When pinholes on the tin surface and copper exposure defects occur on the tin dipping process site, more than 80% of the problems do not stem from the tin bath temperature or lifting speed, but from insufficient matching of the interfacial reaction between the Flux, the substrate copper surface and the OSP film layer. Three consecutive months of tracking data from a HDI board factory in East China with JiajinYuan JY-806A show that when the pinhole rate of a certain competing flux originally used reached 3.2% and copper exposure points exceeded 17 per square meter, after switching to JY-806A, the pinhole rate was stabilized within 0.4%, and copper exposure points dropped to an average of 2.1 per square meter. This result was achieved under the premise of keeping the original tin dipping temperature of 255℃±2℃, tin dipping time of 4.5 seconds, and lifting speed of 1.2 meters per minute unchanged.

The solid content of the flux must be controlled within the range of 18.5%±0.8%. Too high a solid content will lead to insufficient solvent volatilization and residual microbubbles, while too low a solid content cannot support the active components to form an effective covering film on the copper surface. The measured solid content of JiajinYuan JY-806A is 17.9%. Combined with its compound system whose main components are modified rosin ester and organic amine salt, more than 92% of solvent migration can be completed in the 145℃ preheating section, avoiding pinholes caused by bumping after entering the tin liquid. Meanwhile, its acid value is controlled at 42.3mgKOH/g, which is sufficient to activate copper oxide without excessively corroding the OSP film and causing copper exposure.

Copper surface cleanliness and flux wettability must be evaluated simultaneously. We once sprayed JY-806A and a certain domestic flux on the same immersion gold board respectively and then measured the contact angle. The average wetting angle of the former was 27.6°, while that of the latter reached 41.3°. This means that JY-806A can spread out faster at the moment of tin liquid contact and push the interfacial gas to be discharged orderly along the edge, instead of being trapped in the center of the pad to form pinholes. This difference is particularly obvious on fine-pitch BGA pads below 0.3mm, because the gas escape path is shorter and the resistance is greater.

The storage stability of the flux also directly affects the consistency of tin dipping. After JiajinYuan JY-806A was stored in a sealed condition at 25℃ for 180 days, the viscosity change rate was only ±3.7%, while some low-cost fluxes showed stratification and activity attenuation after 60 days of storage, resulting in the same bath solution being usable in the morning but the pinhole rate suddenly increasing to 1.8% in the afternoon. This is related to the addition of ethylene glycol ether stabilizers with a specific molecular weight distribution in its formula, rather than simply relying on increasing the solvent ratio to cover up the problem.

When replacing the flux on site, the tin bath and spray system must be cleaned simultaneously. Otherwise, the residual old flux will undergo a complexation reaction with JY-806A to form gelatinous deposits. We encountered a situation in a factory in Dongguan where three consecutive batches of PCBA had local uneven tin thickness and slight copper exposure due to incomplete cleaning of the nozzle. After re-performing ultrasonic cleaning + 75℃ isopropanol immersion cleaning, the situation returned to normal. This action can stop the loss faster than adjusting parameters.