Understencil Cleaning: The Update

Q: What is the optimal solvent contact time to effectively remove flux?
A: To effectively remove flux and paste from the apertures and the bottom of the stencil, it is critical to solubilize the flux component of the paste. A wet wipe sequence is needed, along with using a well-matched cleaning agent (IPA is never recommended) and providing adequate time to solubilize the residue. It’s not possible to state exactly how much time is needed for optimal results for your specific USW process. Still, based on this testing, KYZEN can recommend that heavier saturation of the paper and longer exposure time yield the best results.
Q: Are current wiping sequences achieving the highest cleanliness and defect reduction level?
A: As this study clearly showed, the answer is no. A dry wipe sequence does not have the ability to solubilize the flux component of the paste. Conventional wisdom has been to minimize paper saturation, while this study showed just the opposite. This study also showed that gantry speed is often too fast, not allowing enough time for flux and paste transfer to the paper.
Q: Could the use of cleaning solvents impact the print process negatively?
A: Solder paste inspection (SPI) was used to validate that the wet wipe sequence (use of a cleaning agent) does not have a negative impact but just the opposite. The results showed an increase in CP and a decrease in SD. So, one can expect a more stable and consistent print process when running a wet wipe USW sequence rather than a dry wipe sequence.