Selecting low-temperature Solder Paste is by no means a simple matter of applying melting point parameters mechanically. Instead, it requires dynamic trade-offs among the temperature resistance threshold of the component body, the Tg margin of the PCB substrate, the precision of stencil aperture, and the response capability of the reflow oven's temperature zones. For example, when thermally sensitive thin-film capacitors are mounted on the backside of the driver IC of an LED transparent display, if Sn42Bi58 (melting point 138°C) is selected, the peak temperature must be kept below 175°C; otherwise, the thin-film layer will blister and delaminate. In the same scenario, switching to Sn64Bi35Ag1 (melting point 151–172°C) allows the peak temperature to be raised to 205°C, which not only improves solder joint wettability but also avoids thermal sensitivity risks.

Among domestic solder pastes, the models that truly withstand production line scrutiny are concentrated in our JJY LFP series. Among them, LFP-JJY2R-64351 corresponds to the 8MTHL/2RTHL series, uses Sn64Bi35Ag1 alloy powder T3/T4, has a halogen content of 800–900ppm ROL1, and has been stably operating on the crystal oscillator SMT production line for over 18 months. Its printing release yield is >99.2%, and the open-circuit rate in ICT testing after reflow is <300ppm. Crucially, its Flux activity is moderate: it not only ensures that the solder climbing height on 0.3mm pitch pads at the bottom of QFN packages reaches 75% of the pad height, but also does not excessively erode the OSP bare copper surface, causing pad copper exposure.

LFP-JJY5RW1A-4258 is a water-washable low-temperature solder paste, matched with Sn42Bi58 alloy powder T2.5/T3, and is specifically designed for welding nickel-plated aluminum fins of heat sinks. Because its flux is an organic acid salt system, post-solder water rinsing leaves no residue or white marks, avoiding the insoluble soap compounds that traditional rosin-based fluxes form on aluminum surfaces, which would cause heat dissipation failure. This model was tested in a copper wire lamp project at a lighting manufacturer: after 1000 thermal cycles from -40°C to 125°C, the solder joint shear strength retention rate was >91%, far exceeding the industry requirement of 85%.

The model that truly pushes the limits is LFP-JJY5RG-In52Sn48, which uses In52Sn48 alloy powder T5/T6/T7 with a melting point of only 118°C. It must be used with nitrogen protection and a precisely temperature-controlled vacuum reflow oven. Currently, it is only applied in small batches in non-reworkable scenarios such as flexible board touch-up soldering for fingerprint lock MCUs and flexible OLED touch FPC bonding. Its advantage lies in minimal thermal impact on the substrate, but the stencil must be replaced with a nickel alloy material to prevent indium in the solder paste from penetrating the stainless steel stencil apertures and causing clogging. Moreover, reflow must be completed within 30 minutes after each printing; otherwise, flux volatilization will cause a sharp increase in solder collapse defect rates.

All low-temperature solder pastes face the unavoidable issue of matching powder particle size with activity. T2.5 powder has good printability but is prone to oxidation, while T4 powder has strong wetting properties but is sensitive to squeegee pressure. When debugging LFP-JJY2R-64351, it was found that after reducing the stencil thickness from 100μm to 75μm, if T4 powder is still used, the squeegee angle must be adjusted from 60° to 55° and the pressure increased from 5.2kg to 6.8kg; otherwise, the solder paste fill rate will be less than 82%. Switching to T3 powder, on the other hand, maintains a 94% fill rate under the original parameters. This shows that solder paste selection is never a single-point decision, but rather an interlocking calibration of materials, equipment, and processes.