Protecting modern-day, complex stencils requires a mechanism overhaul.

Ahh, understencil cleaning: a necessary – but challenging – aspect of the stencil printing process. I’ve certainly discussed cleaning in this space before, as the topic bears revisiting when things change. Now is one of those times. As a subprocess of the overall printing operation, understencil cleaning is employed at specific intervals – after “x” number of prints, as determined by the process and the product details – to clear the aperture area of solder paste. Left unchecked, there is a high probability any smear around the aperture will cause defects. This is especially true if printing anything close to microelectronics-level dimensions such as 0402s, fine-pitch BGAs, etc. With these conditions, the likelihood of bridging, solder balling or some form of defect is relatively high without a robust understencil cleaning regimen. To maintain a centered, high-yield process, thorough cleaning of the underside of the stencil between prints “as and when” is required. (There is no standard, “right” number.)

These facts have not changed in many years. What has changed are PCB designs, dimensions and electronics assemblers’ expectations. As we are all aware, miniaturization has driven stencil thicknesses down to an almost unbelievable 60µm for today’s mobile products. That’s thin! Modern-day stencils are highly complex tooling components with many tens – if not hundreds – of thousands of apertures cut into a paper-thin piece of stainless steel. The material is delicate, to say the least. With these actualities, it is time to reconsider the mechanisms for ensuring thorough, repeatable understencil cleaning that do not damage the stencil, introduce instability into the process or take too long to perform routine tasks. The industry should rethink the understencil cleaning system needed to manage current and future assembly realities. Aspects to consider include:

Controls and motors. To ensure the cleaning head touches the stencil enough to clean it but not so much as to damage it requires finesse, which is difficult to achieve with conventional on-and-off pneumatic-type motor operation. More sophisticated mechanical systems with a pivot mechanism to provide thorough but gentle contact with the stencil underside, as well as coplanarity, allow for more efficient cleaning without damage. Many of these newer controls that leverage linear motors are also programmable for varying speed profiles and smooth delivery of the cleaning cassette using an independent drive for maximum flexibility and accuracy: for example, the cleaning head moving to the stencil at a rate of 300mm/sec., slowing during the cycle, and returning to home at an increased speed.

Easily managed for changeover. Poka-yoke, or “mistake-proofing,” is a common term used in Lean manufacturing. The more the chance for human error is reduced, the more defects are reduced and process efficiency raised. Designing stencil cleaning systems that leave no room for interpretation about how to change the fabric roll, for example, and integrate capability like direct drives on the paper to be advanced by stepper motors, improves outcomes, reduces downtime and eliminates the possibility of offline consumable changes eating up too much time.

Fabric and solvent delivery. Maximizing fabric and solvent use through precision movement and deposition reduces consumable costs and ensures repeatability. Systems that can advance enough fresh fabric to the cleaning head with highly accurate solvent dosing delivered on top of the fabric at the proper speed are important. If the fabric is too damp, solvent can penetrate the apertures and the paste roll. Proper dosing is critical, particularly with ultra-fine dimensions.

Prioritization of health and safety. Operator health and safety are of utmost importance, and manufacturers take many measures to protect the manufacturing environment from harmful substances. The understencil cleaning system should be no different. Exhaust from the understencil cleaner may contain solvents and metal particles from the solder paste. Integrating advanced filters ensures the system captures these contaminants and protects workers.

Nonstop performance and integration with frequency optimization tools. Finally, an understencil cleaning system with solvent reservoirs and fabric capacity to go the distance for a full shift vastly improves productivity and yields. When combined with software systems that can determine the optimal “as and when” cleaning frequency, the duration between required cleans can be further maximized.

As I’ve said many times, the understencil cleaning operation is a process within itself and should be evaluated almost independently of the printer, as the system is integral to achieving printing objectives. Don’t overlook understencil cleaning capability and its role in achieving high-quality, high-volume assemblies, particularly in the age of miniaturization and ultra-thin stencils. 

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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