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


Reading time ( words)

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.

GregSmith-Figure 5.jpg

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.

GregSmith-Figure 6.jpg

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.

GregSmith-Table 3.JPG

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.

GregSmith-Figure 7.jpg

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).

GregSmith-Figure 8.jpg

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.

Share

Print


Suggested Items

Approaches to Overcome Nodules and Scratches on Wire-Bondable Plating on PCBs

07/17/2019 | Young K. Song and Vanja Bukva, Teledyne Dalsa Inc., and Ryan Wong, FTG Circuits
Initially adopted internal specifications for acceptance of printed circuit boards (PCBs) used for wire bonding was that there were no nodules or scratches allowed on the wirebond pads when inspected under 20X magnification. This paper details if wire bonding could be successfully performed over nodules and scratches and if there was a dimensional threshold where wire bonding could be successful.

Practical Implementation of Assembly Processes for Low Melting Point Solder Pastes (Part 1)

07/16/2019 | Adam Murling, Miloš Lazić, and Don Wood, Indium Corporation; and Martin Anselm, Rochester Institute of Technology
Since 2006 and the implementation of the RoHS directive, the interest in bismuth-tin solder alloys—whose melting point around 140°C is very desirable because it allows for the use of lower temperature laminate materials and reduces thermal stress on sensitive components—has only increased as the industry has searched for Pb-free alternatives to the chosen standard, SAC305, which melts at considerably higher temperatures than the incumbent tin-lead alloys.

Size Matters: The Effects of Solder Powder Size on Solder Paste Performance

07/08/2019 | Tony Lentz, FCT Assembly
Solder powder size is a popular topic in the electronics industry due to the continuing trend of miniaturization of electronics. And the question commonly asked is, "When should we switch from Type 3 to a smaller solder powder?" Read more to find out.



Copyright © 2019 I-Connect007. All rights reserved.