Electronics Assembly Knowledge, Vision & Wisdom
Step Stencil Design for Handheld
Step Stencil Design for Handheld
Paper explores a process for using a two step stencil process. A thin stencil for fine pitch components followed by a thinker stencil for other components.
Production Floor

Authored By:
William E. Coleman Ph.D.
Photo Stencil Inc.
Colorado Springs, CO USA
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Transcript
Some of the new handheld communication devices offer real challenges to the paste printing process. Normally, there are very small devices like 01005 chip components as well as 0.3 mm pitch micro BGAs along with other devices that require higher deposits of solder paste.

Aperture sizes for the small devices require a stencil thickness in 2-3 mils range whereas RF shield and SMT connectors would like at least 6 mils paste height. This paper will explore a different type of step stencil for this application using a Two-Print Stencil Process step stencil.

Design guidelines for minimum keep-out distances between the relief step, the fine pitch apertures, and the RF Shields apertures as well relief pocket height clearance of the paste printed by the first print stencil will be provided.
Summary
Some of the new handheld communication devices offer real challenges to the paste printing process. Normally, there are very small devices like 01005 chip components as well as 0.3 mm pitch uBGA along with other devices that require higher deposits of solder paste. Surface mount connectors or RF shields with coplanarity issues fall into this category. Aperture sizes for the small devices require a stencil thickness in the 50 to 75 um (2-3 mils) range for effective paste transfer whereas the RF shield and SMT connector would like at least 150 um (6 mils) paste height. Spacing is too small to use normal step stencils.

This paper will explore a different type of step stencil for this application; a "Two-Print Stencil Process" step stencil. Here is a brief description of a "Two-Print Stencil Process". A 50 to 75 um (2-3 mils) stencil is used to print solder paste for the 01005, 0.3 mm pitch uBGA and other fine pitch components. While this paste is still wet a second in-line stencil printer is used to print all other components using a second thicker stencil. This second stencil has relief pockets on the contact side of the stencil any paste was printed with the first stencil.

Design guidelines for minimum keep-out distances between the relief step, the fine pitch apertures, and the RF Shields apertures as well relief pocket height clearance of the paste printed by the first print stencil will be provided.
Conclusions
It has been demonstrated that "A Two-Print Stencil Process" is an effective solution to print solder paste when different paste heights are required. The spacing between apertures requiring different heights can be as small as 380 um (15 mils). It has also been demonstrated that the depth of the relief pocket in the 2nd print stencil can be as low as 25 um (1 mil) more than the thickness of the 1st print stencil. It should also be noted that aperture spacing between apertures in the 1st and 2nd print stencil is independent of the thickness of the 2nd print stencil. This is very useful when very thick paste heights are required from the 2nd print stencil.

The "Two-Print Stencil Process" offers a new solution to cell phone and hand-held SMT assemblers. Thick solder paste for RF shields can be printed very close to solder paste bricks for very small devices like 0.3 mm uBGA and 01005 chip components. A design decision must be made as to which apertures are to be included in the 1st print stencil and which apertures are to be included in the 2nd print stencil. As a secondary outcome of this testing it is seen, for the particular print set-up, the standard deviation of the 50 um (2 mil) thick 1st print stencil is significantly lower that the thicker 75 um (3 mil) 1st print stencil. It is also observed that generally, the standard deviation for the 1st print stencils is lower for the larger apertures and the thinner stencil.
Initially Published in the IPC Proceedings
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