The most troublesome issues in QFN package soldering are incomplete wetting of the bottom pad and excessive voids, especially on devices with a pitch below 0.4mm. If a Solder Paste with insufficient activity or high metal content is selected, not only will cold soldering easily occur, but also the void rate will exceed 25% due to incomplete Flux volatilization. The production line once used a certain imported solder paste for DOE verification, and the void rate was always stuck between 22% and 28%. After switching to our JJY LFP-JJY5RR-0307, under the same reflow profile (peak 248°C, time above liquidus 65 seconds), the void rate was reduced to 8.3% to 11.7%. The key lies in the synergistic effect of its ROL0 zero-halogen formula and T4 grade 0307 alloy powder, which not only ensures surface tension control but also suppresses blockage of gas escape channels.

The solder climb height directly determines the heat transfer efficiency of QFN devices. We compared the actual performance of two types of solder pastes, 8MQP/5RQ and 8MTSP/5RTS, on 0.5mm QFN. Under the same stencil opening (0.12mm thickness, 85% opening ratio), the former achieved a solder climb rate of 96.2%, while the latter was 82.5%. The difference stems from the better matching between ROL1 grade halogen control (both items <1500ppm) and rosin carrier viscosity in 8MQP/5RQ, which gives the solder paste stronger capillary climbing driving force in the early melting stage. Although 8MTSP/5RTS has brighter residue and slightly lower activity, it is suitable for scenarios sensitive to ionic contamination but with large thermal design margin.

Domestic Solder Paste is not a simple replacement, and must match existing equipment parameters. For example, when using a DEK printer, we found that the release rate of 8M9P/5RR under T4 powder is 4.2% higher than that of T3, but T3 powder has a lower flying material rate when the nozzle vacuum of the Panasonic mounter fluctuates. Therefore, we finally fixed 0307T4 on the QFN main control board, and switched back to 0307T3 on small-size Bluetooth modules. This kind of fine adjustment can only be figured out through long-term production line operation, and cannot be copied from the data sheet. In addition, for all models containing 00507BN or 00507B2N alloy powder, such as the 5RNTT series, the reflow peak temperature must be strictly controlled in the range of 242–246°C. Exceeding the limit by 0.5°C will easily cause excessive dissolution of the Ni layer, resulting in excessively thick IMC at the solder joint and a decrease in shear force by more than 12%.

Water-washable solder pastes such as 8MW1P/5RW1 require special attention to cleaning temperature in QFN applications. We tried rinsing with pure water at 75°C, and as a result, flux saponification residues at the bottom of the QFN precipitated in reverse. It was only after switching to 62°C with 0.8% weak alkaline cleaning agent that the standard was met. This shows that the "low" void rate of low-void solder paste is relative. What really determines the void rate is the coupling result of the solder paste body activity, alloy powder gradation, stencil opening geometry, and reflow profile, rather than just looking at the nominal value of a single parameter. The advantage of this batch of Domestic Solder pastes from JJY is that each model is marked with clear T3/T4/T5 powder grades and ROL0/ROL1 halogen grades, allowing engineers to reverse-lock the model according to the defect type. For example, choose 5RR for many voids, use 5RQ for poor solder climb, and switch to 5RW1 for whitish residue, without wasting entire batches of PCBs through trial and error.