PCB pad treatment for oxidized board rework is not simply a matter of replacing solder joints, but requires simultaneously addressing three obstacles: copper surface oxidation, insufficient Flux penetration, and delayed Solder Paste wetting. Especially on OSP boards stored for over 72 hours or bare copper pads exposed to high-temperature, high-humidity environments, the oxide film thickness can reach 30–50 nm, which conventional flux cannot penetrate at all.

Typical Failure Characteristics of Oxidized Pads

Our continuous tracking of 12 batches of rework orders revealed that cold solder joints are concentrated on QFN bottom pads and the edges of 0201 resistors and capacitors. Under a microscope, the solder climb height is less than 1/3 of the pad width, and the solder joint edges show serrated shrinkage. This indicates that the flux failed to fully activate the copper oxide interface; after the solder paste melts, surface tension dominates the spreading direction, rather than being driven by the spontaneous formation of intermetallic compound (IMC).

Core Mechanism of JJY High-Activity Flux

This flux contains a composite organic acid system and a trace halogen activator. It begins to continuously release H? ions at 150°C, undergoes an in-situ reduction reaction with CuO to form soluble copper salts, and completes interface cleaning before the peak temperature of 220°C. Its activity window is 45 seconds wider than that of ordinary RMA-type flux, which exactly covers the critical stage from solder paste slumping to complete wetting, avoiding solder retraction caused by premature activity decay.

Supporting Processes Must Be Adjusted Simultaneously

Merely changing the flux without adjusting the process is ineffective. We must increase the stencil aperture by 5% to compensate for flux volatilization loss, while stabilizing the Reflow Soldering nitrogen flow at 15 L/min and reducing the oxygen content to below 100 ppm. Otherwise, even with high-activity flux, residual oxides will re-adsorb oxygen during the cooling stage to form a secondary passivation layer.

Pad Pre-Treatment Before Rework Cannot Be Omitted

Using a 3M Scotch-Brite soft grinding head to lightly scrape the pad surface for 0.5 seconds at 0.8 MPa air pressure can remove loose oxides without damaging the underlying copper foil, followed by a two-step cleaning with anhydrous ethanol and compressed air. If this step is skipped and flux is applied directly, post-rework X-ray inspection shows that the IMC thickness fluctuation reaches ±0.8 μm, far exceeding the ±0.3 μm tolerance band allowed by IPC-A-610G.

Real Feedback from On-Site Verification Data

Comparative rework was performed on 216 oxidized BGA motherboards in an automotive-grade production line. The first-pass soldering yield of the group using JJY flux was 92.3%, while the group using a certain Japanese RMA flux in the same period was only 64.7%. Moreover, 11.2% of the solder joints in the latter group developed microcracks after temperature cycling, while the proportion in the former group was zero. This indicates that high activity not only improves initial wetting but also enhances the stability of intermetallic bonding.