Is a wet cycle necessary after every print? Maybe not.
In the previous installment of screen printing hacks, we discussed some proven workarounds for alignment issues. This month – and based on some recent customer observations – the advice centers on understencil cleaning, how lack of control can adversely impact this sub-process of printing and the overall result, and a few suggestions for correcting the problems.
Here’s the backstory: A customer printing very small dimensions – 200µm square apertures with spaces of 130µm, on average – was experiencing sub-4 Sigma results on some NPI designs. Transfer efficiency was low, and there was a large standard deviation across devices and the PCBs, so a lot of inconsistent paste-on-pad volume. Our team developed new stencil designs and tested them in a lab environment with our SPI, yielding excellent results. After making some machine calibration adjustments onsite at the customer and integrating the new stencils, however, there still wasn’t tremendous uptick in the process; improvement was observed but not at the expected level. Let the troubleshooting continue! We turned our attention to the cleaning process.
As is the case with many customers printing very small consumer product boards, protocol is to run an understencil clean after every board is printed. Unfortunate as it is, that’s today’s reality, given the tight dimensions and the impact of board stretch. In this situation, the cleaning routine run after every print was a wet-vac-dry, which is arguably a fairly aggressive approach. If the parameters on this setting aren’t precise, issues such as those we observed can contribute to a lack of process stability. First, we noted the understencil cleaning roll paper was saturated with solvent, even on the indexed areas for the vac and dry cycles, so too much solvent was being applied, and the paper quality was such that the solvent wicked over a broad area. In addition, the cleaning system needed to be leveled to bring it back to spec, and there were some paper feed issues with the cleaning roll. Finally, we recommended taking a less aggressive approach and running dry wipes at a certain frequency in between the wet-vac-dry cycles to maintain stability.
If you suspect the understencil cleaning process may be contributing to less-than-acceptable results, try these remedies.
Ultimately, by addressing paper roll indexing issues, modifying cleaning cycles to introduce dry cycle sweeps in between the wet-vac-dry routine, and optimizing solvent quantity deposited, our customer’s NPI assembly returned to 4-Sigma performance, and we are aiming to improve even more.
email@example.com. His column appears bimonthly.is global applied process engineering manager at ASM Assembly Systems, Printing Solutions Division (asmpt.com);