A new platform employed sophisticated statistical tools, eschewing traditional trial-and-error paste formulation.

No-clean solder pastes use rosins or resins to boost flux activity and to ensure electrical reliability. Rosins are derived from natural sources, such as trees or plants. Resins are either highly refined rosins or completely synthesized chemistries with no basis in nature whatsoever. Rosins present problems because they are naturally occurring substances, and they are subject to a great deal of variation in their makeup and performance. They can contain numerous chemical compounds in varying amounts, depending on their country of origin, area of the country, source plant type – even changes to growing conditions. On the other hand, resins, which are manufactured under controlled conditions, provide far more consistent performance. Synthetic resins have rapidly evolved over the past 10 years as a result of increased polymer research. They promised the potential to eliminate many paste performance variations assemblers have grown to accept, but their implementation required formulators to step away from existing knowledge and experience to embark on a new learning curve.

That’s where the Six Sigma methodologies came into play. Although these principles have been used in the industry for years, for the most part they have been applied to PWB fabrication or assembly processes – not to product development, and certainly not to soldering materials themselves. The recent development of a new SnPb solder paste, however, combined modern raw materials and the application of Six Sigma principles.
The project was a unique endeavor that combined the analytical skills of Black Belts in electronics design and assembly with the expertise of flux and paste formulators. Both parties combined statistical and scientific knowledge to find new ways to apply the proven methods.

Identifying Deficiencies

One of the first steps in defining the path of the project was identifying deficiencies in currently available SnPb solder pastes. These perceived deficiencies include:

• Voiding in BGA components.
• Inability to run fast print speeds, sometimes gating production throughput.
• Reflow defects such as solder beading and balling, tombstoning, and wetting to oxidized copper pads.

These areas were targeted as major performance characteristics of the current solder paste formulation that required improvement, but experimental analysis was not solely limited to this group of outputs. A total of 27 performance characteristics were considered in the final analysis. The multiple inputs and multiple outputs associated with this project required a series of analyses to map the entire system. The primary Six Sigma tools used in the co-development effort included:

• Houses of Quality, used to establish the Voice of the Customer and translate customer requirements into product design requirements.
• Measurement Systems Evaluations, to define variability in the testing. Gage repeatability and reproducibility (GR&R) studies were used to verify certain partially subjective quantification methods such as solder bead count. Standard laboratory measurement techniques with GR&R and calibration records were not requalified.
• Response Optimization, to determine performance of each raw ingredient. Analytical methods that included high-performance liquid chromatography and gas chromatography were used to identify cause-and-effect relationships between raw material components and paste performance. (This correlation process was the cornerstone of understanding the mechanisms at work, shortcutting and outperforming the traditional trial-and-error methods.)
• Regression analysis and mixture DoEs, to determine optimum overall performance. Because the chemical compounds used in solder paste can sometimes have strong interactions with each other, mixture DoEs were preferred over traditional factorial designs to better characterize those interactions.
• Balanced Scorecards and Desirability Curves, which allowed the research team to reduce over 20 outputs to a single factor or variable (desirability) to optimize overall performance.

Results

Balancing solder paste properties can be a daunting task. Improving one property, such as voiding for example, can often impair other key properties, such as wetting. Overall optimization of performance is an intricate balancing act. The trial-and-error process has the potential to become an infinite loop, and eventually compromises must be struck to define a final product. Stacked bar graphs accelerate the process by helping to compile the overall scores for individual properties – the overall desirability – to aid the optimization process.

Figure 1 shows a sample stacked bar graph comparing different formulations. This tool helps provide an overall score, but it is not the ultimate decision tool. The product development team must still be on guard for individual properties performing below minimum acceptable levels. In the ranking system that was applied, a perfect score would equal 28. Normalizing those numbers to a percentage basis, the improvement in overall performance equals 42% over the previous formulation and 21% improvement over a competitive material.
Voiding. Improvement in voiding properties was a key goal of the development process. The final product demonstrated significantly less voiding than the previous paste (Figure 2).

