SMTA International Conference Proceedings


Authors: Michael Roesch and Joerg Franke
Company: Institute FAPS, University of Erlangen-Nuremberg
Date Published: 10/4/2009   Conference: SMTA International

Abstract: Solder paste stencil printing takes up a central position in electronics production. Nearly two-thirds of all process defects originate in the stencil printing process. The continuous increase of requirements in particular, but also the resulting sensitivity for failures of the printing process call for permanent optimization. The complexity of the process results from a great variety of influencing variables and interactions. Two of the most important ones are solder paste and printing stencil.

The results of extensive printing experiments with different stencil technologies and solder pastes are presented within this article. The stencil variation concerns base materials like stainless steel and nickel as well as manufacturing technologies like laser cutting and electroforming. Alternative stencil finishing options like mechanical brushing, electrochemical polishing and a novel and innovative nano coating technology supplement the investigated variations. Furthermore alternative solder pastes with different powder sizes and tackinesses were used within the studies.

This paper will discuss correlations of significant stencil respectively solder paste properties with transfer efficiency. The transfer efficiency, which describes the release behavior of the solder paste, is of particular importance and depends exceedingly on the area ratio of the stencil aperture. The stencil design for robust printing processes demands the definition of the area ratio in consideration of the combination of stencil and solder paste used. Based on the research results stencil design guidelines for robust printing processes can be modified and extended. The recommendations complement the IPC-7525A guidelines and include stencil and paste specific definitions of the stencil design, particularly of the area ratio of the stencil apertures.

Furthermore the article demonstrates properties and potentials of coated stencils. The coating that covers aperture walls and bottom side leads to a decrease in the adhesion between solder paste and stencil aperture. Consequently the transfer efficiency increases when using coated stencils. Considering the stencil design, the area ratio can be reduced up to a value of approximately 0.4. Thus the development of coated stencils contributes extensively to a robust printing process and a continuous miniaturization.

Key words: Stencil Printing, Transfer Efficiency, Area Ratio.

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