Symptoms: After reflow, silver-gray metal balls with a diameter of 20–80 μm are densely distributed around the PCB pads, mostly attached to the solder mask or adjacent non-pad areas, and occasionally splashed onto the IC body. Cause: The 3.0% silver and 0.5% copper content in SAC305 Solder Paste raises the melting point to 217°C, but the matching rosin-based Flux has a wide boiling range, insufficient volatilization during the preheating stage, and violently vaporizes when entering the reflow peak, entraining liquid solder droplets and ejecting them. Practical solution: Tighten the heating slope of reflow oven Zone 3–4 to within 0.8°C/s, maintain the Peak temperature at 235±2°C with a duration controlled at 45–60 seconds, and use a thermocouple to measure the actual board surface temperature difference ≤2°C. Prevention points: Each batch of Solder Paste must be used up within 4 hours after opening, allowed to warm to room temperature in a dark place for 2 hours, manually stirred for 90 seconds before use, and centrifuged for 3 minutes to remove bubbles.

Symptoms: Obvious paste trailing residue on the bottom of the stencil, and irregular burr-like solder paste accumulation appears on the pad edges after printing. Cause: SAC305 Solder Paste Viscosity decays rapidly, dropping by 15% after 30 minutes at 25°C, and the squeegee pressure is not dynamically compensated as the solder paste ages, causing the solder paste to tear and string during demolding. Practical solution: Enable the automatic squeegee pressure closed-loop system, automatically calibrate the pressure value every 10 printed boards, set the initial value to 5.2kgf, increase by 0.3kgf for every additional 20 boards, and simultaneously reduce the stencil wiping cycle to once every 5 boards, using non-woven fabric + 99.8% ethanol for alternating dry and wet wiping. Prevention points: After the solder paste is warmed to room temperature, it must be stored in a constant humidity cabinet (RH45%±5%), the printing machine ambient temperature is locked at 23±1°C, and direct air conditioning blowing on the track is avoided.

Symptoms: The solder ball occurrence rate on QFN device pads on the same PCB exceeds 12%, while SOIC devices have almost no abnormalities. Cause: The bottom thermal pad of QFN has a large area and fast heat dissipation. After SAC305 solder paste melts, the surface tension mutation lags behind the surrounding small pads, causing excess solder to be squeezed out and form solder balls. Practical solution: Perform step etching on the stencil openings of the QFN thermal pad, reduce the opening rate of the central area to 65%, maintain 85% for the peripheral annular pads, and additionally print a 0.1mm thick thermal conductive adhesive gasket. Prevention points: Before reflow, apply a small amount of nitrogen protective agent to QFN devices individually, with the dosage controlled at 0.8nL per solder joint, calibrated using a piezoelectric dispensing valve.

Symptoms: Starting from the 37th board in continuous production, the number of solder balls increases sharply by 4 times, concentrated in the BGA area. Cause: The stencil tension decays to below 32N/cm, the 325-mesh metal powder in SAC305 solder paste slips laterally at the micro-deformed openings, causing uneven collapse after printing and local solder boiling during reflow. Practical solution: Check the stencil tension for the first piece of each shift, replace it immediately if it is below 38N/cm, manually grind the sidewalls of the stencil openings twice with 1200-mesh silicon carbide sandpaper for old stencils, and then back-blow with 0.5MPa nitrogen for 15 seconds. Prevention points: The stencil life is hard-set to 8000 printing cycles, scrapped when exceeded, and cleaning with alcohol soaking is prohibited.

Symptoms: Solder balls are arranged in a chain along the conveyor track direction, with a spacing of about 12mm. Cause: High-temperature carbonized particles from chain grease mix into the solder paste, and the SnAgCu phase in SAC305 forms heterogeneous nucleation cores with carbon particles, lowering the surface energy threshold of the solder. Practical solution: After shutdown, use a high-temperature cotton swab dipped in dichloromethane to wipe the chain joints, reduce the conveyor speed to 72cm/min, and install a 5μm-level magnetic filter screen at the track entrance. Prevention points: Replace the chain-specific high-temperature grease at a fixed time every Wednesday afternoon, use a vacuum pump to suck out residual old grease before injecting new grease, ensuring no visible oil film residue.

Symptoms: Solder balls are concentrated near the V-Cut slots on the PCB panel process edge, with a density 7 times that of the inner board area. Cause: Stress relief from the V-Cut slots causes local pad micro-warpage, hindering the wetting and spreading of SAC305 solder paste, causing the solder to curl and fly along the warped edges during reflow. Practical solution: Uniformly control the V-Cut depth to 45% of the board thickness, immediately lightly grind the slot chamfer with a 300-mesh belt sander after cutting, and apply 0.15mm thick polyimide tape on the process edge before reflow. Prevention points: During the panel design stage, it is mandatory to require the V-Cut line to be ≥3.2mm away from the nearest pad, otherwise switch to mouse-hole depaneling.