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

Estimated reading time: 12 minutes

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 ^[1].

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.

Conclusions

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.

Acknowledgments

I would like to thank Andrea Motley, our summer intern, for the hours of print testing to obtain the data needed for this article.

References

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.

• < Previous Page