Electronics Assembly Knowledge, Vision & Wisdom
Evaluation of Stencil Materials, Suppliers and Coatings
Evaluation of Stencil Materials, Suppliers and Coatings
An experiment is devised to identify the best stencil option for a highly miniaturized, densely populated SMT product.
Production Floor

Authored By:
Chrys Shea
Shea Engineering Services, Burlington, NJ, USA

Ray Whittier
Vicor Corporation - VI Chip Division, Andover, MA, USA
,{url:'http://www.circuitinsight.com/videos/evaluation_stencil_materials.mp4'},{url:'http://www.circuitinsight.com/videos/programs_final.mp4'}], clip:{autoBuffering:true, autoPlay:true, scaling:'scale' } }).ipad();
Transcript
The past few years have brought PCB assemblers a multitude of choices for SMT stencil materials and coatings.

In addition to the traditional laser-cut stainless steel or electroformed nickel, choices now include stainless steel that has been optimized for laser cutting, stainless steel with smaller grain structures, and laser cut nickel.

Available post-cutting processes include electropolishing and nano-coating.   Each option touts advantages over the others.

To identify the best options for the real-world application of a highly miniaturized, very densely populated SMT product, the authors of this paper devised an experiment.

It included different materials, manufacturing methods and suppliers. Stencils were tested in pairs in order to capture the effects of a new hydrophobic coating.

The surface treatment was applied to one stencil of each pair, allowing for direct comparison of print performance with and without the coating. Output variables covered in this paper include print yields, transfer efficiencies, volume repeatabilities, and dimensional accuracy.
Summary
The past few years have brought PCB assemblers a multitude of choices for SMT stencil materials and coatings. In addition to the traditional laser-cut stainless steel (SS) or electroformed nickel, choices now include SS that has been optimized for laser cutting, SS with smaller grain structures, and laser cut nickel. Available post-cutting processes include electrpolishing and nano-coating.

Each option touts advantages over the others. To identify the best options for the real-world application of a highly miniaturized, very densely populated SMT product, an experiment was devised. It included different materials, manufacturing methods and suppliers. Stencils were tested in pairs in order to capture the effects of a new hydrophobic coating. The surface treatment was applied to one stencil of each pair, allowing for direct comparison of print performance with and without the coating.

Output variables included print yields, transfer efficiencies on 0.5mm BGAs and 0201s, volume repeatabilities on BGAs and 0201s, and dimensional accuracy of the stencils.
Conclusions
The stencil technology selected for this production operation is stainless steel with two-part nanocoating applied. Only small differences were noted between types of SS and suppliers in terms of print volumes and transfer efficiencies, but substantial yield improvements were observed on stencils with the surface treatment.

The SS foils offered the best dimensional accuracy. Electroformed nickel foils and stencils varied considerably more than SS in both thickness and aperture size. The positional accuracy of the electroformed stencils also appears poorer than that of the SS stencils, introducing more alignment error into the printing process.

The overall print performance of the SS foils were better than that of the electroformed ones. The actual differences between the optimized SS with different grain sizes need to be further quantified, as the experimental results from them are very close.

Nanocoatings did not improve the transfer efficiency of small apertures with area ratios in the 0.6 to 0.66 range. In fact, all the stencils with the coatings released less paste at this AR than their uncoated counterparts. The paste release for ARs in the 0.70 - 0.80 range were similar with and without the coatings. Nanocoatings improved yields dramatically. The improvement in yields afforded by the coated stencils equates to an undeniable boost in productivity.

The slightly lower transfer efficiencies of coated stencils, and of specialized stainless steel has not been investigated. It is speculated that crisper print definition may account for the small differentials, but no formal analysis has been performed to date.

Concerns of depositing adequate solder volume with a thinner stencil were addressed. Laser-cut nickel stencils with 0.0045" foil thicknesses deposited an average of 250 cubic mils, whereas the SS stencils with 0.004" foil thicknesses deposited an average of 322 cubic mils. Furthermore, the 0.004" SS stencils showed less variation in the volumes than the laser-cut nickel stencils. 0.004" SS foils with modified grain size and surface coating are now used in production for assembly of the test vehicle PCB and many similar products.
Initially Published in the SMTA Proceedings
Submit A Comment

Comments are reviewed prior to posting. Please avoid discussion of pricing or recommendations for specific products. You must include your full name to have your comments posted. We will not post your email address.

Your Name


Company


E-mail


Country


Comments


Authentication

Please type the number displayed into the box. If you receive an error, you may need to refresh the page and resubmit the information.



Related Programs
bullet Revolutionary Printing Solution for SMT Assembly
bullet Low Surface Energy Coatings Rewrite Area Ratio Rules
bullet Past, Present, Future of Solderless Assembly
bullet Stencil Printing of Small Apertures
bullet Jetting Strategies for mBGAs
bullet Pick and Place Placement Rate
bullet Stencil Considerations - Miniature Components
bullet Selective Printing for BGA Components
bullet Assembly Options for Handheld Products
bullet Assembling Boards with BGAs on Both Sides
More Related Programs
About | Advertising | Contact | Directory | Directory Search | Directory Submit | Privacy | Programs | Program Search | Sponsorship | Subscribe | Terms

Circuit Insight
6 Liberty Square #2040, Boston MA 02109 USA

Jeff Ferry, Publisher | Ken Cavallaro, Editor/Business Manager

Copyright © Circuitnet LLC. All rights reserved.
A Circuitnet Media Publication