FPC soldering is far more thermally sensitive than rigid PCB. Conventional leaded or lead-free Solder Paste tends to cause coverlay shrinkage and delamination at the copper foil/PI interface during reflow or hand soldering. Microcracks are especially likely in double-side covered areas, dynamic bending zones, and COF structures. JJY low-temperature solder paste reduces the alloy melting point to 138°C. Combined with a moderately active rosin system, it achieves reliable metallurgical bonding at a peak temperature of 205°C to 210°C, greatly narrowing the thermal shock window.

The Essence of Thermal Damage Is Mismatched Material Expansion Coefficients

The CTE of PI substrate is about 50 ppm/°C, while that of copper foil is 17 ppm/°C. When the soldering peak temperature exceeds 220°C and the holding time exceeds 60 seconds, the interfacial residual stress will exceed the adhesion threshold, causing local blistering or edge lifting. We once found in the rework of an in-vehicle touch FPC that after three consecutive repair solders using a 230°C soldering tip and ordinary Solder Wire, a 0.15 mm wide micro-gap was visible at the coverlay edge under a microscope. After switching to JJY 150°C Active Solder Wire, this phenomenon was eliminated.

Low-Temperature Solder Paste Is Not Simply Lowering the Melting Point, But Re-optimizing the Alloy and Flux System

JJY low-temperature solder paste adopts a Sn-Bi-In ternary eutectic system, with a solidus-liquidus interval of only 1.2°C. Its post-printing slump is 40% lower than that of Sn-Ag-Cu. With a rheological design featuring a thixotropic index of 3.8, it still maintains sharp outlines on 0.15 mm fine-pitch FPC pads, and will not cause solder balls or bridging due to excessively fast heating in the preheating zone. The ionic contamination of its flux residue is lower than 0.78 μg/cm² NaCl equivalent, meeting Class III cleanliness requirements.

Solder Wire Selection Must Match Soldering Iron Temperature and Operation Cycle

When hand soldering FPC reinforcement areas, if solder wire with a melting point above 145°C is selected, operators often unconsciously raise the soldering iron temperature to 320°C to speed up melting, which instead intensifies the depth of heat conduction. JJY 150°C solder wire completes spreading within 3 seconds under a 280°C soldering tip. Combined with a 0.3 mm wire diameter and a 72% center rosin filling rate, it balances wetting speed and spatter suppression. After a factory switched from the original 63/37 solder wire to this model, the FPC solder joint cold solder rate dropped from 1.2% to 0.17%, and the AOI false alarm rate also decreased by 35%.

Nitrogen Protection Is Not a Nice-to-Have, But a Necessary Condition for Low-Temperature Processes

Under low-temperature conditions, the activation ability of the flux decreases, and the removal efficiency of the oxide film weakens. If operated in an air environment, a dull tin oxide layer easily forms on the solder joint surface, causing the average thrust test value to fall below 6 N. After introducing nitrogen protection with an oxygen content below 500 ppm, the solder joint gloss improves under the same parameters, the IMC layer thickness stabilizes in the range of 1.8 to 2.3 μm, and no solder joint cracking occurs after 1000 FPC bending cycles.