Solder Wire, Solder Bar, and Solder Paste are used daily on SMT production lines, but many people mistakenly believe that all products labeled SAC305 can be used interchangeably. As a result, solder joints turn black when solder paste is used for wave soldering, pad wetting is weak when solder bar is used for Manual Soldering, and solder bridges occur frequently when solder wire is used to replace solder paste for printing in Reflow Soldering. The root cause is that the three have completely different requirements for alloy microstructure, oxidation control, and Flux carrying capacity. Solder wire must balance smooth wire feeding and heat transfer efficiency of the soldering tip; solder bar must meet the requirement of stable dross generation under long-term high-temperature immersion in the melting pot; solder paste must be repeatedly calibrated among thixotropy, slump resistance, and metal powder sphericity.

Our production line once melted a certain brand of solder bar into liquid and filled it into a solder paste mixer to try making solder paste ourselves. As a result, obvious slump began 2 hours after printing. A large number of irregular angular tin powder particles were observed under the microscope, indicating that the coarse grains formed during the ingot cooling process of the original solder bar alloy cannot meet the hard requirements of solder paste for metal powder D50 particle size of 4~12μm and sphericity >90%. The trace amounts of bismuth and antimony added in the solder wire drawing process can improve low-temperature wettability, but they will accelerate the attenuation of flux acid value during Solder Paste Storage, leading to a decrease in activity by more than 30% after opening for two weeks.

A more hidden problem lies in the flux system. The rosin-based carrier in solder paste must be dynamically matched with the thickness of the oxide film on the tin powder surface. The outer flux of solder wire penetrates into the core by capillary pressure, while solder bar completely relies on external spraying or dipping for replenishment. The solid content, activation temperature range, and halogen residue of the three fluxes are set backward according to their respective forming processes. Forcibly mixing them is equivalent to making the reflow soldering temperature curve adapt to the flux decomposition rhythm of wave soldering, which will inevitably cause carbonized and whitish residues on the back of the PCB and increase the AOI false alarm rate by more than 17%.

The most reliable method verified in practice is: among the SAC305 series from the same supplier, solder paste must select the printing-specific model with the 'P' suffix; solder wire should preferably use actually qualified batches with a diameter of 0.3mm～0.8mm and a core diameter ratio controlled at 18%～22%; solder bar should only recognize the furnace temperature stability certified version with melting point deviation ≤±2℃ and ash content <0.05%. Even if they are from the same factory and have the same grade, before cross-form replacement, at least 24-hour Solder Paste Viscosity tracking, X-ray void rate comparison of 5 batches of first articles, and EDS scanning of the IMC layer thickness of soldering iron solder joints must be done; otherwise, it is gambling with yield.