Authored By:
Jim Villalvazo
Interlatin
Summary
he 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 driven issues by adding more and more controls to their systems, but the fact is one of the age old processes continues 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 construed as being simple. 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 expected results, the key factor being productivity, for manufacturing. The majority of the SMT community gives direction that stipulates 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.
This paper showcases 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 will 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.
Conclusions
The common challenge is sustaining a paste deposition capable of meeting the SMT assembly requirements based on design. The experiments proved that each of the technologies used for stencils have merit, but not one has a complete scope of the print environment. The constant here is the tools, which must be improved to help produce the results required. This process tool, which with proper design will enhance the printers performance and support the EOL yieldsmeetsthe demands of the manufacturing world, while improving the process and maintaining it.
Beware! All processes must be evaluated prior to actual design. The theory of improving a stencil aperture design to improve manufacturing yields is achieved by improving the age old original stencil. Back to basics.
Initially Published in the IPC Proceedings
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