SCREEN PRINTING
Keeping Count: Accurately Tracking Stencil Life
An RFID tag can log everything from storage location to print strokes.
Outside of sheer printing machine capability, the stencil is arguably the next most important element of the printing process. Stencil material, thickness, aperture integrity, sidewall smoothness (or lack thereof), and tension all play a role in the quality of the solder paste deposit. And, like all consumables, metal stencils have a lifetime: They do not last forever. Unless a stencil is damaged, tension loss is the factor that most often determines when a stencil has run its course. A properly tensioned stencil enables a good, solid release of the paste deposits onto the board. Alternatively, a stencil that has lost tension and has begun to “sag” may result in defects such as “dog ears”1, bridges, or insufficient paste on pad, to name a few.

Today, stencil tension is more important than ever. Historically, when stencil thicknesses averaged 200µm, one was far more likely to retire a stencil from damage than from wear. Now, however, with the exceptionally thin 60µm foils required for miniaturized designs, tension loss can occur sooner, as repeated stencil pressure during the print stroke eventually reduces stencil elasticity. As has been addressed in this column, there is a proven correlation to changing tension and the output of the printing process.2

To ensure that stencil tension remains robust and does not become a defect factor, most manufacturers set a limit on the number of print strokes a particular stencil should see in its useful life, depending on elements such as aperture design, area ratios and application. The printer can certainly track the number of strokes for a particular job, but what if there are several stencils for that one product? Perhaps there’s a spare and another for inspection? How do you know the stroke quantity of each? Manual logs are often kept to manage this, but they, too, are problematic for obvious reasons. Electronically tracking the print stroke quantity per stencil is the ideal solution to ensure that preset print cycle thresholds are not exceeded and quality protocols are reliably maintained.

Aligning with Industry 4.0 procedures, RFID tag technology now offers manufacturers a robust alternative to manual record-keeping to track stencil stroke exposure. It works like this: An RFID tag is bonded to the stencil at the beginning of its life. The printer software then monitors usage and tracks print cycle quantity for each specific stencil. Even if stencils are used on different lines or printers, the RFID technology maintains a precise record of print strokes unique to that stencil foil. The RFID can also integrate other useful information about the stencil, such as manufacturer origin or storage location for quick retrieval. All stencil data on the tag are read via RFID readers installed on factory printers or by a portable handheld scanner. Manufacturers set specific warning and machine stop thresholds to trigger new stencil ordering or halts to production. Authorization to bypass a machine stop can be assigned to specific employees to maintain quality procedures and process stability. Naturally, each stencil will have different thresholds, depending on the stencil design, the end application for the board being built and other quality criteria.

While attempts at implementing such systems have been made in the past, a lack of robustness limited their use. The stencil is moved frequently, put in and removed from storage, and must endure cleaning systems with harsh chemicals. So, the durability of the RFID tag itself and the bonding strength are critical to its effectiveness and longevity. The new technology addresses these necessary requirements, so assemblers can confidently process stencils as normal without concern for malfunction due to any chemical interaction, and while continuously writing important use data to the RFID tag. In today’s dynamic manufacturing environments, where product quality is king and time is money, managing stencil use to ensure robust tension in an automated way is a welcome advance. Now manufacturers have a truly repeatable, Industry 4.0-capable method to track stencil life, provide critical traceability records, verify adherence to production protocols and ensure higher quality outcomes.

References
  1. Clive Ashmore, “Dog Ears and Witch Hats,” CIRCUITS ASSEMBLY, December 2019.
  2. Clive Ashmore, “Stencil Tension: The Proof’s in the Printing,” CIRCUITS ASSEMBLY, August 2019.
Clive Ashmore headshot
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.