The reason why manual rework with Eutectic Solder can be rapidly deployed at JJY customer sites lies fundamentally in the temperature control tolerance advantage brought by its fixed melting point of 183°C. As long as the soldering tip temperature is set between 220°C and 240°C, paired with a contact time of less than 2 seconds, the solder joint can be fully melted without damaging surrounding components. In contrast, conventional tin-lead or lead-free solder requires strict profile matching and is highly prone to overheating-induced warpage.

We typically adopt a dual-zone coordinated temperature control approach: first, use a bottom preheating platform at 80°C to 100°C to uniformly heat the backside of the PCB, then apply a temperature-feedback-controlled constant-temperature soldering tip precisely to the front side of the solder joint. This not only avoids pad peeling caused by localized overheating but also maintains sufficient wetting time for the eutectic solder near the liquidus line, thereby significantly improving the consistency of solder ball formation and the uniformity of interfacial intermetallic compound thickness.

Actual measurements show that when local PCB preheating is insufficient, even if the soldering tip temperature meets the standard, the eutectic solder will exhibit a "false melting" phenomenon, where the surface briefly shines but the interior is not fully fused. Forcibly dragging the chip in this state will cause solder ball spike formation or even fracture. Therefore, before each rework, an infrared temperature gun must be used to confirm that the temperature difference between the center and four corners of the BGA area does not exceed ±3°C; otherwise, the preheating time must be extended by more than 15 seconds before re-verification.

The selection of the soldering tip directly determines the spreading quality of the eutectic solder. We only use beveled flat tips with a diameter of 0.8mm to 1.2mm and regularly maintain the iron-plated layer with fluorine-containing polishing paste. If the tip body oxidizes and turns black or the plating peels off, even if the temperature display is normal, the heat transfer efficiency will drop by more than 30%, leading to defects such as solder joint shrinkage cavities and wetting angles greater than 60°. This type of problem is particularly prominent when reworking 0.4mm pitch CSP devices.

The selection of Flux must match the activity of the eutectic solder. JJY recommends using no-clean RMA-type rosin-based soldering paste, whose activation temperature range is 150°C to 175°C, exactly covering the entire solid-liquid phase transition process of the eutectic solder. If a high-residue water-soluble flux is mistakenly used, white ionic residues will form after cooling, which not only affects the conduction rate of ICT testing but also induces electrochemical migration failure in high-temperature and high-humidity environments.

Finally, it must be emphasized that the cooling rate after eutectic solder rework cannot be artificially intervened. It is strictly prohibited to use an air gun for forced cooling or immerse the assembly in low-temperature liquid. The solder joint must be allowed to cool naturally in still air to below 80°C; otherwise, uneven solidification shrinkage stress will cause microcracks to propagate along the IMC layer. Such defects are invisible under X-ray but will manifest as intermittent functional interruptions after on-machine aging.