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.

• Adjust cleaning cycles and frequency. Our company’s screen printers provide up to six options for cycle combinations on the cleaner. As in this customer’s case, a wet cycle may not be required after every print and, in fact, could be contributing to inconsistent paste release. While very tight dimensions may dictate a clean after every print, try using a dry-only cycle until the SPI indicates a more aggressive routine should be implemented. Then settle on that sequence frequency. The goal is to remove debris without adding much cycle time or introducing instability. New self-learning technology can make initial recommendations on cleaning parameters and ensure the process stays in check during production.
• Analyze cleaning parameters. It’s not only about the sequence; multiple cleaning system parameters need to be optimized. Pressures, speeds and valve open times are all critical. If too much solvent is delivered to the paper, your wet-vac-dry could turn into a wet-wet/vac-wet, and then all bets are off. Ensure the characteristics of the solvent are aligned to the paper and how it wicks, so a dry wipe is indeed dry. (IPA has been known to contribute to solder slump and solder balling.)
• Confirm quality of inputs and consumables. Let’s all say it together: “Good inputs equal good outputs.” That’s my mantra, as you know, and is as true for cleaning as for any other process. High-quality fabric and cleaning chemistries that enhance (instead of undermine) the process – especially with these ultra-small dimensions – are critical. Some understencil fabrics are lint-free, are constructed to trap solder paste particles, instead of smearing them, and help control contamination. Similarly, cleaning solvents should be highly compatible with solder flux chemistry. Less expensive does not always mean less costly!
• Verify understencil paper positioning and indexing. Understencil cleaning systems require some manual intervention to reload paper, manage solvent, etc. Naturally, this takes time and can be prone to error. If the paper isn’t loaded properly, for example, it will not present as flat to the stencil, which may result in the stencil not being thoroughly cleaned. Operators should confirm the paper is correctly positioned, so flatness is maintained and indexing occurs as intended.

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.

Clive Ashmore is global applied process engineering manager at ASM Assembly Systems, Printing Solutions Division (asmpt.com); clive.ashmore@asmpt.com. His column appears bimonthly.

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