Print speed. Another key area for desired performance improvement was the squeegee speed used in printing. The previous paste formulation demonstrated a maximum print speed of 2"/sec. (50 mm/sec.). Four inches/sec. (100 mm/sec.) was required of the new formula. Figure 3 shows three different types of paste prints at 100 mm/sec.

Reflow performance. Reflow considerations included solder balls, solder beads, tombstones, and wetting to OSP. Tests for these properties were performed on the Benchmarker II test vehicle. Figure 4 shows the difference in reflow performance between the existing and new formulations. The new formulation reduced solder balls by 72% and solder beads by 67%. The study did not produce enough tombstones to provide statistically significant results. The previous formulation generated one tombstone defect, and the new formulation generated none.

To gauge wetting performance, the test vehicle used four different patterns (Figure 5). Results from each test are scored. The more points that a paste formulation scored, the better it wetted.

Figure 6 shows results of the wetting test. The use of modern ingredients to improve soldering is obvious in the results. The previous formulation only scored points on the simplest test pattern, and scored no points in the more challenging patterns. The new formulation substantially outperformed the previous formulation in all four test categories.

The target characteristics defined by the customer included BGA voiding, print speed and reflow properties. In the cases of print speed and solder balls, actual values are reported; in the case of voiding and wetting, outputs were normalized by assigning points on a performance scale. Table 1 summarizes the outputs.
The original goals also included improvements to compatibility with direct pressure print heads, resistance to high humidity and resistance to hot slump. Performance in all major categories met or exceeded the original project’s goals.

Conclusions

The project changed the way the customer and solder paste supplier worked with each other. In the past, the customer would conduct periodic benchmarking. A list of desired attributes would be provided to the supplier, which would then choose what they felt was the best candidate based on the list. Unfortunately, this meant the solder paste supplier would independently determine the tradeoffs prior to submitting candidates to the customer for testing. The customer would test the product to determine its overall acceptability, and very little technical dialogue regarding performance benefits or tradeoffs occurred. The best candidates for the job could have possibly gone untested and unrecognized in this scenario, because the true needs of the product were not communicated on a functional basis. Furthermore, natural variation could not be captured or understood by the customer because its benchmarking occurred at a single point in time.

A major benefit for the supplier was correct use of experimental techniques and the ability to understand the outputs of solder paste formulations as functions of their inputs. Prior to this project, formulators were more familiar with factorial designs of experiments, and had limited exposure to mixture designs. Only through the customer-supplier interaction were the real benefits of mixture designs appreciated and maximized.

Both the supplier and customer immediately began realizing the mutual benefits of understanding the Y = f(X) relationships spanning from the flux’s raw material stage through the assembly process and on to the final solder joint. Dialogue regarding potential production issues and their resolutions were catalyzed by the mutual understanding of how multiple inputs (Xs) affected multiple outputs (Ys). Furthermore, the supplier began applying the methodologies from the SnPb co-development effort to the next generation of Pb-free solder paste products.

Even the assembler that lacks the resources to support a full Design for Six Sigma co-development effort can realize a cost savings by updating its process chemistries. Most of today’s no-clean SnPb solder pastes are based on formulations developed in the 1990s. Ten-year advances in the raw materials’ technologies have opened new pathways to improved material stability, with direct results in greater process stability. Upgrading a process chemistry, such as solder paste, can reduce the costs of defects and rework, improve assembly line utilization, and reduce expensive false failures at test.

Bibliography

H. Sanftleben, L. Lewis, K. Stark, M. Young and M. Skrzat, “The Value of Joint Customer and Supplier Quality Function Deployment (QFD) and Design for Six Sigma (DFSS) Toolset Applications,” SAE World Congress & Exhibition, April 2008.

Richard Lathrop, “The Digital Solder Paste,” IPC Apex Proceedings, April 2009.

Derek Moyer is an applications and customer support engineer with Heraeus (heraeus.com); derrick.moyer@heraeus.com. Brian Bauer, Steve Ratner, Martin Lopez, John McMaster, Frank Murch and Mike Skrzat are with Heraeus.