During the Reflow Soldering process, Solder Paste must undergo a complete phase transition from solid to liquid and then back to solid. If the peak temperature in the reflow zone is below 217°C, the Sn63/Pb37 Eutectic Solder paste cannot be fully melted; if it exceeds 245°C, it can easily cause PCB warpage or MLCC capacitor cracking. Therefore, we usually control the reflow peak temperature of JJY SG-8800 series solder paste within 230°C±3°C, and ensure it is maintained at this temperature for 60 to 90 seconds to guarantee sufficient wetting of the solder on pads and pins, as well as stable growth of the intermetallic compound (IMC) layer.

The wind speed consistency of each temperature zone inside the furnace directly affects the heating uniformity of the solder paste. When dust accumulation on the fan in a certain heating zone causes the wind speed to drop by more than 15%, the measured temperature difference at adjacent chain rail positions will expand to 8°C. This directly leads to tombstoning of small-sized resistors and capacitors on the same PCB, while QFP devices have cold solder joints. Therefore, we must use an anemometer to detect each zone weekly and clean the air duct filter with compressed air.

The time interval from solder paste printing to entering the reflow oven must be strictly controlled within 60 minutes; otherwise, the rosin resin in the Flux will oxidize and deactivate in advance. Even if the subsequent reflow temperature meets the standard, abnormal increase in surface tension will lead to more solder balls and a higher probability of bridging. We use a time label + scanning code system on the production line to real-time monitor the circulation time limit after stencil printing for each batch.

An excessively slow cooling rate in the cooling zone will cause excessive growth of the IMC layer. Especially when the thickness of Cu6Sn5 exceeds 3.2 μm, the brittleness of solder joints increases significantly; too fast cooling may cause microcracks in solder joints at the bottom of BGA. Therefore, we set the cooling slope to -2.5°C/s, and regularly use thermocouples to verify whether the actual speed of the three-stage cooling fans at the furnace tail matches the PLC set value.

When it is found that the soldering yield of the same LOT solder paste on different ovens differs by more than 3%, first compare the thermocouple calibration records of the two reflow ovens, then check whether the conveyor chain tension is consistent. Because tension deviation will cause the PCB posture to shift inside the furnace and thus change the heating path, this problem is particularly common in dual-track parallel furnace models.

The metal content of JJY solder paste is 89.5±0.3%, which directly determines the volume stability of solder joints. If incoming inspection finds that the deviation exceeds the tolerance range, even if all reflow parameters are qualified, the final X-ray inspection will still show excessive dispersion of solder joint height. Therefore, IQC must conduct specific gravity method measurement on each batch of solder paste and retain samples for sealing.

Reflow soldering is not simply a process of melting solder paste, but through precise regulation of heat energy input, the solder particles in the solder paste complete atomic-level diffusion under the activation of flux, ultimately forming a strong metallurgical bond between the pad copper layer and the component terminal. Any minor out-of-control in any link will be exposed as low-probability occasional defects in AOI or ICT testing.