Evaluation of Stencil Materials, Suppliers and Coatings

Chrys Shea
Shea Engineering Services, Burlington, NJ, USA

Ray Whittier
Vicor Corporation - VI Chip Division, Andover, MA, USA

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.

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.