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Stencils and the Phonograph Stylus
December 31, 1969 |Estimated reading time: 4 minutes
Remember the phonograph? Back when phonographs were part of every stereo system, a simple rule applied: if the stylus wasn’t top–quality, the musical reproduction wouldn’t be either. A stencil has much the same effect on surface mount assembly. The analogy involves the tenets of modern quality theory. Defects should be eliminated, not repaired, and defects “amplify” as they move downstream in production. The stencil printing process is one of the first steps in assembly and one of the first opportunities to eliminate defects in production.
Defects that occur during this step “amplify” downstream in the form of soldering problems, test failures, and perhaps worst–case field failures. It seems reasonable, therefore, to assume that quality of stencil design and manufacture are critical to the assembly process. It also is important to point out that, although a stencil is in some ways a consumable, it clearly is not a commodity. As such, the support services and process expertise of the stencil vendor are as critical as the product itself.
A stencil is as unique to the assembly process as the PCB itself. Its layout mirrors that of the board, and it has a number of design parameters that must be compatible with the assembly. Stencil considerations include the smallest aperture and pitch to be printed, paste transfer efficiency, paste volume control, print volume, stencil longevity, and the price/performance ratio. All of these factors should be reviewed with the stencil vendor to ensure that process yield targets are met within the guidelines of cost–effectiveness.
To help with sorting through these parameters, a stencil performance hierarchy may be considered. This can be thought of as a pyramid of performance ranging from the chem–etch process at the base to sophisticated technologies, like electroformed stencils, at the apex. Laser cutting, different base alloys, and stencils with various post–processing treatments fill in the rest of the pyramid. The key in using the performance hierarchy as a selection tool is to match stencil capability to the demands of the process, while factoring in operational considerations like stencil longevity.
The most demanding applications — printing with ultra–fine pitch, micro–BGAs and flip chips, wafer bumping — require the high–level capability of electroformed stencils and quality manufacturing expertise to maximize the technology. One form of this technology provides trapezoidal apertures, nickel surfaces, high tensile strength, and quality performance. Special–alloy stencils are well suited to high–volume production. Even basic technology, like chem–etch, can be enhanced with electropolishing and nickel plating. Ideally, stencil vendors will offer a wide range of performance capabilities and can help with selecting the appropriate stencil design to maximize assembly yields and value.
The stencil vendor is as important as the stencil itself. In this era of rapid response, quick turnaround, and short product lifecycles, prompt and effective technical support from the stencil vendor can be a lifesaver to the process engineer. Along with in–depth knowledge of the printing process and stencil design, the vendor should be able to provide stencil specification tools that translate design parameters, like area ratio, into useful selection guidelines. Closely linking the inside sales, applications, and design teams at the stencil vendor’s site can facilitate the exchange of customer information and ensure efficient and effective design support. For less–experienced process engineers, some suppliers offer informative workshops that not only examine their technology, but also provide useful design–for–manufacturability (DfM) criteria that can be incorporated on future projects.
The stencil vendor’s internal manufacturing capabilities are critical as well. The precision of laser cutting and the handling, control, and application of potent chemicals must be monitored and regulated within the rigor of a well–defined and implemented quality system. An analytical laboratory and expert, skilled metallurgists should support the process.
Conclusion
A competent stencil vendor should be an integral part of the end–user’s supply chain. The luxury of long lead times is not acceptable. Therefore, the vendor should be able to offer quick turnaround service and, most importantly, meet or beat delivery commitments on a regular basis. Multi–facility, multi–national customers should demand the ability to “duplicate exactly” from various stencil vendor facilities. Technology and rapid user/vendor data communication can facilitate consistent delivery and product repeatability. Support services such as stencil design archives and customer–specific stencil aperture libraries should be provided by the vendor. These services help the process engineer and reduce the potential for error in order entry.
Although the cost of a stencil is trivial when compared to that of a production line — or even the cost of many individual circuit board assemblies — its impact on production yields is tremendous. Stencil technology, manufacturing quality, design, and vendor support are key determinants in optimizing the printing process. As with the phonograph stylus, a stencil can be the weakest link in the process, or the foundation of its success.
Keith Favre, president and CEO, Photo Stencil, may be contacted at kfavre@photostencil.com.