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Mass Imaging Performance with Lead-free Solder
December 31, 1969 |Estimated reading time: 7 minutes
Manufacturers face the rapid approach of the European Waste in Electrical Equipment (WEEE) Directive. Materials scientists have had plenty to say about lead-free alloys and soldering processes, and component and PCB industries believe they are ready for the new age. But assemblers now need hands-on experience, particularly at pre-placement, to implement workable processes in time for WEEE.
By Clive Ashmore
Pre-placement process performance has a profound impact on end-of-line yield. Approximately 60 percent of surface mount assembly defects can be traced back to this stage of assembly. Numerous studies with lead-rich materials have identified the major influences within the mass imaging process, and assemblers can now establish extremely accurate, repeatable performance. This is especially true when using an enclosed head system. But lead-free solder alloys change the paste properties considerably. And because mass imaging relies strongly on the process engineer's "feel" for what is happening, lead-free solder pastes present a new learning curve for mass imaging.
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Re-learning the Process
The printing process can be boiled down to two sub-processes: an aperture filling process, and an aperture release one. The first process is a function of parameter settings related to the action of the enclosed head. The second is an interaction of aperture characteristics (geometry) and the behavior of the solder paste material.
To begin to build a picture of the effects of lead-free pastes on the mass imaging process, the experiment described in this article set out to identify optimal process parameters for leading lead-free pastes. These parameters were then used to analyze paste release performance for each material. A leading lead-rich material also was included in the study to allow a meaningful comparison with established mass imaging performance.
This article is not aiming to present a definitive set of process recommendations. Rather, it aims to stimulate further practical research into optimal mass imaging process settings and selection of solder pastes. Engineers must build personal knowledge empirically — ideally off-line, in a suitable environment.
Experimental Conditions
Printing machine. Before embarking on the design of experiments (DoE) described in this article, the printing machine* was calibrated mechanically. Using the manufacturer's defined procedure, Cp and Cpk values were verified to pass the minimum 1.6 value. To reduce statistical "noise," the same machine, interface and transfer head were used throughout the experiment. The same substrate also was used throughout, for measurement purposes.
Materials. The solder paste samples used in this investigation are shown in Table 1. All samples had the same particle range Type 3 (25 to 45 µm), suspended in a no-clean flux medium. The alloy types and rheology packages are the main variable parameters. All samples shown in Table 1 are commercially available.
Stencil. To complete the process window experiment, a laser-cut metal stencil of 100-µm thickness was selected, featuring apertures complying with established design rules for 0.4-mm pitch QFP and 0.5-mm pitch CSP packages. Aperture geometries are described in Table 2.
For the paste release experiment, a stencil was created with a range of aperture geometries to demonstrate the variation of paste release performance with diminishing aspect ratio. Studying stencil geometry in relation to paste release is important, because stencil designers choose progressively lower aspect ratios to print fine details when using stencils that are also thick enough to achieve suitable paste volume for soldering large components, such as tantalum capacitors or connectors. Table 3 shows the range of aperture characteristics included in the second stencil of 125-µm thickness.
A 300-mm dual-chamber transfer head** with stepped metal wipers (200 to 300 µm) was used throughout the investigation. The stencil featured apertures arranged both parallel and perpendicular to the transfer head. The print quality and defects responses were measured with a solder paste inspection system.***
Design of Experiments
To begin, a DoE was created with three factors (print speed, paste pressure and separation speed) and two levels (high and low), with a center point (medium). This resulted in nine experiments, as shown in the matrix of Table 4. Levels were chosen that would allow investigation of the capability at the limits of each parameter, especially at the higher speeds. All other process parameters, such as print gap and system pressure were constant. The operating temperature and humidity were stable at 23°C and 40%, respectively.
Using the three factors mentioned in Table 4, the results can be presented in a 3-D diagram, where each corner of the cube represents one of the nine experiments. By analyzing paste deposits using the solder paste inspection system, average solder paste volume and standard deviation were calculated across all deposits for each solder paste being tested. Color-coding the experiment results allows process parameters to be concisely compared. Those experiments that yielded poor results for paste volume with unacceptably high standard deviation are shown in red. The experiment is colored yellow where acceptable results are achieved. Green areas highlight the point at which paste volume is closest to the nominal value, with the lowest standard deviation; that is, a close to ideal process. Graphical results from the DoE for each solder paste are shown in Figures 1 to 4.
