SMTA International Conference Proceedings


Authors: Kevin Liticker, MS, CSSBB
Company: Rockwell Automation
Date Published: 10/24/2010   Conference: SMTA International

Abstract: Electronics assembly continues to evolve towards smaller and more complicated designs, requiring more control over all aspects of the assembly process, and in particular the stencil printing process. Smaller paste deposits, hidden solder joints, and the trend towards elimination of electrical testing all highlight the importance of optimizing the print process as a critical factor in the quality of the completed product.

Achieving a minimum aperture area ratio (pad area divided by aperture wall area) of 0.66 is a commonly accepted indication of how well a paste deposit will transfer from the stencil aperture to the circuit board. This becomes difficult to achieve for all components on designs with mixed technology. Balancing different paste volume requirements within the same stencil often requires the stencil designers to choose between optimizing stencil design for large parts, which can lead to poor print quality on the very small apertures, and optimizing for the small parts, which will starve the large parts of the necessary paste volume for a complete solder joint.

A variety of stencil technologies are available which will purportedly reduce the acceptable area ratio well below the 0.66 limit, with the correct choice of materials and/or aperture forming (cutting) methods. The purpose of this work is to evaluate several of these stencil technologies and aperture forming methods as they relate to paste transfer efficiencies for small apertures. Stencil materials under evaluation are “PhD”, Fine Grain Stainless, and Nickel. Forming methods are pulsed YAG laser, Fiber Optic laser (with and without electro-polish), and Electro-Form.

Key words: Stencil Printing, Stencil Materials, Stencil Aperture Cutting Methods.

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