Impact of Stencil Foil Type on Solder Paste Transfer Efficiency for Laser-cut SMT Stencils (Part 2)

Estimated reading time: 12 minutes

Ceramic nano-coated metal stencils are becoming more widely used in today’s assembly environment to achieve the best print possible, especially for low area ratio printing. It has been shown in previously published papers these coatings improve transfer efficiency by 10% up to 24% ^[3] based on the size of the aperture and the brand and particle size of solder paste being used. To properly evaluate the different metal materials being used to manufacture SMT stencils, it is important to include the ceramic nano-coating technology in this study. The objective is to evaluate if specific material types improve the effect of the coating technology.

Initially, all seven metal foils were analyzed for all area ratios. Again, the top performers were identified based specifically on transfer efficiency. Figure 9 shows the results of both uncoated and coated stencil materials for all area ratios combined.

Figure 9: Transfer efficiency for coated and uncoated stencils for all metals and area ratios.

The top performers for ceramic nano-coated stencils for all area ratios measured are Materials 1 and 2. When measuring the mean transfer efficiency of the coated stencil versus the uncoated stencil, Material 1 improves transfer efficiency by 8.2%. Material 2 shows an improvement with a coating of 6.5% versus the uncoated material. Comparing coated stencil transfer efficiency, Material 1 improves transfer efficiency by 10.2% more than Material 2. Material 2 improves transfer efficiency more than Material 4, the third-best performer, by just under 4%. One can also see that the improvement in transfer efficiency created by the ceramic nano-coating technology closely follows the release characteristics of the base metal being cut. This phenomenon shows the importance of selecting the best possible base material in the stencil manufacturing environment.

To further evaluate the ceramic nano-coating technology, it is critical to look at small area ratio printing defined in this article as apertures with area ratios of 0.3, 0.4, and 0.5. Figure 10 shows the improved release characteristics with the addition of the ceramic nano-coating.

Figure 10: Transfer efficiency for coated and uncoated stencils for all metals with 0.3, 0.4, and 0.5 area ratios combined.

The coated material exhibiting the best mean transfer efficiency for area ratios of 0.3, 0.4, and 0.5 combined is Material 1. When averaging these three area ratios, an increase in mean transfer efficiency with the ceramic nano-coating is 16% versus the uncoated stencil. Material 2 with the coating technology had the second highest mean transfer efficiency improvement of just under 16% as well. Overall, a larger improvement in transfer efficiency is seen in small area ratios with the application of the ceramic nano-coating technology versus the larger area apertures. Again, it should be noted that the improvement in solder paste release from the nano-coated stencil follows the transfer efficiency of the base material, especially on small area ratio apertures.

Currently, most stencil providers limit lower area ratios to 0.6 to maintain proper release and volume to achieve acceptable solder fillets after reflow. Observing the data in Figure 11, one can see that Materials 3, 5, 6, and 7 are close to 80% transfer efficiency on 0.5 area ratio apertures with no coating (blue bars) and Materials 1, 2, and 4 are just at or over 90% with no coating (blue bars). When the ceramic nano-coating is added, the transfer efficiency mean for Material 1 increases by 28–125% (orange bars). With the best base material and the ceramic coating technology, small aperture printing at 0.5 area ratios is now possible.

Figure 11: Transfer efficiency of coated and uncoated stencils for all metals and 0.5 area ratio.

Transfer Efficiency: Grain Size Comparison

Almost all metals are crystalline in nature and contain internal boundaries known as grain boundaries. As new grains are nucleated during processing, atoms line up in a specific pattern common to the crystal structure of the alloy. Each grain eventually impacts others and forms an interference where the atomic orientations are different ^[4]. These areas are known as grains. Grain size is normally determined by processes such as heat treatment and cooling rates during the alloy extrusion process. Typically, it is accepted that most mechanical properties improve as the size of grains decrease. An example of a grain structure is seen in Figure 12.

Figure 12: Example of metal alloy grain structure.

For several years, SMT stencil vendors have offered “fine grain” metals to the industry with the benefit of improved print processing. Initially, only one vendor offered this material to stencil manufacturers, and over the past several years, more vendors have offered “fine grain” metals to the industry. This investigation identifies “fine grain” material as a foil with grain sizes of less than five microns. To better understand print performance with these “fine grain” alloys, we have divided grain sizes into three categories. Category A materials have a grain size of 1–5 microns, Category B materials have a grain size of 6–10 microns, and Category C includes materials with grain size more than 10 microns.

Figure 13 shows the transfer efficiency of all area ratios based on grain size. Both Category A, grain sizes 1–5 microns, and Category B, grain sizes 6–10 microns, produce higher transfer efficiency results than Category C with grain sizes of higher than 10 microns. The uncoated stencil shows a slight improvement in transfer efficiency for Category A versus Category B when looking at all area ratios.

Figure 13: Transfer efficiency versus grain size for all area ratios.

Looking more closely at the effects of grain size on print performance, Figure 14 shows transfer efficiency results for small area ratios, 0.3, 0.4, and 0.5 based on the grain size of the metal. Both Categories A and B show very similar solder paste release characteristics and both exhibit improved transfer efficiency versus Category C grain sizes. It should also be noted that adding the ceramic nano-coating improves Category C transfer efficiency more than the others. Finally, when the transfer efficiency of the two nickel materials is averaged together the nickel material releases solder paste similar to the Category C grain size stainless steel before coating. However, the nickel alloy does not release paste as well as the stainless-steel alloys with the addition of the coating technology.

Figure 14: Transfer efficiency by grain size for 0.3, 0.4, and 0.5 area ratios.

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