Concerned about stencil life? Use care--don't abrade, bend, staple, twist, mishandle or overheat the stencil!

Q: I am looking for guidelines to determine the estimated lifetime of my solder paste stencils. IPC documents do not answer my question. Is there any quantitative measurement that can be used to determine when to replace the stencil and how the life of the stencil can be extended?

A: Design change is one of the most frequent drivers of short stencil life, particularly in the early stages of product life. However, your concern appears to occur later in the cycle. Many operations, with good practices for stencil design, handling and cleaning and few design changes, enjoy stencil lives of several years. In many cases, obsolescence results in more stencil replacements than wear and tear. Also, the less complex the board design, the longer the stencil life.

Typical Causes of Damage

In addition to a design change, visible damage to the stencil, poor-quality solder paste deposits, variable solder paste deposits and smear of the deposited solder paste are indicators that a stencil replacement is needed. However, these undesirable attributes can be caused by factors other than the stencil itself, such as paste properties, control of board and stencil alignment and stencil machine operation

The majority of stencil damage is caused by improper handling during the equipment setup process--solder paste deposition, stencil cleaning and maintenance processes. Dents and nicks in the print area are major reasons for stencil replacement. Such factors will very often result in poor deposits, as described. Good storage facilities and care in handling will extend the life of the stencil.

Shorter stencil life may sometimes result from dense board designs with very narrow webs between apertures. These narrow webs are weak spots and can be easily damaged or wear out with heavy use, poor handling or haphazard storage practices. When the soldering process is fine-tuned, and more solder paste is desired, the process engineer may elect to open the aperture and overprint the pad. This practice can lead to narrower webs and increased susceptibility to stencil handling damage.

The type of squeegee material used (metal or polyurethane) and the stencil machine setup may also affect stencil life. Proponents of both metal and plastic materials exist, but the metal squeegee appears to be the most popular. Misuse and poor setup can result in stencil damage and/or poor results with either type of squeegee.

Cleaning Issues

Stencils must be cleaned to maintain the stencil's printing properties and the stenciling equipment and to achieve satisfactory solder paste deposition. If not performed properly, the stencil cleaning operation can result in significant damage to the stencil, the elastomer frame or the bonding materials. The stencil pattern may also be damaged when removing excess solder paste via uncontrolled manual brushing methods or the actual stencil cleaning process. With very fine-pitch components and dense packaging, the thin webs between apertures may be damaged through use of high pressures for application of the cleaning and rinsing solutions. Selection of a robust, integrated, equipment/chemistry stencil cleaning system requires compatibility of the chemistry and the process with all stencil materials used.

Other Factors

Degradation of stencil tension, due to incompatible chemistry (hot, high-pressure cleaning solutions), may require early replacement of the stencil. Even minor, thermally-induced distortions severely affect the performance of a stencil.

Ultrasonics is sometimes applied during the stencil cleaning process to assure removal of paste and solder balls. Long-time exposure to the ultrasonics, and the hot cleaning solution, can increase the potential for stencil property damage. Following the recommendations of the cleaning equipment and chemistry manufacturer is a must.

Conclusion

Good stencil handling, cleaning and storage practices--combined with conscientious, trained employees--can extend the life of the stencil, but cannot eliminate the impact of design changes.

Send your process, technology or training question, to [e-mail]. Please type "ASK LES" in the subject line and indicate your name and company or institute affiliation. All questions may not be answered.

Les Hymes is the owner of Les Hymes Associates, Surprise, AZ; [phone]; e-mail: les.hymes@ worldnet.att.net.