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Improvements in Pb-free stencils: nickel has inherent benefits as a stencil medium, and refinements make it a superior choice to steel.

Nickel is proving to be a valuable element aiding the move to cost-effective Pb-free assembly at high production volumes. When screen printing with Pb-free (SAC) solder pastes, stencils made from pure nickel or a nickel alloy help to achieve paste transfer efficiency comparable to the levels achieved using SnPb pastes with stainless steel laser-cut stencils. Combined with the efficiencies of the frame-mounted interchangeable foil stencil system, nickel is the optimal stencil medium for maximum Pb-free process repeatability and productivity.

Laboratory tests have highlighted a reduction in paste transfer efficiency because of the higher coefficient of friction between SAC pastes and common stainless steel laser-cut stencils. (1) This friction is greater because the revised flux composition for Pb-free soldering produces higher adhesive characteristics compared to SnPb pastes. Figure 1, taken from the results of recent Pb-free investigations, (2) shows that laser-cut stencils made using high-nickel-content stainless steel perform better than regular stainless steel stencils. Moreover, pure laser-cut nickel stencils display even higher efficiency. The experiments also included tests with pure nickel electroformed stencils, and discovered only marginal improvement in paste release efficiency compared to laser-cut nickel samples with identical apertures.

[FIGURE 1 OMITTED]

Nickel Stencil Production

Nickel production processes are considerably different from steel. This reflects both the lower production volumes for nickel products, and the different handling and processing requirements. Cold rolling can produce stainless steel sheets in large quantities within sufficient tolerances with regard to the gauge for stencil applications. The only proviso for precision stencil use is that the steel blanks should be cut from the center of the stock, where the gauge is most uniform. This was essential for ensuring uniform aperture thicknesses across the face of the stencil.

[FIGURE 2 OMITTED]

On the other hand, pure nickel stencils are built up electrolytically. This is true whether creating a stencil blank for laser-cutting or a turnkey electroform stencil. However, when producing a blank for laser-cutting, there is no requirement to first generate the complex mandrel that dictates aperture positions and dimensions. The blanks can be formed more quickly and without the cost of the mandrel--a process less expensive than electroforming.

[FIGURE 3 OMITTED]

Important new knowledge of the effects of laser-cutting on nickel blanks has further refined the process. For example, whereas an electroform stencil is composed of pure nickel, organic hardeners are added in the production of laser-cut blanks. These influence the surface hardness and brightness of the resulting nickel alloy, which have an important effect on the laser-cutting process. The addition of organic hardeners permits the blank to withstand the extreme heat of the laser-cutting process, and is important both in preventing cracking of the blank and in ensuring optimal aperture characteristics. Nickel producers are experienced in using these hardeners in numerous Ni-based products, but stencil developers have worked successfully with the industry to define optimal proportions for nickel in laser-cut stencil applications. As a result, nickel blanks for laser-cut stencils are unique, drawing on the combined knowledge of nickel producers and the screen printing community.

Surface Finish 'Sweet Spot'

The electrolytic process for generating nickel blanks results in a uniform, fine-grained and stress-free material structure that responds well to an optimized laser-cutting process. The laser produces a fine internal aperture surface (Figure 2) with a roughness in the range of 1 to 1.5 [micro]m. In practice, this now appears to be close to the optimal surface characteristic for Pb-free paste release.

When combined with the adhesive qualities of Pb-free pastes described earlier, this fine wall roughness has a beneficial effect on paste release behavior. The apertures of a stainless steel laser-cut stencil, in contrast, display a typical surface roughness in the region of 3 [micro]m, which appears to be sufficient for the paste to "key" into and thereby prevent release when the board is separated from the stencil.

The aperture surfaces of an electroform stencil (Figure 3) display roughness as fine as 0.5 [micro]m. The fine aperture dimensions achievable through electroforming are essential for ultra-fine-pitch printing. When used with Pb-free pastes formulated for ultra-fine-pitch work, including chip scale and wafer-level semiconductor assembly processes, electroform stencils deliver the optimum print resolution and deposit repeatability. (3) When printing with Pb-free pastes for current mainstream SMT assembly, laser-cut nickel-alloy stencils have price-performance benefits.

The laser parameters for cutting nickel blanks also differ from the established settings for stainless steel. Following laboratory investigations, strategic technical centers have been the first to master the knowledge and competencies for producing laser-cut nickel stencils. Over time, demand is expected to draw these capabilities closer to customers.

Combining nickel electroform and laser-cut stencil fabrication techniques with the stencil frame system streamlines the manufacture of stencils using this technology by eliminating several of the assembly processes for standard mesh-mount stencils. After laser-cutting or electroforming, the nickel foil is immediately fitted with extruded aluminium profiles at each edge, which are then secured using four interlocking plastic corners. For 'customers who require total control over Pb-free implementation, these corner blocks can be color-coded green to ensure operators do not inadvertently cross-contaminate product. The stencil is then ready for market. The profile and corner pieces are easily removed at end-of-life, thereby enabling complete disassembly for recycling.

Cost differences. Nickel stencils--particularly electroform nickel stencils--are considerably more expensive than laser-cut stainless steel. However, early adopters in the field are reporting increased durability, which, over the life of the stencil, may offset its higher initial purchase price. In terms of overall price and performance, laser-cut nickel stencils potentially present an even more attractive option.

References

1. Clive Ashmore, "Mass Imaging of Lead-Free Materials: The Impact of Stencil Technology Choice," Global SMT & Packaging, October 2004.

2. Ibid.

3. Ibid.

Clive Ashmore is global applied process engineering manager at DEK (dek.com); cash-more@dek.com. Michael Zahn is VectorGuard product manager, Europe, at DEK.
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Title Annotation:Nickel Stencils
Author:Zahn, Michael
Publication:Circuits Assembly
Date:Dec 1, 2006
Words:968
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