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RoHS Implementation: An OEM and EMS Case Study

Published: 07 December 2005
 by Guy Martindale and H.R. Chai
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The process by which a Pb-free line was evaluated is detailed.

Global Sourcing
Each OEM/EMS team approach to Pb-free conversion will be different. These differences can be generated by limiting factors such as company resources, RoHS budget, influence within the supply chain and logistics. Due to the smaller size of each company in this Pb-free case study, these limiting factors affected how roadmaps, testing and implementation plans were developed.

This article delineates the process by which a component designer and EMS firm worked together toward RoHS compliance. The companies were Rabbit Semiconductor (formerly Z-World), a fabless semiconductor company, and SEP Productionworks Ltd., a PCB supplier and contract manufacturer.

Figure 1
FIGURE 1: Cross-functional team diagram.
The first step toward Pb-free implementation was to set up cross-functional teams (Figure 1) with participation from all departments. Cooperation and buy-in at all levels, especially executive management, is critical to success. The established teams defined a mission:

An integral element was to gain as much knowledge as possible on the RoHS initiative. Each team member was required to attend workshops, seminars, training courses, online presentations and teleconferences. Most team members accumulated 60 to 90 hours of training and education during the first six months of process development. Starting in mid 2004, the majority of training materials detailed the specifics of the RoHS and WEEE directives and presented much the same information. In the past six months, the available information on RoHS has become more informative with detailed hands-on experience and explanations of what has or has not worked. The tone has become more collegial and team-oriented.

The team defined a company roadmap which included input from all suppliers. Due to the logistical difficulties with conversion, varying time frames and due dates will impact time-to-market and customer delivery. The roadmap should detail an overview of the RoHS and WEEE directives, company compliance goal, RoHS team mission, high level steps to becoming compliant, dates on Pb-free product availability and end-of-life (EOL) determinations.

As the team moved forward, critical impact areas were defined, with cross-functional teams were set up to handle each. Using a standard spreadsheet template, each team defined all tasks, actions and projects needed to address the critical impact area. These spreadsheets were rolled in to a master task sheet that was reviewed on a weekly basis for status updates and prioritization.

The Materials Compatibility team looked into the interaction between Pb-free paste, conformal coating and any heatsink compounds. The team developed a plan for dual processing on the manufacturing floor, running both a leaded and Pb-free process simultaneously. PCB surface finish and laminate compatibility were defined.

The Process and Equipment team defined the impact on the current process. Two five-zone ovens and one seven-zone oven were in use for Pb-free reflow. Whether to use an N2 blanket was discussed. With a current water-soluble process, the team defined an opportunity to convert to no-clean paste along with Pb-free. This would require ROI analysis for eliminating the wash process and performing both clean and no-clean Pb-free testing. A major concern about wave soldering was also defined. With a significant portion of the product line requiring wave soldering, an ROI analysis had to be performed on whether to replace the wave pot, pump and accessories, or to invest in a laser soldering system. This also becomes a business decision on though-hole products due to potential EOL cases. The impact on manual soldering irons and rework stations was outlined. Pb-free, it was determined, would require new tools, tips, heads and redefined profile programs for rework.

Components pose perhaps the biggest data management issue for Pb-free. The Component team looked at Pb-free part availability, certification and declaration of conformance on those parts. A decision had to be made on how to handle Pb-free part numbers. Do you change the internal part number for Pb-free? Most component suppliers are not changing manufacturing part numbers. Initially, detailed part number information requests were submitted to suppliers. This included a request for the percent composition of the six banned RoHS substances for each part number. Due to the level of detail required to answer these part number information requests, suppliers were stymied by the volume of data requested and did not respond in most cases. Apparently, component suppliers did not have the data readily available and were struggling with the Pb-free conversion. Follow-up requests that asked only for the part number and some signed documentation that the part was RoHS-compliant yielded better responses.

