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Lead-free: Screen Printing - Where to from Here?
December 31, 1969 |Estimated reading time: 6 minutes
The deadline for lead-free electronic assembly is less than 12 months away. Time is running out to get “production-wise.” This article reviews understandings of screen printing using lead-free materials, and discusses the final pieces of the puzzle.
By Richard Heimsch
With the July 2006 deadline for lead-free electronic assembly less than 12 months away, manufacturers must prioritize hands-on experience using production-standard, lead-free processes to be manufacturing at maximum yield and 100% confidence by that time.
Paste Technology
The industry appears to have agreed on the most suitable solder alloy - Sn96.5Ag3.0Cu0.5. However, the overall composition of the paste, including flux and other compounds, may vary considerably from one producer to the next. Users can expect to observe differences in wetting performance during reflow and in the strength and appearance of resulting solder joints. At pre-placement, the effects of these differences in paste composition are manifested as subtly different rheology, leading to different results in terms of aperture filling and paste release. Engineers must be able to find optimal machine and process settings to make valid comparisons, thereby identifying the paste that best matches their production and environmental constraints. When evaluating pastes and developing new processes, manufacturers should also bear in mind that new stencil design rules are likely to emerge. The existing rules were developed around SnPb pastes, which are known to display quite different physical properties than current lead-free alternatives.
Some valuable information about optimal materials and fabrication methods for a lead-free stencil also has been discovered. The message is that apertures with low-friction internal surfaces will perform better in terms of paste transfer and paste-release efficiency.
Process Parameters
Clearly, changing to a lead-free paste will demand changes in machine setup. For example, experimental data has shown that separation speed must be considered,1 whereas SnPb pastes are insensitive to variations in this parameter.
Figure 1. Typical process window for lead-free solder paste.
Figure 1 provides an indication of the interdependencies of all machine settings when working with lead-free pastes. Each corner of the cube represents one set of experimental settings. The average solder paste volume and the standard deviation across all deposits for each solder paste were used to derive a concise assessment of each process. Experiments that yielded poor results for paste volume with unacceptably high standard deviation are color-coded red. Where acceptable results are achieved, the experiment is colored yellow. Green areas denote a close-to-ideal process, where paste volume is closest to the nominal value and the standard deviation is lowest. This is a graphical representation of the results from a design of experiment (DoE) carried out to analyze parameters influencing lead-free screen printing performance. As well as highlighting the newly apparent importance of separation speed, this investigation also showed that lead-free pastes generally respond well to higher excursion speeds and lower transfer pressure when using an enclosed print head.
Pastes and Stencil Design
Systematically exceeding the limits of known stencil design rules, in all axes, provides the basis for a methodical approach to investigating if a new set of design rules may exist for screen printing with lead-free pastes. Tests using stencils displaying apertures in a variety of shapes and sizes have been used to explore the limits of paste-release efficiency.2 By measuring the volume of paste released from each individual aperture accurately and expressing this as a percentage of the actual aperture volume, paste-release efficiency can be plotted against the aspect ratio of the apertures.
What has become clear from such experiments is that established SnPb pastes support greater process robustness than their lead-free successors. Only when the aspect ratio is reduced below the practicality threshold of 0.6 did paste release for the SnPb control paste in the previous experiment fall below that of the lead-free formulations.2 However, the performance of lead-free pastes in this respect will almost certainly improve with further development because they are in the beginning of their evolutionary cycles.
Pastes and Stencil Technology
It is common sense that friction between the paste and the aperture walls has a strong influence on paste transfer. This has implications both for materials and fabrication technologies. For example, the electro-forming process results in inherently smooth aperture surfaces. This is enhanced by the low coefficient of friction displayed by nickel, from which electro-formed stencils are generated. On the other hand, laser-cut apertures present a relatively high friction surface, even though they are demonstrably adequate for many applications. If market pressures demand, stencils cut by laser from nickel-rich stainless steel may provide a workable balance of outright paste-release efficiency, turnaround time and overall stencil cost.
Figure 2 shows deposit height and area data, as well as the volume of paste released by individual apertures in a set of test stencils fabricated using electro-form and laser-cut processes. Because paste volume is closely related to the quality of the mechanical joint created between the device interconnect and the land during reflow, this investigation makes valuable data available in the quest to develop high-yield assembly processes based on lead-free materials.
Figure 2 confirms the benefits of the electro-form nickel stencil. However, the results are very close; and the hybrid stencil shows remarkable release characteristics with two different lead-free pastes.
Calculating Cp and Cpk from the accumulated average volume, area and height data for the two different lead-free pastes, as well as a leading SnPb paste, demonstrates the interdependencies between stencil technology, aperture design and solder paste selection. It appears that stencil technology - comprising the manufacturing process and base material - is the dominant variable. The electro-formed stencil has excellent capability, the hybrid stencil runs a close second and the ordinary stainless-steel, laser-cut stencil shows a marked fall in performance when used with lead-free paste.
Conclusion
A series of methodical investigations into the performance of emerging lead-free pastes in relation to all possible screen printing process variables has highlighted the areas in which process engineers should concentrate efforts over the coming months. Among the most intriguing points to emerge is that separation speed is a valid parameter for scrutiny and experimentation, and must be examined alongside excursion speed and pressure. At the same time, it also has become clear that higher excursion speed and lower transfer pressure tend to be the most suitable direction in which to move these variables when optimizing process settings for lead-free.
It is tempting to bemoan the passing of SnPb pastes. They have indeed served the industry well for many years, and still outperform lead-free solders in many respects. By contrast, new pastes have a lot of development ahead of them. Also, many other variables are changing simultaneously. Stencil technologies are likely to change such that engineers can expect to see greater numbers of nickel-electro-formed and high-nickel-content laser-cut stencils on the factory floor. Stencil design rules developed for SnPb pastes, which now appear poorly matched to some new pastes, will also drive further investigations to identify new and better-optimized design rules for lead-free applications.
The next step for stencil designers is to investigate the impact alignment offsets and aperture profile have on the assembly process of packages ranging from 0201 devices to 0.4-mm-pitch ICs, to arrive at robust processes for assembling the most advanced package types entering mainstream assembly.
Assemblers also should continue studying lead-free technology and seeking every possible opportunity to familiarize themselves with the behavior of lead-free pastes. This is the only way the industry will be truly ready for July 1, 2006.
REFERENCES
- Investigating Mass Imaging Lead-free Materials using Enclosed Print Head Technology; Ashmore, Goldsmith; 2002.
- Further Investigation into Mass Imaging Lead-free Materials using Enclosed Print Head Technology; Ashmore; 2004.
Richard Heimsch, president, DEK International, may be contacted via e-mail: rheimsch@dekusa.com.