B2PCOE Pb-Free Manhattan Project Report Phase I - Recommendations
From B2P Portal
9.1 Overview
This report documents the current baseline practices for Pb-Free electronics mitigations and usage along with corresponding technology and manufacturing gaps. Based upon this compilation of information, the following summary recommendations are provided for the purpose of minimizing risks associated with the use of Pb-free electronics in A&D products. Additional details are contained in the appropriate sections within this report.
9.2 General Design Recommendations
- As part of a risk management strategy, employ and use a structured Pb-free control plan, incorporating the existing GEIA documents as a guideline, updating where necessary for new materials and processes. See specific section on design for further details.
- Develop a Pb-free material compatibility guideline that incorporates all facets of the material selection process, which will include specifications and recommendations for design on the basis of environmental requirements.
- Product designers should consider the complications incurred by the increased mitigation efforts to reduce whiskers, as well as increased incidences of electrical failures incurred by higher manufacturing process temperatures. Also as a result of this, increase the frequency and number of both "on-board" analysis and subsequent field inspections.
- Insist that the environmental conditions be defined clearly. No cognitive design recommendations for Pb-free electronics can be made without this information. This will also affect the type and severity of the test conditions for ESS where the qualification parameters are defined.
- Expand on the criteria and data needed for a preferred parts list where necessary, and define the conditions under which a Pb-free electronic part can be added to a BOM.
- Design rules should include a strategy to handle modification of the way printed wiring boards are designed, subsequently assembled, and will include a selection of alloys and substrates potentially more suitable than what presently exists. Specialized designs for RF should also be considered. Some of the key areas:
- Attachment Alloys
- Component Finishes
- Printed Wiring Boards
- PCB Finishes
- Mechanical Parts
- Make provisions in the design of electronic systems for field repairs, which will inevitably be subject to different soldering materials and conditions.
- An effective and rapid method of validating new designs needs to be developed which utilizes test parameters derived for Pb-free materials and conditions, and includes a variety of accelerated stress tests.
- Include tin whisker risk as part of the design criteria. Use the GEIA standards as guidelines.
9.3 General Manufacturing Recommendations
- Provide a comprehensive DFM plan which includes assessing all the potential processes, materials, and in-testing modifications that will be required for the introduction of Pb-free material into the manufacturing stream. Include an emphasis on first article inspection as a critical event before proceeding to manufacturing.
- Investments will have to be made to update equipment . This includes reflow process equipment, in-process inspection, and analytical equipment. Additional investments will be needed in the area of training, which includes personnel from all the areas of manufacturing.
- It is inevitable that engineering time will have to be invested to determine the parametric process window for Pb-free electronics, with an emphasis on optimizing around reflow conditions and cleaning cycles.
- Rework processes must be re-qualified with careful consideration to the additional thermal energy that will be used as part of the processes.
9.4 General Sustainment Recommendations
- Sustainment involves a large number of interactive areas, all with issues regarding Pb-free electronics. An infrastructure presently exists to accommodate the various elements of the sustainment process, but a greater concerted effort will be needed to ensure that the details – often missing for accurate management of configuration control – are available for Pb-free electronics. The particulars for the proper course of action should be decided upon collaboratively among sustainment, manufacturing, design, and the customer. The specifics should include:
- A Lead Free Control Plan
- Repair Procedures and Metrics
- Pb-Free Assessment Plan
- Feedback from the Field
- Contingency Plan for Unavailable Parts
- Inventory Plan to Maintain Overage
- An Assessment of COTS as a Suitable Replacement for Repairs
- There will be heavy pressure upon repair depots to sustain Pb-free materials sets beyond their capability to handle them. It is highly recommended that repair facilities restrict their material usage to one or two selected Pb-free solders, and more critically, that the original repair requirements be modified to reflect the change.
- The original designer of the assembly must be notified of repair modifications, which may have been necessitated by an unexpected change in SnPb component or material availability. Communication between the repair depot and the original assembly designers is critical.
9.5 General Testing Recommendations
- The upper and lower boundaries of many of the thermal cycling tests are not adequate to simulate field failures, and must be established. The same can be said for isothermal conditions for long term aging or as a prerequisite for further thermal stress testing.
- Parameters for mechanical testing (e.g., vibration) need modification; appropriate test parameters must be defined.
- Validated computational models to predict fatigue field failure must be developed. This requires detailed information on test vehicle design, test parameters, and field results.
- The JEDEC standards can be used as a guideline in the interim, but they are insufficient as an acceptance document for A&D applications.
9.6 General Reliability Recommendations
- Investment in further research and characterization of Pb-free materials is needed to prevent the reliability uncertainties due to the disparity between the existing SnPb knowledge base and the more limited Pb-free electronics data pool.
- Avoid using Pb-free attachments in mission critical A&D applications.
- Research is needed on the accumulative damage to Pb-free electronic assemblies from environmental conditions under varying loads. Essentially, there is non-liner degradation that is occurring which is not accounted for in present ESS models.
- The low temperature damage (-40 °C) to Pb-free assemblies is an issue. Constitutive models will have to be developed.
- For applications where assemblies are subjected to sustained loads, SAC alloys with < 3% silver are not recommended.
9.7 Summary of Recommendations for Phase II of the Lead Free Electronics Manhattan Project
Using the corporate knowledge collected in the Phase I deliverable, develop a research and development roadmap for future projects.
This requires a thorough vetting of where the gaps are and how they interact with each other. For example, selection of a particular material set based on the current baseline practices may produce acceptable first tier results, but it may impact other areas that are not readily obvious. For example, selecting SAC305 solder is the commercial "alloy of choice" as a best manufacturing practice. Through a gap analysis for A&D applications, the likelihood exists that at some strain value, the composition of SAC305 may not be suitable for high mechanical stress applications. Therefore, further work on strain point limitations would be needed, precipitating a recommendation that SAC305 may not be the A&D alloy of choice under high strain conditions.
Develop statements of work and budget estimates for the research and development projects.
Each specific area of focus (Design, Materials, Manufacturing, Sustainment, Testing, and Reliability) has tangible deficiencies. Statements of work for projects will need to be developed, estimated, and implemented to address these deficiencies.
Document results in a "Pb-Free Electronics Risk Mitigation Research Plan."
Once the data is collected on Pb-free electronics mitigation efforts, the documented results will need to be compiled, analyzed and added as part of a continually updated Pb-free electronics research and development program.
Develop a communication package to brief the customers.
Through the efforts of the B2PCOE, the PERM, and other consortium, a concerted, organized communications plan is needed to train providers and vendors about the best Pb-free manufacturing practices, and offer a briefing on the specifics of the technological roadmap.
Use the Lead Free Electronics Manhattan Project model for other industry-pervasive issues.
The model for conducting the Lead Free Electronics Manhattan Project has proven to be a successful method for integrating a diverse body of subject matter experts and creating an environment of synthesized information exchange and capture. It is recommended that this process be identified as a best practice and applied to other pervasive issues.
