SMTA International Conference Proceedings


Authors: Chrys Shea, Valentijn Van Velthoven, and Ron Tripp
Company: Cookson Electronics
Date Published: 9/21/2003   Conference: SMTA International

Abstract: It is not uncommon for SMT yields to drop when a product’s manufacturing is transferred to a new location, whether it is from a launch center to a production site, or from one production site to another. Resolving the yield fluctuation often requires costly resources for both the CEM and the OEM. To limit fluctuations, many manufacturers have migrated to standardized processing techniques: similar equipment, similar line configurations, and similar chemistries. This proactive standardization philosophy has saved assemblers billions of dollars by improving product transfer efficiency.

One source of variation in SMT manufacturing that has not been resolved until very recently is the solder stencil. Stencil printing is commonly blamed for up to 75% of the defects in an SMT assembly line. Despite the efforts to standardize the printing process, the stencil itself is subject to two main avenues of variation: its design and its manufacture. The stencil’s design can often be modified by a manufacturing site to address local preferences. The stencil’s manufacturing process is subject to tolerances that vary from one stencil producer to another, and may differ based on their equipment’s age, condition, or manufacturer.

A system has been developed to address both sources of stencil variation on a global basis, by providing tools to control design changes and manufacturing variations. An intelligent database system manages design information, while regular statistical process capability studies help to calibrate and control the manufacturing process. This paper reviews the information management process and discusses the options and safeguards it provides. It also discusses the statistical methods employed to guarantee similar positional and dimensional accuracy of every stencil produced, regardless of its manufacturing site.

Key words: stencils, screen printing, variation, SMT manufacturing.

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