Figure 1. (left) Process window for material W. Figure 2. (right) Process window for material X.
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Figure 3. (left) Process window for material Y. Figure 4. (right) Process window for material Z.
These diagrams clearly show that each factor has a significant effect on process performance. For each solder paste, a "sweet spot" can be found. This demonstrates that all lead-free pastes are compatible with enclosed head mass imaging technology. Moreover, high excursion speed and low print pressure generally delivers better results. Separation speed also appears to be a critical factor in the search for a robust process.
Paste Release Analysis
Using the second stencil, measuring 125 µm thick and featuring apertures ranging from 150 to 220 µm in width for a 0.4-mm-pitch QFP, and from 200 to 350 µm in width for a 0.5-mm-pitch CSP, the optimal process settings for each paste type were used to analyze paste release. Results recorded by the solder paste inspection system accurately identified the volume of paste released by each individual aperture. Plotting paste release efficiency as a percentage of the nominal paste volume against the aspect ratio of the apertures resulted in the paste release curves of Figures 5 and 6. The results for rectangular QFP and round CSP apertures are shown separately to clarify paste performance in each application.
Figure 5. Release curve for rectangular apertures.
Within the industry, an aspect ratio of 0.66 is generally accepted to be the lowest useable value for production purposes. By including the lead-rich solder paste, material Z, this experiment shows that established SnPb pastes support greater process robustness than lead-free at all practical stencil geometries. Only when the aspect ratio is reduced below 0.6 does paste release for lead-rich paste fall below that of lead-free formulations. Given that the lead-free pastes are relatively new, and the technology is at the beginning of its lifecycle, this is perhaps to be expected.
Figure 6. Release curve for round apertures.
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Conclusion
The results achieved establish that enclosed head mass imaging is a valid technology for use with lead-free solder pastes. Second, by analyzing the effects of key process parameters, the first part of the experiment showed that separation speed has a critical impact on process performance. Process engineers working in commercial environments are unlikely to have identified this dependency, as traditional lead-rich pastes are relatively insensitive to changes in this parameter.
The paste release experiment allowed a clear performance comparison with current lead-rich solder pastes. Manufacturers can now begin to form expectations for the new lead-free pastes. Finally, the chief disadvantage that lead-free pastes currently display — noticeably lower paste release efficiency — has been identified and, to some extent, quantified. This data can be used to direct further investigation. Potential areas of study include improvements to process settings, mass imaging techniques, stencil technologies and design rules, and material properties.
Further Work
The move to lead-free SMT assembly is potentially the most significant change in manufacturing since the arrival of the BGA package. By focusing predominantly on soldering processes and joint performance, solder paste vendors are closing in on a small number of suitable paste formulations that will meet reflow and electrical/mechanical performance criteria. OEMs and outsource electronic assemblers, who will be responsible for working with these new materials and processes, must now begin in earnest to finalize an implementation plan for each stage of SMT assembly, from pre-placement to reflow.
This article provides an introduction to the issues governing pre-placement performance with lead-free solder pastes, but mass imaging is a decidedly hands-on process. Much intuitive input must be gained "on the job." But acquiring this expertise is not easy, since few lead-free processes are in action at commercial sites today. It is certainly expensive, and usually impossible, to take over a commercial line for lead-free studies.
One company**** makes its laboratory facilities available on request for visiting engineers to benefit from the lead-free experience of its own applied process engineers, and to create their own studies using company equipment. The lab is equipped with printing and inspection equipment, and enables testing to full IPC requirements, including slump and solder balling analysis. The value of such one-to-one contact with lead-free processes, in an off-line and equipped facility, cannot be overstated.
* DEK Horizon printer.** ProFlow.*** CyberOptics SE300.**** DEK.
Clive Ashmore, global applied process engineering group manager, DEK, may be contacted at cashmore@dek.com.