B2PCOE Pb-Free Manhattan Project Report Phase I - Conclusions
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Contents |
8.1 Overview
The Phase I Manhattan Project Team has collectively documented a set of baseline best practices for addressing risks associated with the use of Pb-free, within the context of a product life cycle model. Drawing upon this collective body of expertise and knowledge, the following sections provide a summary of the major conclusions documented within the body of this report.
The effect of introducing Pb-free materials in A&D applications presents potential risk issues. An appropriate amount of scrutiny and resources should be utilized to address the surplus of unknown performance and reliability of Pb-free material. The problem is real and may manifest itself from the innocuous to the catastrophic.
The efforts of the Lead Free Electronics Manhattan Project – Phase I have shed considerable light onto the uncertainties regarding the consequences of utilizing Pb-free materials, intentionally or otherwise, in A&D programs. The various electronic material suppliers are not aware, or have little control over the supply chain and cannot prevent the admission of Pb-free products into the manufacturing flow. The challenge in mitigating the entry and effects of Pb-free materials is uniformly distributed across the product life cycle.
The best current baseline practices, outlined in this report, are to be considered a foundation for the subsequent research which will be used to solidify the gaps in Pb-free electronics. What can be validated through experimentation and data collection, must be part of a collective and comprehensive effort on the part of industry, DoD, consortiums and academia. The risk of a fragmented effort will only lead to both the replication and omission of critical projects, at a much higher cost, and more importantly, a remission in finding plausible solutions.
8.2 Design
The process of managing Pb-free risk begins in the early stages of an assembly or CCA design, where a greater impact can be made toward controlling and mitigating effects. Based on the findings of the Phase I Manhattan Project Team SMEs in the field of design, the following conclusions were drawn.
- The implementation of a Lead Free Control Plan is a necessity.
- Designers must be acutely aware that there are no heritage products from which to derive design rules.
- Single drop-in replacements do not currently exist that will conform to present SnPb baselines.
- Established practices and analytical techniques such as FMEA can be used, but with extreme caution. Modifications to these tools will need to be made to help improve detection of intermittent electrical failures.
- An accurate accounting of the environmental conditions is required for Pb-free.
- There is a critical need for controlled documentation, verification and selection process for the construction of a BOM.
- More stringent procedures need to be implemented for product qualification due to latent damage to PCBs subjected to Pb-free processes. This may require a larger sample of qualification lots.
- Designing to mitigate the effects of vibration will be paramount in Pb-free systems. The placement location of key components and stabilizing structures onto the assemblies may be needed.
- Computational models are needed to develop test parameters for Pb-free.
- The designer should expect that new failure modes will occur with Pb-free.
8.3 Manufacturing
The manufacturing process plays a fundamental role in developing the structure and morphology of solder joints, which will eventually form into an interconnection. The manufacturing processes for Pb-free are at a greater maturity level than the other supporting sectors of electronic assembly production, but face difficult challenges nevertheless. Engineering changes to the manufacturing process will be necessary for adaptation to Pb-free materials, since a mixed material set of SnPb and Pb-free will need modified process parameters to form a proper solder joint. Faced with these prospects, the manufacturing experts conclude:
- The use of Design for Manufacturing (DFM) tools are critical in establishing processes and procedures that will guard against the potential consequences during the Pb-free transition. It is of paramount importance that first article inspections are adhered to with close scrutiny of the results.
- Some amount of process development will be needed to optimize manufacturing for Pb-free materials. There are no existing "drop-in" processes.
- Plan for the need to modify equipment capability. This is particularly true for reflow ovens, wave solder, and hand soldering tools.
- The Pb-free solder will behave differently and will look different. Because of this, training of operators and other personnel will become crucial.
8.4 Sustainment
The reduced availability and increased cost of SnPb parts has made the task of sustainment more difficult, and has had a profound effect on all the elements of logistics management. Everything from configuration control, design traceability, part replacements, and material compositions has been affected by the transition of the COTS electronic industry from SnPb to Pb-free. A common conclusion that appears throughout the entire report is the failure of suppliers to document when changes have occurred; assuming the posture that small changes in material composition (e.g., component finish) do not violate the fit, form, and function directive for initiating an Engineering Change Notice (ECN). This conclusion is relevant to all areas, including sustainment, where traceability of original assemblies and materials is crucial to the rework process. The sustainment experts concur that:
- There needs to be a sustainable effort to ensure that data and information regarding any constituent part or material that goes into the construction of the electronic assemblies is known, adequately documented, and properly communicated through the product life cycle.