The Quality Assurance team outlined the new standards and inspection criteria for Pb-free solder joint inspection. IPC-A-610D addresses Pb-free solder joints; all operators would be certified to this standard. Long-term reliability of Pb-free solder joints is difficult to quantify. Thermocycle and highly accelerated life testing (HALT) will be performed to gather data on theoretical long-term reliability. An additional step was to work with customers that could beta-test Pb-free products, providing feedback from the field. With the increase in reflow temperatures, moisture sensitivity levels for components are changing, making it more critical to track MSLs.

The Engineering Design team looked at the cost of EOL vs. redesign for specific products. Components such as aluminum electrolytic capacitors, which cannot meet Pb-free reflow temperature requirements, may drive EOL or redesign. The team identified potential product performance issues with RoHS parts to be evaluated. All documentation, such as DfM, needed to be updated for new parameters, rules and guidelines for RoHS.

It was critical to have executive management buy-in on the RoHS program. The costs of conversion, EOL, changes in the supply chain, impact to marketing, sales and customers, and legal issues with RoHS need to be addressed from a high-level business perspective. These subjects need to be addressed early in the RoHS program. Last-minute decisions will cause failure.

EMS Process Qualification

Integral to the success of converting to Pb-free/RoHS was the inclusion of our contract manufacturer on the team from the start. Being able to share knowledge and prepare for qualification testing made the process easier. The advantage to working with an EMS is that a Pb-free/RoHS process is qualified and a source for producing Pb-free/RoHS product by July 2006 is defined. For the EMS, it is an opportunity to gain market share in the Pb-free/RoHS assembly business early in the industry transition.

Since SEPP is based in Shenzhen, preliminary setup and process parameters were set by the engineering staff at SEPP to establish a process before onsite test runs were performed. Procurement of RoHS parts was planned in advance to ensure timely delivery and to maximize the potential of having a 100% Pb-free BoM. Working with the paste supplier, SEPP ran tests on various modified profiles and stencils for a SAC305 paste. The 10-zone reflow profile was set up with one soak period and had a peak reflow temperature of 248°C with 70 sec. above liquidous. A 0.005" stencil was used and the aperture design was modified for 105% in the width and 100% in the length.

A four-day time frame was planned to perform the testing. To ensure complete separation of Pb-free and leaded processing, the Pb-free process is on a separate floor in the manufacturing plant. This aided in controlling potential logistical issues of mixing the two processes. The test plan evaluated 280 assemblies made up of two products – high and low Tg RoHS compliant laminate – and used >95% RoHS compliant components. Five components were not Pb-free/RoHS:

Although the headers did not meet the reflow temperature specification, they did not show signs of melting or deformation. Red and blue soldermask on the PCBs distinguished the high and low Tg RoHS laminate (Figure 2). Testing high and low Tg RoHS laminate was critical due to the characteristics of the test assemblies and the potential for panel warpage. The test vehicle had:

Figure 2
FIGURE 2: High (left) and low (right) Tg circuit boards.
Overall, the testing was successful. Initial results indicated that the Pb-free process was qualified.

To fully quantify the quality of the Pb-free solder joints, a sampling of the Pb-free assemblies was sent to a third-party laboratory for cross-sectional analysis. Each sample was inspected and photomicrographed. Samples were cross-sectioned, selectively etched and photographed using a scanning electron microscope (SEM). XED analysis was performed on the solder joints to determine composition by weight percent. Table 1 is an example of the analysis results.

Table 1

To further test the reliability of the Pb-free assemblies, the samples will be subjected to thermocycle and HALT.

In-House Process Qualification

From information gleaned in seminars and conferences, the impression preceding the initial test phase was that Pb-free reflow could not be performed in anything smaller than a seven-zone oven. With two five-zone oven process lines, this was a major concern and needed to be evaluated specifically with our products. In addition, the current process uses water-soluble paste. To reduce process steps and production cost, conversion to a no-clean process was evaluated.

Phase IA testing evaluated the following:

As Pb-free assemblies were built, specific in-process responses were evaluated. These were mainly visual observations including paste consistency, print and placement quality, wetting, solder joint appearance, panel warp and twist, and any component problems (Figure 3).

