When we encounter honeycomb pores on the surface of tin bars on the production line, our first reaction is not to immediately judge them as scrap, but to synchronously check the melting and casting records of this batch of tin bars, the fluctuation curve of nitrogen protection pressure, and whether the standing time before casting is less than 120 seconds. This is because the formation of honeycomb pores is often not caused by a single factor, but the result of the combined action of three factors: insufficient escape of residual hydrogen in the melt, excessive addition of aluminum-magnesium deoxidizer, and uneven cooling rate of the continuous casting machine.

In actual troubleshooting, we found that the density of honeycomb pores in the same heat of tin bars near the sprue is significantly higher than that at the tail end, which indicates that the possibility of secondary air entrainment at the end of the runner is extremely high, rather than a problem with the purity of the Tin Ingot itself. At this time, if a 30x magnifying glass is used to observe that there is no oxide film on the pore wall, the pore diameter is less than 0.15mm, and the drawing test is continuous without breakpoints, the risk of unqualified solder appearance can be basically ruled out.

However, if the honeycomb pores are accompanied by gray-white dross on the surface, a dark base exposed after scraping, or obvious shrinkage cavities and wetting lag after immersion in molten tin at a constant temperature of 245°C for 3 seconds, this batch of tin bars must be taken out of service. This is because behind this phenomenon, there is a high probability that the content of SnO? suspended particles in the tin liquid exceeds the standard. Such oxides not only hinder the activation of the Flux, but also aggravate the nozzle clogging frequency during the wave soldering process. A customer once caused two consecutive AOI false alarms of cold solder joints, reaching 17%.

We once verified the effect of different protective gases on honeycomb pores in a factory in Dongguan. The incidence of honeycomb pores in tin bars produced under the conditions of 99.995% nitrogen purity and a stable flow rate of 8L/min decreased by 82% compared with that cast in ordinary workshop air. However, if the nitrogen moisture content exceeds 30ppm, the honeycomb pores are denser and the pore walls are burr-like, which indicates that hydrogen generated by water decomposition is more destructive than air entrainment.

Therefore, the key to judging whether the honeycomb pores on the surface of tin bars constitute a quality defect is to see whether it causes abnormalities in subsequent processes, such as the Reflow Soldering wetting angle after SMT placement being greater than 65°, difficulty in tin sticking to the soldering iron tip during Manual Soldering, or residual flux carbides in the pores after ultrasonic cleaning. As long as any of these phenomena occurs more than three times, the supplier's PPAP must be initiated to re-evaluate its melting process parameters.

Additionally, we remind front-line colleagues not to directly dig honeycomb pores with fingernails to judge the depth, as this operation can easily introduce grease and contaminate the tin bar surface. The correct approach is to use a 0.1mm thick stainless steel probe to lightly touch the edge of the hole to feel the change in resistance. If the probe can slide in without resistance and there is slight elastic feedback at the bottom, it indicates that it is a gas escape channel rather than an inclusion cavity. In this case, the tin bar can still be used for LED bracket welding scenarios that do not have high requirements for Solder Joint Strength.