When we encounter poor wettability of lead-free Solder Paste, we never start by flipping through the data sheet. Instead, we directly use a microscope to check whether shrinkage cavities, copper exposure, or spherical residues appear at the edges of solder joints, and then compare the wetting differences of components at different positions within the same reflow oven. If there is large-area non-wetting at the bottom of a QFN but 0402 resistors are normal, we can basically rule out solder paste batch issues, and instead check whether the stencil aperture size is 15 μm smaller than the pad, whether the squeegee pressure has increased from 2.8 kg to 3.5 kg causing insufficient solder volume, and simultaneously confirm that the actual temperature in reflow zone 3 is 12 ℃ lower than the set value.

The wettability of the solder paste itself is determined by the Flux system. Rosin-based flux spreads faster than water-soluble flux on OSP boards but is more difficult to clean. In no-clean flux, if the concentration of organic acid activator is lower than 0.8%, its wetting ability on nickel-gold layers will be significantly weakened. In addition, the Sn96.5Ag3.0Cu0.5 alloy in lead-free solder paste has a melting point as high as 217 ℃, so the peak temperature must reach 235±5 ℃ to allow the liquid solder to flow fully. However, many factories keep the peak temperature below 230 ℃ to ensure BGA reliability, with the result that the solder solidifies as soon as it melts, leaving no time for wetting.

The surface condition of PCB pads has an even greater impact on solder paste wettability than the solder paste itself. If the gold thickness of electroless nickel immersion gold boards exceeds 0.1 μm, an excessive barrier layer will form. If the tin surface of hot air leveling boards is oxidized and darkened, or the OSP film thickness fluctuates by more than 30?, the wetting angle will deteriorate from the ideal 25° to over 70°. At this point, even switching to high-activity solder paste will have little effect. It is necessary to work with the supplier to adjust the pre-treatment process, for example, shortening the OSP film formation time from 30 seconds to 22 seconds and strictly controlling the temperature at 25±2 ℃.

The variables in the printing process are more trivial but fatal. After the stencil thickness is changed from 120 μm to 100 μm, the solder paste volume decreases by 18%. If the squeegee angle is not adjusted synchronously from 45° to 60°, the pad coverage will be less than 65%. If the ambient humidity is long-term higher than 60%RH, the rosin resin in the solder paste will absorb moisture and burst violently in the early stage of reflow, blowing the un-wetted solder beads directly off the pads. This phenomenon is especially common on production lines during the plum rain season. The corresponding measure is to extend the solder paste warm-up time from 2 hours to 4 hours and enable nitrogen-protected reflow.

Truly effective countermeasures always come from cross-validation. For example, if a certain LOT of solder paste shows poor wetting on line A but is normal on line B, we should immediately compare the stencil tension values (line A 28N/cm2, line B 36N/cm2), chain speed (line A 72cm/min, line B 65cm/min), and nitrogen purity (line A 98.2%, line B 99.6%) of the two lines, rather than rushing to ask the supplier for a new DSC thermal analysis report.