Impact of Stencil Foil Type on Solder Paste Transfer Efficiency for Laser-cut SMT Stencils (Part 2)
Editor's Note: Read the Part 1 of this article here.
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 . 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.
Transfer Efficiency: Ceramic Nano-coated Metal Stencils
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%  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 . 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.
Variation in the Print Process
Transfer efficiency is one key indicator of stencil print performance; however, one must also investigate whether specific metals improve variation in the print process. The coefficient of variation (CV) is the standard deviation of the print volume measurement divided by the mean of the measurements. Comparing the CV of each material with and without nano-coating will provide another tool for identifying the best performing materials. A CV of 10% or less will be considered acceptable for this comparison and is typically considered to be good .
Figure 15 shows CV percentages for the 0.5 area ratio both with and without the coating. When looking at top-performing materials, this percentage must be less than 10%.
Figure 15: CV by metal type.
Looking at the previous graph, it can be seen that uncoated CV percentages are below 10% except Material 4. Although Material 4 performed well when observing transfer efficiency, it is the worst performer when looking at print variation. Material 1 once again exhibits the best results when specifically looking at print variation. Another observation when looking at this data is the stencils with ceramic nano-coating all exhibit lower CV percentages except Material 6. Material 4 exhibited the largest decrease in CV with the ceramic coating technology, lowering the CV by 54%. Overall, CV percentages were lowered for each material by 32–57% with the addition of the coating technology. Since Materials 1 and 2 exhibited the overall best transfer efficiency results and also have CV percentages below 10%, they are the two top contenders for best-performing stencil material when evaluating both transfer efficiency and print variation (Table 4).
Table 4: Transfer efficiency (TE) and CV for all metals with 0.5 area ratio.
Aperture Sidewall Images
SEM photographs were obtained for each of the material types, and an attempt was made to correlate these images to print performance. Figures 16 and 17 show SEM of aperture sidewalls of the coupons previously described. Material 1 was the best performing material for both transfer efficiency and print variation (Figure 16). The second-best performing material was Material 2. An SEM of the aperture sidewall of the coupon (Figure 17).
Figure 16: SEM of uncoated aperture sidewall, Material 1.
Figure 17: SEM of uncoated aperture sidewall, Material 2.
To understand these SEM images, one must understand the laser cutting process. When laser cutting SMT stencils, the laser always penetrates the foil from the bottom or board side of the stencil. This is the side that has the smoothest cut at the foil surface. Paste release is optimized with the smoothest cut side facing the PWB during printing. Initially, the laser penetrates the foil away from the center of the aperture. As the laser beam melts through the metal, an assist gas pushes the molten metal away from the foil. Once the beam burns through the metal, it moves toward the edge of the aperture and follows the path of the aperture design.
The laser cuts with a series of energy pulses. You can see these pulses in the SEM photos. As the molten metal is removed by the assist gas, some material may freeze just at the surface, and most stencil manufacturers remove this with a secondary process. By properly maintaining the laser settings including focus and energy settings, the optimal cut quality will result. For both Materials 1 and 2, both sidewalls are clean, and the corners are smooth. When comparing these two SEM photographs to the worst-performing material (Figure 18), one can see that aperture wall smoothness—or in this case, roughness—correlate to lower transfer efficiency and higher CV.
Figure 18: SEM of uncoated aperture sidewall, Material 3.
In Figure 18, one can see more defined striations and overall a rougher surface. This surface tends to “hold” the solder paste and prevent good release. Materials 5 and 6 were average performers in this analysis. These images are shown in Figure 19.
Figure 19: SEM of uncoated aperture sidewall, Material 5 and 6.
Finally, Figure 20 shows the aperture sidewall after coating with the ceramic nano-coating technology. The coating fills in the striations created during the laser cutting process and creates a smooth surface that is both hydrophobic (repels water-based materials) and oleophobic (repels oil-based materials). This smooth surface not only allows the solder paste to release from the apertures more easily than an uncoated surface but also repels the fluxes in the solder paste to allow the surface of the PWB to easily pull the solder paste from the apertures. The results, as seen in the data presented, are better transfer efficiency and reduced CV.
Figure 20: SEM of ceramic nano-coated aperture wall.
There are many choices of stencil material for SMT stencil manufacturers to utilize in their process and many of these materials claim to be “fine grain.” This study looked at seven different materials and quantified those materials for overall print performance. Material 1 was the best overall performer when measuring transfer efficiency and CV. This material fell into the “fine grain” category. Material 2 was the second-best performer and did not fall into the “fine grain” category. It was also observed that some “fine grain” materials, such as Materials 6 and 7, did not perform as well as others.
Ceramic nano-coating technology was also investigated and exhibited both improved transfer efficiency on all materials tested. It also reduced CV in the print process for all but one material. These improvements in transfer efficiency also followed the base material results. One can conclude that choosing the best base material and then applying the nano-coating technology produced the best performing stencil.
Finally, it was shown through SEM analysis that laser cut wall quality changed by only changing the base material. Certain materials exhibited smoother wall quality surfaces after the laser cutting process and showed improved transfer efficiencies. Others exhibited a rougher aperture side wall and lower transfer efficiencies. Overall, it was shown that by choosing the best base material and applying a ceramic nano-coating technology, transfer efficiencies can be optimized and print variation reduced in the assembly process.
I would like to thank Andrea Motley, our summer intern, for the hours of print testing to obtain the data needed for this article.
1. Shea, C., & Whittier, R. “The Effects of Stencil Alloy and Cut Quality on Solder Paste Print Performance,” Proceedings of SMTA International, October 2014.
2. IPC. “IPC-7525B 2011-October Stencil Design Guidelines.”
3. Bath, J., Lentz, T., & Smith, G. “An Investigation into the Use of Nano-coated Stencils to Improve Solder Paste Printing with Small Stencil Aperture Area Ratios,” Proceedings of IPC APEX EXPO 2017 Technical Conference.
4. Voort, G. “Committee E-4 and Grain Size Measurements: 75 Years of Progress,” ASTM Standardization News, May 1991.
Greg Smith is manager of stencil technology at BlueRing Stencils and an I-Connect007 columnist. To read past columns or contact Smith, click here.