The essence of Solder Paste printing tenting is that the solder paste fails to completely detach from the stencil aperture walls at the moment of demolding. It is common in pads of fine-pitch components such as 0201 and 0402, and is particularly prominent when the ambient humidity is higher than 60%RH or the solder paste has been opened for more than 12 hours. At this time, the rosin carrier in the solder paste precipitates, and the thixotropy decreases, leading to weakened cohesion. In addition, the residual micron-level oxide film on the bottom of the stencil will aggravate the solder paste tailing.

When the squeegee angle is set between 55° and 65°, if the pressure exceeds 6.5kg and the printing speed is higher than 25mm/s, the solder paste will produce the elastic hysteresis effect of non-Newtonian fluid under the action of shear, resulting in insufficient rebound at the moment of detaching from the stencil. Moreover, using a polyurethane squeegee with a hardness of 85A is more likely to cause tenting than 90A, because the former has slow deformation recovery and fits too tightly to the stencil surface, which instead hinders the release of solder paste.

When the stencil thickness is 0.12mm, if the aspect ratio of the aperture is less than 1.5, or the secondary laser repair leads to the aperture wall roughness Ra＞0.35μm, the capillary resistance of the solder paste during demolding increases. Combined with the uneven OSP film thickness on the PCB pad surface (measured deviation＞8nm), continuous stringing will form at the pad edge. We once found on a customer's production line that after the same DEK printer switched to different batches of solder paste, the tenting rate suddenly rose from 0.3% to 2.7%. After testing, it was confirmed that the slump of the new batch of solder paste was 18% higher than that of the old batch at 25℃, indicating that its high-temperature fluidity is excessive and its room-temperature retention is insufficient.

The solution must be advanced in three parallel lines. First, stabilize the Solder Paste Storage temperature at 22±2℃ and strictly control the rewarming time to no less than 4 hours. Second, reduce the squeegee pressure to 5.2kg, fine-tune the angle to 62°, and lower the printing speed to 22mm/s. Synchronously, gently wipe the bottom of the stencil with 1000-mesh silicon carbide paper to eliminate the electrostatic adsorption layer. Finally, use X-ray to check whether the stencil tension is maintained at 45N/cm and the deviation of the four corners is ≤3N/cm; otherwise, even parameter optimization will hardly eradicate tenting.

Production line verification shows that when the solder paste printing is observed under a microscope and the solder amount distribution on the pad is "mountain-shaped" with the top diameter less than 60% of the pad width, it can be basically determined that the tenting risk has been eliminated. At this time, the AOI false alarm rate drops to within 0.08%. However, forcibly increasing the squeegee pressure to try to "flatten" the tenting will only accelerate stencil wear and induce solder bridging. This kind of drinking poison to quench thirst has been repeatedly disproved in the 37 production lines served by JiajinYuan.