The Reflow Soldering process window for lead-free Solder Paste is narrower than that of leaded systems. The liquidus temperature of JJY SAC305 Solder Paste is 217°C. In actual production, the peak temperature must be ensured to fall between 235°C and 245°C. If the temperature is lower than 235°C, insufficient wetting of solder joints and low thrust will result; if it is higher than 245°C, it will accelerate the oxidation of PCB pad copper foil and induce cracking of component bodies, especially QFN and LGA devices are extremely sensitive to over-temperature.

Coupling effect of soak zone temperature and time

The soak zone does not simply maintain a constant temperature, but should evenly raise the PCB surface temperature to above 183°C, while completing Flux activation and organic solvent volatilization. JJY solder paste recommends setting the soak zone temperature to 175°C～195°C for a duration of 60 seconds～90 seconds. If the soak temperature is as low as 165°C and the time is compressed to within 45 seconds, the oxide film on the surface of solder paste particles cannot be fully reduced, and spherical residues and cold solder joints are likely to form during subsequent reflow.

Matching logic between peak temperature and heating rate

The heating rate from the end of the soak zone to the peak temperature should be controlled between 1.5°C/s and 3.0°C/s. Too fast will cause the gas inside the solder paste to escape too late and form voids; too slow will cause the flux to be exhausted in advance and increase the surface tension of the solder. We found during the commissioning of a customer's production line that when the rate dropped below 1.0°C/s, the void rate of BGA solder joints in X-ray inspection soared from 12% to 37%, and after adjusting the rate to 2.2°C/s, the void rate fell back to within 9%.

Furnace temperature profile verification must be based on actual measurement rather than theoretical setting

Each time the solder paste batch is changed or PCBs of different thicknesses are switched, KIC or DataPaq furnace temperature testers must be used to actually measure at least three sets of thermocouple data. One set is attached to the thickest heat dissipation pad area, one set is placed under the thinnest fine-pitch BGA, and one set is placed at the edge small resistor and capacitor position. Relying solely on the equipment's default profile or directly reusing the parameters of the previous order will inevitably lead to local underheating or overheating. The thermal response difference of JJY solder paste under different carriers can reach more than 8°C.

Linkage adjustment principle of conveyor speed and zone power

When it is found that the peak temperature is overall high, the Zone 5 power cannot be lowered alone, but the conveyor speed should be synchronously fine-tuned to increase by 2%～3%, otherwise it will cause a double imbalance of insufficient preheating in the front section and too fast cooling in the rear section. When we processed a large batch of USB-C interface boards at a factory in Dongguan, we reduced the power of Zone 3～4 by 5%, increased the conveyor speed by 2.5%, and increased the wind speed of Zone 6 by 15% to control both the solder joint gloss and the IMC layer thickness within the IPC-A-610G Class 2 standard.