Figure 3
FIGURE 3: No-clean Pb-free solder joints. Pastes used were Sac305 (left and center) and SAC387 (right).
After top and bottom SMT were completed, all defects were recorded, a failure rate was determined and each sample was functionally tested. Phase IA testing results indicated significant differences between the pastes with regard to flux residue and solder ball defects. The Tg of the laminate did not have any impact on the test results.

Phase IB testing further evaluated the “best” paste from Phase IA. Prior to sending samples to an independent laboratory for cross-sectional analysis of the joint microstructure, additional reflow profiles were tested. If cross-sectional analysis of Phase IA samples indicated that the solder joints were unacceptable, there would be no indication on which direction to adjust the process.

Based on the profile from Phase IA, the profile was set lower on the maximum temperature by -10°C and higher by +15°C. With the five-zone oven, the actual changes in peak temperatures were -6° and +8°C from the standard profile.

Samples of each test configuration from Phase IA and IB were sent to an independent laboratory for cross-sectional analysis. Although no significant difference in the average composition of the solder between samples was evident, the lab results indicated a significant difference in the microstructure of the solder joint materials. This was due to the low, standard and high temperature reflow profile and the effects on the rate of cooling. Even with the microstructure difference, all the joints were acceptable as compared to standard leaded joints (Figure 4).

Figure 4
FIGURE 4: Cross-section and microstructure Pb-free no-clean solder joints.
Phase II testing evaluated the water-soluble versions of the pastes from Phase IA. The same visual and defect responses were recorded. Although consistency was different among the three paste suppliers, no print or placement issues were observed. The major defect with no-clean was solder balls; with water-soluble paste, it was shorts. The water-soluble pastes rated exactly the opposite of the no-clean. Thus, paste vendor selection would change based on clean vs. no-clean processing. Lab analysis results indicated good intermetallic structure and that all joints were acceptable overall.

Thermocycle and HALT testing will be performed on all samples from each phase of testing to provide more data on reliability and solder joint integrity. Based on the results thus far, a five-zone oven can be used for both clean and no-clean Pb-free processing for this specific product form factor. Additional testing will be performed on optimizing specific pastes and testing of alternate product families which include BGA technology.

Lessons Learned

RoHS and WEEE requirements are not set in stone and interpretations of the directives vary. This makes it difficult to develop complete company transition plans. How does a company apply for an exclusion based on the broad scope of electronics these directives encompass? Feedback from an industry teleconference in April 2005 indicated that the approval process for exclusions would take longer than the time pending before the July 2006 compliance date.

Sorting component supplier specification data on Pb-free parts is time-consuming. Requests for detailed data on Pb-free parts not likely be answered in a timely fashion. In addition, not all suppliers are creating new part numbers for Pb-free.

Quantifying the cost associated with converting to Pb-free can be difficult. The cost impacts every level of a conversion plan, including PCB laminate and surface finishes, components, independent lab testing, capital equipment, training and education, consultants, re-design and EOL.

The appearance of the Pb-free solder joint is dull and grainy compared to that of a leaded solder joint. Operators, inspectors and customers have to be trained to new criteria. Investment in certified training has an associated cost. The top concern from a quality standpoint is probably reliability. Long-term reliability testing will aid in determining the most reliable Pb-free process but defects such as tin whisker growth are still not totally understood for Pb-free soldering.

Ed.: This paper was first published at Nepcon South China in August 2005 and is used with permission of the authors.

References

  1. Douglas C. Montgomery, Design and Analysis of Experiments, 5th edition, John Wiley & Sons, 1997.

  2. Ray Prasad, Surface Mount Technology: Principles and Practice, 2nd edition, Chapman & Hall, 1997.


Guy Martindale is outsource program manager at Rabbit Semiconductor (rabbit.com); guy_martindale@rabbit.com. H.R. Chai is assembly manager at SEP Productionworks Ltd. (Contact Bob Ellis, director U.S. operations, SEP Productionworks Ltd.; sepusa1@sbcglobal.net.)

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