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Developing a green product development strategy

Posted: 01 Aug 2008 ?? ?Print Version ?Bookmark and Share

Keywords:green electronics? green design and manufacturing?

Research efforts for RoHS conversion by the New England Lead Free Consortium of companies and academia, which was created by the author, began in 1999 and continue today. The research followed the broad guidelines outlined above. The concerns for RoHS conversion were focused on three parts: reliability, quality, and manufacturability. The RoHS-compliant alternatives had to meet and/or exceed current nongreen materials and processes in terms of reliability; they should be able to produce virtually defect-free products and be implemented in a typical manufacturing process line with standard machines and processes, including repair and rework.

The projects conducted for the Pb-free conversion are comprised of four major phases to date, with each phase taking about two years. The lengthy time is due to the consensus needed to be achieved by the consortium members, and the funding requirements, as most materials and actual manufacture and tests were donated or performed by member companies, on a voluntary and pro bono basis. More information, including papers published on these projects, could be obtained from the author or the TURI website at www.turi.org. More detailed information on these phases is available in the next chapters, showing the technical background and reasoning behind the decisions made.

The phases were as follows:

1. Feasibility (phase 1)During this phase, the feasibility of Pb-free soldering was explored as a viable alternative to leaded solder. The goal of this phase was to provide knowledge on the major issues of Pb-free implementation during that time frame (1999 to 2001): What solder composition is the best alternative to tin/lead [(tin/bismuth, tin/silver/copper (SAC), and tin/silver]? What about the higher melting temperatures? Can the thermal energy to melt the solder be integrated in time to lessen the impact of the thermal shock on electronic components? Can a Pb-free process deliver zero defects under controlled conditions?

TAL refers to time above liquidus. The project material and process selection was limited to a small number of laminate and component finishes. The selection was also organized in a set of partial factorial experiments to lessen the time and effort involved. The reliability testing was performed through pull tests after 2,000 typical thermal cycles of 0C to 100C in 1-h cycles, and the number of solder defects was analyzed on a ppm (parts per million) basis, according to IPC standards.

The results were very encouraging. Reliability data (thermal cycling followed by pull tests) showed that the Pb-free joints were stronger than legacy tin lead joints, and the quality data indicated that zero defects were possible with Pb-free soldering in certain combinations of solders and PCB surface finishes. Manufacturability issues of thermal profiling were also shown to be of little or no significance.

2. Wide material selection (phase 2). In this phase, the experience gained from the first phase was used to narrow some of the choices such as the SAC solder formulation and a common thermal reflow profile, while the alternatives for laminate finish (5) and component type surface finishes (4) were expanded. Manufacturability concerns such as the use of nitrogen were also added. A full factorial experiment was performed to make sure that there was no confounding of interactions. A baseline of legacy tin lead finish components was also produced and compared to the Pb-free alternatives. This experiment was much larger in scope and effort than the first experiment. The same reliability assessment of thermal cycling and pull tests as well as quality consideration of 100 percent visual testing according to IPC standards was used, in addition to some investigations of the inner metallic layers using SEM.

The results were very similar to those of phase 1, in that certain combinations of materials and processes produced near zero solder defects. Many of the factors were not significant in either reliability or quality testing.

Figure 1.1 is a Minitab plot that shows the distribution of visual defects versus the selected factors of surface finish, solder suppliers, and reflow atmosphere. Subsequent analyses showed the selections of the finishes were not significant to one another in quality or reliability data.

An interesting subtest of the analysis was that under certain combinations of materials and processes, the quality and reliability of Pb-free soldering were not affected by whether nitrogen was used in the reflow process. This is an important finding in terms of reducing the cost of electronics manufacturing.

3. Manufacturing process optimization (phase 3, also called TURI TV3 in Chap. 6)Building on the previous phase, the material and process selection was narrowed down to the successful candidates gleaned from phase 2, and then the project focused on simulating actual manufacturing conditions. A larger test PCB was manufactured, at the standard panel size of 16 x 18 inches as opposed to the 6-inches x 9-inches from phase 2.

Although a full factorial experiment was conducted, the total number of test PCBs that were analyzed was at 24 Pb-free PCBs as well as 12 for a leaded PCB baseline, compared to 120 PCBs from phase 3. The testing performed was similar to phase 2, with the addition of vibrations and multiple reflows (to simulate rework and repair) to the mix. The reliability and quality results for surface mount technologies (SMTs) indicated that with the factor selection used, there were no significant differences between the Pb-free and the leaded baseline PCBs in reliability as measured by the pull tests, and in quality, as measured by visual inspection. The significant differences in pull tests were in three areas: lead finish (tin versus tin-bismuth), pull direction (up or down, because of the location of the pull gage), and the interaction of laminate finish and laminate type (one of the two SAC solder suppliers performed significantly different with the three laminate types shown).

Figure 1.2 is a plot generated by Minitab to show the various interactions present in the DoE of phase 3. Quality analysis showed a significant lower quality for through hole and rework conditions, necessitating a closer look at these conditions in phase 4, and recounted in Chap. 6 in this book.

4. Manufacturing and technology optimization extension (phase 4, also called TURI TV4 in Chap. 6). In this phase, the unresolved issues of phase 3 were further investigated. These include the through hole technology and the rework methodology. In addition, new materials that might be included in future green efforts such as halogen-free laminates are being investigated against a baseline of halogen-based materials. Similar efforts were undertaken to comply with RoHS requirements for other prohibited materials, including hexavalent chromium, done in the University of Massachusetts Lowell in association with Tyco Electronics. Several alternatives were examined, and a replacement chart for each application was generated based on extensive testing in harsh environment, all meeting company and industry standards. This successfully concluded project followed the general guidelines given above.

In conclusion, the RoHS conversion process for prohibited materials is an excellent opportunity to examine the material replacements and their manufacturing processes and to apply Six Sigma and quality principles to improve quality and lower manufacturing costs.

- Sammy G. Shina
Author, Green Electronics Design & Manufacturing"

Note: Excerpted with permission from McGraw-Hill Professional from Green Electronics Design & Manufacturing by Sammy G. Shina (McGraw-Hill Professional, 2008).


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