We have encountered too many instances where the production line suddenly shuts down at 3 a.m., with the SMT pick-and-place machine having only completed half of the component placement on the PCB. After the Solder Paste has been left standing on the stencil for two hours, it begins to slump, 0201 resistors slide directly off the pads, and the rework rate soars to 12%. However, after switching to long-tack solder paste, even with the same 3.5-hour shutdown before restarting placement, the component position deviation remains within ±0.1mm, and the component drop rate is zero. Behind this is not simply an increase in viscosity value, but the result of the synergistic optimization of the thixotropic agent compounding ratio and the glass transition temperature of the rosin matrix in the solder paste.

The tack performance of long-tack solder paste must be verified within the rhythm of a real production line. For example, a customer's production line schedules two 15-minute line change shutdowns and one 40-minute equipment inspection per shift. For traditional SMT Solder Paste, boards printed after the second shutdown showed a 37% increase in solder paste edge burrs and a 0.08mm decrease in height consistency as detected by AOI. Under the same working conditions, the solder paste profile retention rate of long-tack solder paste still reached 94.6% after three shutdowns. The key is that its initial tack at 25℃ reaches 185kPa, and the residual tack after baking at 60℃ for 20 minutes is no less than 110kPa. This data directly corresponds to the threshold of component vibration resistance before reflow.

The tack decay of SMT Solder paste is never a linear process. When the workshop humidity rises from 45%RH to 62%RH, the tack retention time of ordinary solder paste will be shortened by 40%, while that of long-tack solder paste is only reduced by 11%. This is because its modified rosin system has lower water vapor adsorption activity, and the three-dimensional network structure formed by micron-sized silica particles inside the solder paste is more resistant to shear disturbance. We conducted actual measurements at a factory in Dongguan and found that, using the same DEK printer with the same squeegee pressure and speed parameters, long-tack solder paste still showed no obvious release stringing after continuous printing of 86 boards, while conventional solder paste already exhibited solder paste tailing exceeding 0.3mm by the 53rd board.

It must be reminded that long-tack solder paste is not suitable for all scenarios. For example, the laser solder paste jet printing process, due to its instantaneous high shear characteristics, will instead destroy its thixotropic structure, leading to a non-linear sharp drop in tack. In addition, in nitrogen-protected reflow, if the peak temperature exceeds 245℃, some high-tack formulations will experience aggravated Flux carbonization residue. Therefore, when selecting a solder paste, it is essential to conduct cross-validation based on your own furnace temperature profile, shutdown frequency, and component package type, rather than only looking at the single tack value provided by the supplier.

We once conducted a comparative test on an automotive ECU production line in Suzhou. Using the same 0.1mm-thick stencil to print 0402 LEDs, after a 90-minute shutdown followed by placement, the component drop rate of ordinary solder paste reached 8.3%, while that of long-tack solder paste was 0. However, when the shutdown was extended to 5 hours, the latter also showed a minor component drop rate of 0.7%, indicating that the tack of any solder paste has a physical upper limit. A truly reliable solution is to combine long-tack solder paste with precise temperature and humidity control and reasonable shutdown window management, rather than treating it as a universal panacea.