Our factory switched through three batches of Sn99.3Cu0.7 Solder Bars last year. After running the same JT wave soldering machine continuously for 8 hours at 265°C, we found the solder joint fillet lifting rate on the backside of PCBs rose from 1.2% to 4.7%. After switching to Sn99.0Ag0.3Cu0.7 containing 0.3% silver, the fillet lifting rate stabilized below 1.5% under the same operating conditions. This shows that the inhibitory effect of silver on the surface tension of molten solder is irreplaceable during long-term thermal exposure. However, volatile silver prices add 8,300 yuan to the cost per ton of Solder Bars, so the procurement team must calculate the amortization cost based on the monthly usage of 2.3 tons.

The most direct feedback from the production line is poor soldering on OSP boards. When using Sn99.3Cu0.7, the cold solder joint rate for connectors with 0.6mm pin pitch reached 0.89%, which dropped to 0.11% after switching to the silver-containing alloy. This is because silver accelerates the uniform spreading of the Cu6Sn5 intermetallic compound at the interface, while pure SnCu solder has slow wetting initiation under the slightly oxidized state of the OSP layer. Especially when the conveyor speed is increased above 1.4m/min, the Flux activation window is compressed even shorter, leaving no time to complete the interface displacement reaction.

Last week, we debugged a double-sided mixed-mount board with 0201 resistors on the front side and 1.27mm pitch pin headers on the back side. When using Sn99.3Cu0.7 for the second-side reflow, the solder joint bridging rate of the pin headers was as high as 6.3%. After switching to silver-containing solder bars, the bridging rate was reduced to 0.9%. The reason is that silver lowers the viscosity of molten solder, making excess solder retract more easily at the moment of wave separation. But this is not a universal solution. If the preheating temperature is too low, below 125°C, silver-containing solder bars will instead cause solder joint discoloration due to premature decomposition of the active flux.

We tested Sn99.3Cu0.7 solder bars from five suppliers, and the measured copper content fluctuated between 0.68% and 0.73%. Among them, two batches with copper content near the upper limit produced 22% more wave soldering dross than the average. This shows that even if the copper content exceeds the tolerance by only 0.03 percentage points, it will accelerate the precipitation rate of the Sn-Cu eutectic phase, thereby speeding up the formation of oxidized dross. Therefore, incoming inspection must measure the copper content for every batch and cannot rely solely on the factory inspection report.

The dross morphology of silver-containing alloys is also different. The floating dross generated by Sn99.0Ag0.3Cu0.7 has finer particles and is more likely to agglomerate into clumps, allowing the dross skimming frequency to be extended from once every 2 hours to once every 3.5 hours. However, silver accelerates the corrosion of the stainless steel nozzle. We measured that after a certain brand of silver-containing solder bar was used continuously for 15 days, the wear amount of the nozzle throat diameter reached 0.18mm, which has exceeded the equipment's allowable limit of 0.15mm. The nozzle must be replaced in advance, and this hidden maintenance cost is often overlooked.

One final reminder: for all silver-containing solder bars, when restarting the furnace after a shutdown of more than 40 minutes, the surface condensed hard crust must be scraped off first before stirring. Otherwise, the silver-enriched layer will enter the pump cavity along with the solder circulation, causing gear pump clearance jamming. We once had to stop the line for emergency repairs twice due to this, losing production capacity of 217 boards each time. Sn99.3Cu0.7 does not have this problem; its solidification structure is more uniform, and it has a higher fault tolerance for restart operations.