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Knocking Down the Bone Pile: Tin Whisker Mitigation in Aerospace Applications, Part 3
Impact of Stencil Foil Type on Solder Paste Transfer Efficiency for Laser-cut SMT Stencils (Part 2)
May 8, 2019 | Greg Smith, BlueRing StencilsEstimated reading time: 12 minutes
Editor's Note: Read the Part 1 of this article here.
Results
Transfer Efficiency: Uncoated Metal Stencils
Initially, all seven materials were printed and the uncoated stencil data was analyzed for all area ratios of apertures. The top performers were identified based specifically on transfer efficiency in this analysis. The results are shown in Figure 5. Materials 1 and 2 exhibit better print transfer efficiencies with uncoated apertures than the other materials.
Figure 5: Transfer efficiency of uncoated stencils for all area ratios and metal types.
Since small area ratio printing is key in product miniaturization, it is important to determine which uncoated material performed the best from 0.3–0.5 area ratios. These area ratios are defined as small area ratio printing because they are below the recommendation in IPC7525B standard of 0.66 [2]. Figure 6 shows the results for 0.3, 0.4, and 0.5 area ratio apertures only.
Figure 6: Transfer efficiency of uncoated stencils for all metals and 0.3, 0.4, and 0.5 area ratios.
As shown previously, Metal 1 has the highest transfer efficiency results versus the other metals for the 0.3, 0.4, and 0.5 area ratio prints. It also outperformed the second-best material, Material 2, when comparing the means by over 15%. Material 2 shows a 5% improvement over the third-best material when comparing mean transfer efficiencies (Table 3).
Table 3: Mean transfer efficiency of uncoated stencils for all metals and 0.3, 0.4, and 0.5 area ratios.
Another interesting observation is that at 0.5 area ratio, the differences in transfer efficiency results increase significantly versus the 0.3 and 0.4 area ratios with Materials 1, 2, and 4 easily surpassing the 80% transfer efficiency numbers typically required to pass SPI. Using Tukey-Kramer HSD, Material 1 is statistically the best performing material when measuring transfer efficiency on small area ratio apertures (Figure 7), and Material 2 are statistically in the second-best performing group for transfer efficiency with the highest mean transfer efficiency in that group.
Figure 7: Tukey-Kramer HSD on transfer efficiency for 0.3, 0.4, and 0.5 area ratios.
The final analysis of uncoated stencil foils is to examine larger area ratios to understand if material type affects transfer efficiency. All materials were observed printing at area ratios 0.6, 0.7, and 0.8. The following chart shows the results (Figure 8).
Figure 8: Transfer efficiency of uncoated stencils for all metals and 0.6, 0.7, and 0.8 area ratios.
Once again, it can be observed that Metals 1 and 2 outperform the others when measuring transfer efficiency for the larger area ratios. Mean transfer efficiency for Metal 1 was greater than the mean of Metal 2 by just under 5%, and the mean transfer efficiency for Metal 2 was 5% better than the next best performing Metal 4. Again, we see a large increase in transfer efficiency when moving from 0.6 and 0.7 area ratio printing to 0.8 area ratio printing.
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