If obvious non-spreading areas are found at the edge of the pad after Solder Paste printing and before reflow, and solder paste particle accumulation is visible under a microscope without a tendency to climb toward the copper pad surface, this is usually an early sign of insufficient solder paste wettability. The root cause is attenuation of Flux activity or excessive oxidation of solder powder. The practical solution is to take the same batch of solder paste, dot a 0.3mm diameter paste dot on a bare copper plate, and observe whether the spreading diameter is ≥1.2mm after 60 seconds of constant temperature at 235℃. Preventive points include verifying whether the production date matches the shelf life during incoming inspection, storing in sealed nitrogen protection after opening, and gently shaking and mixing before each use to avoid stratification.

After reflow, a large number of unilateral wetting defects occur on QFP devices accompanied by solder ball residue. The symptom is that the component terminal is only partially tinned and the surface is matte and granular. The cause is that the softening point of rosin resin in the solder paste is too high, resulting in an excessively narrow melting and spreading window. The practical solution is to use the same stencil to print new and old batches of solder paste on the same PCB respectively, pass through the reflow oven synchronously, and compare the difference in terminal climb height. If the difference exceeds 0.08mm, the wettability of this batch is judged to be abnormal. The preventive point is to require suppliers to provide a wetting angle test report after -40℃ to +125℃ thermal shock during the SMT incoming inspection stage.

0201 resistors and capacitors exhibit batch tombstoning with the upright angle concentrated at 75°±5°. The symptom is that one end of the component is completely wetted while the other end is suspended, and the solder joint is crescent-shaped. The direct cause is unbalanced solder paste wettability, causing the surface tension difference between the two ends to exceed the critical point of the component's self-weight. The practical solution is to place the solder paste to be tested and the standard sample paste in the same temperature and humidity environment for 2 hours, then use a squeegee to drag at a constant speed at a 45-degree angle on a nickel-plated copper sheet, and measure the difference in continuous length of the drag mark. If it is more than 8% lower than the standard sample paste, reject it. The preventive point is that SMT incoming inspection must include comparison of dynamic viscosity change curves within 24 hours after opening.

OSP-treated PCBs exhibit dense pad copper exposure after reflow, especially concentrated in the BGA bottom pad array. The symptom is that there is a clear copper-colored ring at the edge of the solder joint, the center has sufficient solder volume but does not cover the entire pad boundary. The cause is insufficient reaction between the halogen activator in the solder paste and the OSP film. The practical solution is to apply 0.5g of solder paste to a designated area of the OSP board, heat it at 180℃ for 90 seconds under nitrogen protection, and then use XRF to detect the residual chlorine content on the pad surface. If it is higher than 85ppm, it indicates insufficient wetting promotion ability. The preventive point is that SMT incoming inspection needs to randomly sample 3 cans for a 48-hour room temperature open exposure test before re-testing wettability.

There are significant differences in the wetting state of CHIP components at different positions in the same reflow oven. The side near the oven inlet has good wetting, while more than 30% of the terminals on the outlet side exhibit solder shrinkage. The symptom is that the solder joint shrinks into a sphere and detaches from the pad edge. The root cause is that the volatilization rate of the low-boiling-point solvent in the solder paste does not match the oven temperature profile. The practical solution is to print the solder paste to be tested on an FR4 test board, pass through the oven according to the customer's actual temperature profile, and then use a confocal microscope to measure the ratio of the maximum solder joint height to the pad width. If it is lower than 0.21, the wetting response is judged to be delayed. The preventive point is to add a dynamic slump test simulating production line speed in SMT incoming inspection, requiring the solder paste to not bridge within a 0.5mm gap when held at 120℃ for 15 seconds and have no slump after cooling.