Improperly mixing two Solder Bars made of different Alloy Solders in a wave soldering machine is as dangerous as blending diesel into engine oil. On the surface, they both appear as silver-gray metal ingots, but in reality, their lead-tin ratios, trace silver and copper elements, and activator types are all completely different. Once they coexist in a molten state, problems such as the precipitation of low-melting-point eutectic phases, suspension of high-melting-point metal particles, and imbalance in the decomposition rate of Flux will occur. Last week on Line A, we encountered a situation where, after mixing Sn63/Pb37 and Sn96.5/Ag3.0/Cu0.5, solder joints turned gray and failed thrust tests for three consecutive hours.

The root cause is that when the liquidus temperature difference between different solder bars exceeds 8°C, the 255°C constant temperature zone set for the wave soldering machine simply cannot accommodate the complete melting and sufficient metallurgical reaction of both. Some solder joint areas are actually in a semi-solid pasty state, leading to an increased wetting angle and a higher probability of bridging. What's more troublesome is that residual unmelted particles will become stubborn stains in subsequent PCB cleaning processes, especially causing irreversible wetting obstacles for boards with OSP surface treatment.

Some people still think that "since they are all tin-based," they can be mixed and used, only to find that the amount of dross is 2.3 times higher than usual, and the skimming frequency has changed from once every 4 hours to once every 90 minutes. This is because the migration rate of copper ions in different alloy solders varies greatly: the SnPb system accelerates the dissolution of Cu Pads, while the SAC system inhibits this process. After mixing, local copper-rich micro-zones are formed, directly catalyzing the nucleation and growth of dross. Moreover, what the furnace temperature sensor feeds back is actually the average temperature, which cannot monitor the real thermodynamic fluctuations caused by microscopic composition gradients within the molten pool.

We tried using a spectrometer on Line B to measure the molten pool composition in real time, and found that after 30 minutes of mixed feeding, the Sn content deviation reached ±1.7%, and the Ag fluctuation was ±0.42%. Such fluctuations far exceed the ±0.15% requirement for batch consistency of alloy solders specified in IPC-J-STD-006. Therefore, it is not a question of whether it can be done, but rather that after mixing, parameters will be out of control, defect rates will soar, and there will be no traceability. The cost of stopping the line to adjust the machine is far higher than the price difference between buying two solder bars of the same model.

If you really need to switch alloy solders, you must empty the entire solder pot and thoroughly clean the residual crystalline layer on the inner wall of the crucible. Use pure tin for two transition passes before introducing new solder bars. Otherwise, even a residual of 0.5% of the old solder bar composition will reduce the oxidation resistance lifespan of the new batch of solder bars by 40%. This data comes from our three-year tracking of material change records for 27 wave soldering machines, not theoretical values simulated in a laboratory.