The biggest risk in rework soldering of FPC flexible boards is not cold solder joints, but thermal damage. The glass transition temperature of the polyimide substrate is only around 260°C, and once the reflow peak exceeds 220°C, local bulging, cover film shrinkage, and even copper circuit lifting can easily occur. Conventional Leaded Solder Paste has a narrow reflow window, while lead-free Solder Paste peaks are generally stuck at 235°C to 245°C, making it impossible to complete reliable soldering without sacrificing the structural integrity of the FPC. Therefore, it is necessary to switch to low-temperature solder paste with a low melting point, moderate activity, and low residue.

JJY LTS-180 series solder paste has a measured reflow peak stably between 178°C and 182°C. With a nitrogen protective atmosphere, the time above liquidus can be controlled within 45 seconds, which not only ensures sufficient solder melting and controllable growth of the interfacial intermetallic compound IMC, but also avoids FPC delamination caused by prolonged high-temperature baking. We once compared three batches of boards from the same lot during FPC rework for the camera module of a flagship model. After reworking with ordinary solder paste, the AOI missed detection rate was as high as 12.7%, while after switching to this low-temperature solder paste, cold solder joint defects dropped to 0.9%, and no microvoids or poor wetting were found by X-ray.

The stencil opening ratio must be adjusted synchronously during the printing process. The 1:1 opening design of conventional solder paste cannot be copied. The opening corresponding to the FPC pad area should be reduced to 85% to 90% of the original size; otherwise, excessive solder will form bridges or tombstoning during low-temperature reflow. Meanwhile, the squeegee pressure needs to be reduced from the conventional 5.5kg to around 4.2kg to prevent FPC deformation during printing, which would cause solder paste misalignment. We once ignored this point and had consecutive solder bridges at the same position on a production line for three days. Finally, we found that it was a composite misalignment caused by the stencil not being re-opened, the squeegee pressure not being adjusted, and the flatness deviation of the FPC back support fixture being 0.08mm.

The reflow oven temperature zone settings must be re-mapped. The heating rate in the preheating zone must be kept below 1.2°C/s; otherwise, uneven heating of the FPC will aggravate curling. The soaking zone temperature is set to 150°C±3°C, and the time is controlled at 90 to 120 seconds to fully activate the Flux and discharge volatiles. The heating rate in the reflow zone should be steeper to ensure crossing the melting point range within 35 seconds, avoiding wetting lag caused by the solder being in a semi-molten state for too long. The wind speed in the cooling zone should not be too high; otherwise, sudden thermal stress will induce microcracks in solder joints.

Before rework, be sure to confirm that the FPC surface is free of fingerprint oil or ionic residue. Otherwise, the flux activity will be insufficient under low-temperature conditions, and white residue will easily form at the root of the BGA solder ball, accompanied by a local wetting angle greater than 90°. We measured with an ion contamination tester that the difference in hand sweat residue before and after the same operator wore gloves could reach 3.8μg/cm² NaCl equivalent, which directly determines whether the low-temperature solder paste can achieve effective metallurgical bonding.