SMTA International Conference Proceedings

How Creating a Stencil Process That Focuses in the Aperture Design Improves Cp and Cpk Values of Any Printer Process

Author: Jim Villalvazo
Company: Interlatin
Date Published: 9/17/2017   Conference: SMTA International

Abstract: The SMT assembly process is continuously challenged by the factors which enhance circuit board performance and limit productivity. The pick and place and reflow systems reflect these issues by adding more and more controls to their systems, but the fact is one of the age-old processes has continued to operate within the same rules since the dawn of the SMT assembly world: The SMT screen printing.

The SMT print process is very old and often considered a simple one. The process is not complicated but controlling the outcome is complex. Given all the factors involved in boards and technologies, the printer environment is a major factor to control and improve yields from the beginning of the SMT assembly process.

Changing stencil materials (SS, Ni, Electroform or coated, including Nano) is one historically prescribed solution to improve the SMT screen printing process. However, this direction has not given the desired results, the key of which is productivity, for manufacturing. Most the SMT community agrees that stencils today drive the outcome of the SMT assemblies.

The Nano coated stencil concept improved paste release to pad which is said, resulted in improved stencil cleaning performance, but as with all coatings there is a productivity negative factor: it wears off, and the coating must go somewhere when it does.

This paper describes a new stencil process that was discovered by reverting to the basics; understanding the reason for each stencil material process; focusing on detailed finishes and a disciplined aperture design process; maintaining original designs, and making the correctly designed apertures to control the paste deposition. The test results drove us to focus the efforts on the aperture walls. In this paper, we demonstrate with lab tests SMT process results, how the improved paste release results in improved SMT print process performance and its positive impact on SPI yields and EOL performance.

Key Words: 

stencil surface, e-fab, nano coating, printer process, SMT, EOL, screen printing, paste release, SPI yield, paste deposition

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