appearance can also be caused by vibration or shock during the solidification 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 in accordance with the process control plan. “Draining” the flux in the solder pot due to dross in the solder pot is usually not necessary, since in conventional applications it is required to add solder to the solder pot, so that the solder pot is always full. In the case of depleted tin, adding pure tin helps maintain the desired concentration. To monitor compounds in tin pans, a routine analysis should be performed. If tin is added, it should be sampled for analysis to ensure that the solder composition is correct. Too much scum is another thorny problem. There is no doubt that dross is always present in the solder pot, especially when soldering in the atmosphere. Use “Chip Wave” which is helpful for soldering high density components, because the solder surface exposed to the atmosphere is too large, which oxidizes the solder, so more dross is generated. 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 created due to the turbulence and flow of the waves in the tin pan. The recommended conventional method is to skim the dross, and frequent skimming will create more dross and use more solder. Scum can also be trapped in the wave crest, causing instability or turbulence in the wave crest, requiring more maintenance of the liquid composition in the solder pot. If the amount of solder in the tin pot is allowed to be reduced, dross on the solder surface will enter the pump, which is likely to happen. Occasionally, granular solder joints can contain dross. The scum initially found, may have been caused by rough wave crests and had the potential to clog the pump. The tin pan shall be equipped with an adjustable low volume solder sensor and alarm.