The change over to lead-free creates an excellent opportunity to evaluate new materials and upgrade manufacturing processes that will result in improved quality and a lower manufacturing cost.

The conversion of electronics manufacturing processes to comply with RoHS regulations provides a great opportunity for process improvement in order to provide efficient, as well as environmentally friendly, design and manufacturing processes. Six Sigma techniques for process measurement, analysis and improvement can be used to select optimum RoHS-compliant processes that will increase quality and reduce cost. The methodologies outlined here were used by the New England Lead-Free Consortium created by the author, and jointly supported by funding and support from member companies, the Toxics Use reduction Institute (TURI) and EPA.

The goals of Six Sigma are to create a design and manufacturing environment for products that are virtually defect free in manufacturing, beginning with incoming materials through every step in the process. Though Six Sigma today has morphed more into a qualitative methodology incorporating lean manufacturing methods to reduce inventory, work in process and throughput times, its original intent was to focus on improving quality through analyzing design quality as well as manufacturing defects. A strong component of Six Sigma is the Design of Experiments (DoE) methodology, which allows for a quick and comprehensive evaluation of alternatives in materials and processes. Combined with the opportunity of new materials mandated by RoHS, the Six Sigma and DoE techniques can be used for successfully selecting material alternatives for products and developing optimum new processes to manufacture them, thus providing a unique opportunity to improve quality and reduce cost.

The RoHS directive bans several key elements and materials that were widely used in the electronics industry, with some exemptions for certain industries such as military and medical products. However, as the component manufacturers began their compliance process by eliminating banned substances such as lead in the component finishes, the exempted industries found that they could not easily obtain their traditional leaded components and are being forced to make necessary changes to their components and processes to be RoHS compliant. Therefore, it appears that the impact of the RoHS regulation is a universal switch away from the banned substances for all industries, giving every company the opportunity to make optimum material and process improvements.

The implementation of RoHS offers companies several dilemmas as well as opportunities. This is because of the myriad alternatives being proposed by their suppliers with conflicting claims and the pace of technological progress in material technologies. What is a hot RoHS material substitute today might be out of favor tomorrow due to subsequent developments. An example of this is Bismuth-based solders, whose lower melting temperatures were attractive, but have fallen out of favor due to contamination problems with lead. Tin based solders and finishes were suspect as lead-free replacements because of the resulting tin whiskers. In addition, technological developments in material technology, as in any other, tend to produce leapfrog effects: A supplier who claims to have the best results for his materials might be overtaken with another supplier with a newer technology. So what is a company trying to implement RoHS to do?

The larger companies have developed their own research programs with a multitude of talent and resources to solve the problems of material and process conversion for RoHS compliance. They might work with their contractor and material supplier to implement RoHS compliance using a variety of reliability and test methods, as well as complex analytical tools such as vibration platforms, long term environmental testing to failure and electronic scanning microscopes to see the interfacing layers of RoHS compliant materials. These sophisticated analysis tools and extensive DoE matrices were used to evaluate a large number of candidate material replacement and process parameters.

Medium and smaller companies could not afford these massive programs, but could develop cost effective conversion programs using Six Sigma principles for RoHS conversion projects. A set of simple guidelines to effectively manage a successful conversion could be as follows:

1. Avoid the NIH (not invented here) syndrome. There are many resources available for identifying successful materials and process replacement for RoHS prohibited materials. These include national consortia and standards setting organizations for electronic products such as iNEMI, SMTA and IPC. However, these organizations might recommend the general composition of the replacement materials, but not the specific process parameters to handle the local compliment of equipment and product mix that the company uses.

2. Use standard performance criteria and test methods for the RoHS material whenever possible. Standards for reliability testing using temperature and humidity cycling, vibrations, electrical conductivity and mechanical stress exist for many materials from the same sources mentioned in the preceding paragraph. In addition, use testing methods that are standardized in industry, either by using techniques that were outlined in the standards or using commonly available commercial testers to perform the test.

3. When standardized or commonly used testing methods are not available, the current processes could be used as the baseline when comparing RoHS-compliant materials to the current process. Comparing to the baseline can also be used when there is not enough time or resources to properly conduct the testing for the RoHS materials. This could be the case for shorter term environmental testing. Examples of this would be to use less temperature, humidity cycling and to not test failures, but to compare pull tests of RoHS materials to baseline leaded counterparts. Vibration testing could be on specific spectrum lines and not the full spectrum of frequencies. For example, statistical significance testing could be used to compare lead-free results to the current product leaded manufacture process baselines.

4. Use the latest material selection available from leading material suppliers, knowing that today’s RoHS material champion might not be the champion tomorrow, and that the tests might have to be repeated down the road as material technology keeps improving. Avoid propriety or patented materials.

5. Realize that the RoHS conversion effort might not be performed in one single large continuum project, but might be a succession of smaller projects that builds on the knowledge acquired in the previous project. For example, in lead-free soldering there might be an initial project to deal with surface mount components, with a distinct portion for BGAs, then follow on projects for through-hole and rework.

