-
- News
- Books
Featured Books
- smt007 Magazine
Latest Issues
Current IssueBox Build
One trend is to add box build and final assembly to your product offering. In this issue, we explore the opportunities and risks of adding system assembly to your service portfolio.
IPC APEX EXPO 2024 Pre-show
This month’s issue devotes its pages to a comprehensive preview of the IPC APEX EXPO 2024 event. Whether your role is technical or business, if you're new-to-the-industry or seasoned veteran, you'll find value throughout this program.
Boost Your Sales
Every part of your business can be evaluated as a process, including your sales funnel. Optimizing your selling process requires a coordinated effort between marketing and sales. In this issue, industry experts in marketing and sales offer their best advice on how to boost your sales efforts.
- Articles
- Columns
Search Console
- Links
- Events
||| MENU - smt007 Magazine
Using Design for Six Sigma to Develop Surface Mount Materials
December 31, 1969 |Estimated reading time: 6 minutes
By Michael Skrzat, Heraeus Incorporated
While most of the solder paste industry has recently been focused on lead-free products, a unique opportunity arose for a joint development project to create an improved no-clean tin/lead eutectic solder paste with a large automotive manufacturer. The current tin/lead paste has been qualified in the automotive industry for more than ten years. However, due to changes in surface mount technology including smaller components, new devices, and new surface finishes on both PCB and components, along with increased process changes such as enclosed print head and thru-hole applications, greater demands have been put on the performance of solder paste. The original solder paste formulation was not designed nor optimized for these new applications. Given how difficult it is to qualify a formulation change, we agreed to optimize the current paste formulation. Both companies decided to work together using a design for six sigma (DFSS) methodology to improve the solder paste performance.
Typically, DFSS is a design approach and toolset applied to develop new products or processes. It focuses on the voice of the customer and removing variation in the design of a product. In classical Six Sigma methodology, the define, measure, analyze, improve, and control (DMAIC) model is most common. However, in this DFSS project we chose the initiate, define, develop, optimize, and verify (IDDOV) methodology. Both methods follow a similar path from defining or initiating a project through control or verifying results. The entire process uses existing Six Sigma tools such as the supplier, inputs, process, outputs, and customer (SIPOC) construct; thought maps; process maps; measurement system evaluation (MSE); design of experiments (DOE); and house of quality (HOQ) to define voice of the customer (VOC).
Initiate ProjectThe joint project began when we met with the customer to establish a business case and project infrastructure. A DFSS team was established and a thought map initiated to outline the program. The DFSS team included process experts from both solder supplier and automotive manufacturer teams to establish the functional characteristics and importance values to ensure that the final product would be suitable for all process applications.
Define RequirementsThe DFSS team reviewed the VOC by establishing a series of HOQ matrices. The HOQ1 translates the customer's needs and expectations of solder paste into functional requirements of the paste. The HOQ2 translates the functional requirements into design parameters of the paste. The design parameters can then be translated to the specific ingredients in the paste. The voice of the customer also is used to evaluate the importance of each parameter in the HOQ. The result of the HOQ is a set of metrics that will measure the performance of the paste beginning with shipping stability and continuing through printing performance, reflowed defects, and long-term stability. In total, 27 properties were identified and measured.
Identifying variability in processes and test methods is essential to defining requirements. To help identify variables, process maps were completed for flux and paste manufacturing as well as several test methods. Process maps help identify inputs that are critical and also noise parameters that must be held constant. To accurately identify variability, we called in the "experts," operators who use the equipment every day. The team also walked through the process to uncover any hidden factors that might hinder the overall outcome. Several MSEs were completed to establish variability in reproducibility and operator-to-operator repeatability. The DFSS team felt confident that if variability could be controlled, then the results from the DOE would be accurate.
Develop ConceptsAfter several weeks of defining requirements, the team finally was ready to begin solder paste development. The process began with screening experiments of the currently used formulation, changing one factor at a time (OFAT). When measuring screening experiments, the team would not measure every property from the HOQ. They focused on critical properties that would be affected by the change in a raw material in a solder paste. Regression analysis was used to statistically show the relationship between the raw material and the response measured.
In some instances, the DFSS team would need more than one component of the same functionality. For example, it is possible that the team would want more than one activator or more than one resin. To test these effectively, a mixture DOE was used. A mixture DOE differs from a classical DOE because each component does not behave independently. In a classical DOE, certain factors can be adjusted without affecting other factors. For example, in a solder paste printing DOE, changes can include print speed, squeegee pressure, screen separation speed, etc. In a mixture DOE, all factors must add up to 100%, so when one factor is reduced, one or more of the other factors must increase. Complicating the design even more is the fact that the solder paste must be functional. A simple mixture DOE may be a blend of fruit juices, for example, made to find an optimum blend of juices with respect to taste. Solder paste DOEs are more complex. Because the paste is a blend of liquids and solids, care must be taken in selecting factor levels so the paste will be able to function on a printer. Using screening DOEs, the team was able to optimize the proper ratio of some components and use them as one factor in the optimization DOE.
OptimizeAfter the final components were selected for the paste formulation, the levels of components of the final formulation needed to be optimized. More than 100 trials are necessary to understand all interactions of a seven-component DOE. Using current software, the design can be optimized to fewer trials if evaluating every higher-order interaction isn't required. Reducing the number of trials makes the optimization a manageable task. After completing the optimization design, the team analyzed the results using regression. The factors selected showed a significant effect on 15 of 27 total responses measured. Adjusting the factors in a certain way may have positive effects on some responses and negative effects on others. Tradeoffs between responses need to be evaluated. Using the importance and specification limits from the HOQ, a response optimization was deployed to balance the score of the pastes. A graphical representation of the response optimization can also show the effect each factor has on the responses. Finally, the optimized levels of the factors can be used to derive a math model for the DOE.VerifyThe optimized formulation and repeatability experiments were performed to verify the results of the design. Benchmark comparisons show that the new DFSS paste significantly outperformed both the existing paste and a competitor's paste. Finally, a robust optimization design is being performed on-site at the customer's location to measure the effect of variability during processing.
ConclusionThe DFSS paste outperformed the existing tin/lead paste in nearly every response. The stacked bar chart (Figure 2) also shows a 38% improvement over the competitor's paste. As the result of this process and the continuing RoHS exemptions, this may be the last tin/lead solder paste designed with significant attention to optimum performance and reliability.
We've experienced a six sigma methodology explosion. Many of the tools now are used to identify variability in other areas, such as quality and technical applications. This directly translates into improvements in products and applications. The joint ownership between solder manufacturer and customer to create a superior product translates into competitive advantages for all parties involved. We developed a more synergistic relationship with the automotive customer, with a greater level of solder paste formula comprehension as a direct result of applying the DFSS toolset. Michael Skrzat is the quality engineering manager for assembly materials at Heraeus Incorporated in West Conshohocken, Pa. Skrzat is responsible for material development, sourcing, and qualification as well as production and quality support at Heraeus. He has over 15 years experience in the electronics industry, and holds a Green Belt in Design for Six Sigma.