During our production line verification of JJY halogen-free Solder Paste, we found that it must simultaneously meet three hard requirements—low halogen content, high wettability, and cold solder resistance—for FPC soldering; otherwise, the persistent defect of coexisting cold solder joints and bridging is highly likely to occur in 0.4mm pitch gold finger areas. When this solder paste is used under a nitrogen atmosphere with a peak temperature of 235°C, the IMC layer thickness of solder joints is stably controlled within the range of 2.1–2.8μm, which not only avoids brittle cracking caused by excessive growth but also ensures conduction reliability after 100,000 bending cycles.

The squeegee pressure in the printing process must be set within the range of 5.5–6.2kg, paired with a 75° squeegee angle to ensure complete transfer of the solder paste on the nickel-gold surface covering the PI substrate. If the pressure is below 5.0kg, the filling rate at 0.25mm apertures will be less than 83%, and the dispersion of solder joint height after reflow will exceed ±12%, which directly causes the AOI false alarm rate to soar above 17%. The thixotropic index of this JJY solder paste model is maintained between 0.68 and 0.73, which exactly matches the shear response characteristics brought by the 12μm bottom surface roughness of domestic stencils.

The reflow oven chain speed must be locked within the range of 1.15–1.22m/min. When the chain speed is higher than 1.25m/min, the actual residence time in the soaking zone is compressed to 62 seconds, resulting in insufficient decomposition of the rosin system; local whitening of residues on the back of the FPC appears during the 85°C/85%RH aging test. Conversely, a chain speed lower than 1.10m/min will cause carbonization of the polymer activator, and a 0.04mm wide shadow band is visible at the edge of solder joints under X-ray inspection. The modified dicarboxylic acid used in the JJY formula can complete gradient activation within the range of 155–175°C, which exactly covers this window.

We once conducted a comparative test using the same FPC on identical equipment. When using halogen-containing solder paste, the solder joint peel force attenuation reached 38% after three high-temperature and high-humidity cycles, while after switching to JJY halogen-free solder paste, the attenuation was only 9.2%. The measured surface resistivity of its Flux residue is 2.1×10¹¹Ω, which is far higher than the 1×10⁶Ω threshold specified in IPC-J-STD-004B, meaning that false triggering caused by ion migration will not occur during EMC testing of FPCs for automotive millimeter-wave radars.

Stencil apertures must adopt a stepped design, with the opening size in the 0.3mm pitch area executed with an 8μm single-side indentation from the pad edge; otherwise, the lateral flow of solder paste in the early stage of reflow will exceed the edge of the FPC cover film by more than 0.06mm, resulting in poor wetting of the underfill during subsequent COB packaging. The viscosity of this JJY solder paste is 780±30Pa·s at 25°C, and paired with its unique spherical powder particle size distribution D50=22.4μm, the post-printing volume change rate can be controlled within ±4.7%, thereby ensuring full solder joint bridging even when the FPC mounting offset is ≤±0.025mm.