In the New England Lead Consortium of companies and academia, research efforts for RoHS conversion began in 1999 and continue today. The research followed 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 materials and processes in terms of reliability; it 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 project was to investigate alternatives to materials and processes that are common to consortium members. The typical compliment of component technologies (including SMTA and through-hole), types and finishes that were used by the consortium members were considered. In addition, suppliers of incoming tools and materials (laminate types and finishes, stencils, etc.) and the manufacturing processes for the members should also be included in the alternative research.

The projects conducted for the lead-free RoHS conversion is comprised of four major phases to date, with each phase taking about two years. The lengthy time frame is due to the agreement by the consortium members and the funding requirements, as most materials and actual manufacture and test 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 web site at www.turi.org. The phases were as follows:

Phase 1 – Feasibility. During this phase, the feasibility of lead-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 RoHS lead-free implementation during that time frame (1999-2001). What solder composition is the best alternative to leaded tin/bismuth, tin/silver/copper (SAC) and tin/silver? What about the higher melting temperatures? Can the thermal energy used to melt the solder be delivered integrated in time to lessen the impact of the thermal shock on electronic components? Can a lead-free process deliver zero defects under controlled conditions?

Table 1 [PDF format] shows the experiment matrix for Phase 1, showing the selection of five factors in 27 experiments. The 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 experiment to lessen the time and effort involved. The reliability testing was performed through lead pull tests after 2,000 typical thermal cycles of 0 to 100?C in one hour cycles, and the number of solder defects used on IPC standards were analyzed on a part per million (ppm) basis.

The results were very encouraging. Reliability data (thermal cycling followed by pull tests) showed that the lead-free joints were stronger than leaded, and the quality data indicated that zero defects were possible with lead-free soldering. Manufacturability issues of thermal profiling were also shown to be of little or no significance. Table 1 shows the design matrix, which gives an indication of the scope of experimentation involved.

Phase 2 – Wide Material Selection. In this phase, the experience gained from the first phase was used to narrow down some of the choices such as the SAC solder formulation and a common thermal reflow profile, while the alternatives for laminate finish and component types surface finishes were expanded. Manufacturability concerns such as the use of nitrogen was also added. A full factorial experiment was performed to make sure that there is no inferred significance. A baseline of leaded components was also produced and compared to the lead-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% visual testing according to IPC standards were used, in addition to some investigations of the inner-metallic layers using SEM.

The results were very similar to Phase 1, in that certain combination of materials and processes produced near zero solder defects. Many of the factors were not significant either in reliability or quality testing. Figure 1 is a Minitab plot that shows the distribution of pull tests performed on QFP component types vs. the selected factors of surface finish, solder suppliers and reflow atmosphere. Subsequent analyses showed the selection of the finishes were not significant to each other in quality or reliability data.

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

Phase 3 – Manufacturing Process Optimization. Building on the previous phase, the material and process selection was narrowed down to the successful candidates gleaned from Phase 2, then the project focused on simulating actual manufacturing conditions. A larger test PCB was manufactured at the standard panel size of 16 x 18 as opposed to the 6 x 9 from Phase 2.

Though a full factorial experiment was conducted, the total number of test PCBs that were analyzed was at 24 lead-free PCBs as well as 12 for a leaded PCBs 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 SMTA technologies indicated that with the factor selection used, there were no non-obvious significant differences between the lead-free and the leaded baseline PCBs in reliability as measured by the pull tests and in quality, as measured by visual inspection. The obvious significant differences in pull tests were in three areas: lead finish (tin vs. tin/bismuth), pull direction (up or down, because of the location of the pull gage) and the interaction laminate finish (one of the two SAC solder suppliers performed significantly different with the three laminate types shown). Figure 2 is a plot generated by Minitab to show the various interactions present in the DoE experiment of Phase 3. Quality analysis showed a significant lower quality for through-hole and rework condition, necessitating a closer look at these conditions in Phase 4.

Vibration tests were done on a random basis, with varying maximum input to try to distinguish between the combinations of materials and processes. In addition, the components were subject to several thermal cycles to simulate production rework and repair conditions. The tests were somewhat inconclusive, partly because of some production issues of the laminate, and also because of the random nature of the vibrations source.

Phase 4 – Manufacturing and Technology Optimization Extension. In this phase, the unresolved issues of Phase 3 were further investigated. These include the through-hole technology and rework methodology. In addition, new materials that might be included in future RoHS directives 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 chrome used at the University of Massachusetts Lowell in association with MA/Com Tyco Electronics. Several applications of hex chrome were examined and a replacement chart for each application was generated based on extensive testing in harsh environment, all meeting company and industry standards. The 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 replacement and their manufacturing processes, and to apply Six Sigma principles to improve quality and lower manufacturing costs. PCD&M

Dr. Sammy G. Shina, PE, is a professor of Mechanical Engineering at the University of Massachusetts Lowell. He can be contacted at This email address is being protected from spambots. You need JavaScript enabled to view it..

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