“Bargain” materials can result in subpar outputs.

We’ve covered understencil cleaning topics in this column many times. Admittedly, if the stencil printing process were perfect, understencil cleaning wouldn’t be required. But it is not an ideal world; we have board stretch, interspace challenges, and the compromise of printing speeds versus pressures to achieve balanced aperture filling.

Here is the basis for some of these challenges: If all the apertures were identical, the filling process (print speed/print pressure) could be optimized around one architecture. But, with the reality of different aperture shapes and sizes, the filling process must have a middle ground. The larger apertures fill more efficiently; thus, the filling process can deliver slightly too much fill on certain apertures, causing excessive material which may lead to solder paste bridging.

Conversely, the smaller apertures have a lower filling efficiency and, under the same speed and pressure setting, can be starved of solder paste, causing an insufficient fill. Insufficient deposited material can also result from an aperture with a low/tight associated area ratio. A low area ratio aperture has a lower transfer efficiency, which means that some of the material may remain in the aperture after filling. Eventually, this unreleased material can block the aperture, requiring understencil cleaning.

PCB to stencil misalignment is another challenge that cleaning helps mitigate. Even though printing machines have exceptional alignment capabilities down to 12µm, current PCB fabrication methods cannot equal this precision. Observing board stretch in the tens of microns across a standard PCB of 200 x 200mm is not uncommon. This mismatch between the stencil and PCB causes bridging due to the apertures not being presented directly in line with the component pads. Indeed, there are a host of other root causes that we have addressed in previous columns – from tooling to incorrect solder paste types to poorly manufactured stencils. That is why we must clean and expect that understencil cleaning will help alleviate potential defects.

Cleaning doesn’t always offer the remedy that’s intended, unfortunately. Recently, a customer experiencing bridging and insufficients told us, “I’m cleaning, but it’s not helping.” While the root cause of those issues may be one described above, the fact that the cleaning “fix” wasn’t providing any correction is what we’ll address here. Even with bridging and/or insufficients, understencil cleaning helps stabilize the process. This customer was not getting any stability post clean.

So, what does one do if the cleaner isn’t delivering the desired result? There are some general pre-flight checks I would advise. First, verify the cleaner is calibrated and touching the stencil. It seems obvious, but if the fabric roll isn’t parallel with the underside of the stencil or making complete edge-to-edge contact, there won’t be any cleaning going on! Though rare, it does happen. Second, check the solvent levels in the tank and the solvent chemistry. Some stencil printing platforms have sophisticated software that alerts users if solvent or fabric requires replenishment. If your platform isn’t equipped with this feature, ensure you are checking supply regularly. The cleaning chemistry is also critical. While IPA is used in lab settings frequently for manual cleaning, it is not recommended for automated understencil cleaning systems because it is fast drying. It can take a bite out of the flux chemistry as all cleaners do, but then it flashes off quickly and leaves a sticky mess that gets smeared on the stencil’s underside. IPA also has a high flash point, which is not safe. Special chemistries are fit for that purpose, so use whatever the machine supplier recommends for its understencil cleaning tool.

Back to our customer. Upon inspection of the understencil cleaner, fabric and solvent, our team noticed that the fabric lacked pores for absorbency. Without a porous structure, the fabric cannot accept the solvent to present it to the underside of the stencil. Ideally, the amount of solvent sprayed on the fabric roll is tightly controlled, the fabric becomes damp, and there is enough material to break down the flux vehicles. This was not happening, and because the fabric was not absorbent, the cleaning solution was rolling off. Once the culprit had been identified, a few minor changes to the understencil cleaning process inputs resulted in a thorough and effective clean. The solder paste bridging and insufficient material deposit challenges were alleviated. There is more work to do to help correct the root cause of those issues, but for now, this customer’s stencil printing process is in spec.

Some advice for what it’s worth: While it is tempting to try to reduce costs by sourcing perceived “bargain” consumables, stencils, and other materials, subpar printing outputs may be the result. Remember that everything that goes into sophisticated stencil printing equipment is an engineered product and should be evaluated just as diligently as the printing platform. You’ve invested in advanced machine technology that requires high-quality inputs. Make wise investments in the process support products, too.

CLIVE ASHMORE is global applied process engineering manager at ASMPT (asmpt.com); clive.ashmore@asmpt.com. His column appears bimonthly.

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