- There should be a flow of communication between the repair process group, logistics, and configuration management to ensure that repair documentation is updated, materials for assembly repair are defined, and part replacements are made available.
- Training of depot repair and other related sustainment personnel on Pb-Free will be required.
8.5 Testing
The failure to document changes to the material composition from SnPb to Pb-free creates a risk when assessing the reliability of the assemblies. ESS and other testing procedures have been structured to provide the most accurate characterization of SnPb in actual field conditions. The test conditions simulate the long term field effects that have not been defined for Pb-free materials, thereby creating an uncertainty in the accuracy of the results in predicting failure when using SnPb test conditions. The team has concluded the following:
- Though equipment presently exists to adequately test for Pb-free assemblies, test conditions and protocols have not yet been defined to provide adequate computational models to simulate service lifetime conditions.
- Current test methods are not adequate for Pb-free A&D electronic assemblies.
8.6 Reliability
Analysis of the reliability of Pb-free electronic assemblies has shown a consistent theme, asserting that the use of SnPb constitutive and predicative models cannot behave as adequate substitutes for characterizing Pb-free material behaviors. Essentially, the characteristic material properties of Pb-free are sufficiently different from SnPb, as to invalidate any assumptions predicated on the hypothesis. This is the case when subjecting both materials to thermo-mechanical stress or shock – the reliability results are unchanged. There are a number of factors which are integral in providing a validated model to predict lifetime behavior of a solder joint; unfortunately, at the present time, there is insufficient information about Pb-free solders to make a reasonable lifetime prediction. The reliability team concludes:
- Data to support reliability models for Pb-free is insufficient to make predictions on the long-term behavior of Pb-free assemblies for A&D programs.
- Pb-free materials are more susceptible to mechanical shock and vibration than SnPb.
- Thermo-mechanical behavior of Pb-free materials at varying temperatures is different than that of SnPb, especially at the low temperature extremities. This is a major concern for A&D products.
- There may be a need for certain pre-conditioning steps prior to ESS to properly asses the effect of the respective environmental test.
- There is no exacting method of mitigating tin whiskers at the present, other than to avoid pure tin.
8.7 Tin Whiskers
Pb-free solder joint reliability is one major concern for the electronics industry, especially in A&D applications; the risk associated with tin whiskers is equally serious. Tin whiskers continue to be a source of consternation for high reliability product providers. The history and the subsequent effects of tin whisker growth have been well known in an anecdotal manner over a number of years. The list of documented failures due to the discovery of tin whiskers continues to grow as the cause of catastrophic failures in avionic and high level infrastructure systems, while many failures due to whiskers go unreported due to liability concerns. The importance of the problem has not gone unnoticed, but in recent years has been highlighted due to the Pb-free transition.
A partial list of costly electronic equipment losses and availability losses is highlighted here to underscore the serious risk associated with this phenomenon.
- Nuclear Utilities Unplanned Shutdown – availability loss
- Space Shuttle Fleet Main Engine Gimbal Avionics – availability loss
- Seven Satellites: complete microprocessor failures – hardware loss
- Patriot Missiles – availability loss
- Six Other Missile Programs: complete failure – hardware loss
- Heart Pacemakers: complete failure – hardware loss
- Heart Defibrillators: complete failure – hardware loss
- F-15 Radar – availability loss
- Several Other Military Planes – availability loss
- Telecom Equipment – availability loss
There is a myriad of activity that centers on resolving the issues of tin whiskers, from predicting their cause to growth, to mitigating the effects. Not one organizational body has the resources to address the underlying solutions to the problem on their own, while the quality of information to resolve the issue of tin whiskers is varied and contradictory. As a result, an "each man for his own good" approach has been an obstruction to discovering universally feasible solutions.
8.8 Universal Gaps
The subject matter experts on the team recommend that a more inclusive, thorough research and development roadmap be developed and implemented. Generally acknowledged areas where gaps exist on a universal scale are:
- Commercial vs. Military
- The effects of product reliability and sustainment due to Pb-free electronics usage are not well understood. Additional empirical evidence on Pb-free electronics, especially long term reliability of greater than ten years, is needed. While some commercial telecommunications products specify lifetimes of ten years, typical A&D products such as military aircraft and missile weapon systems require much longer lifetimes, often measured in decades.
- Technical Knowledge
- There have been numerous Pb-free electronics research publications by the various consortia, industry and academia, representing a wide array of technical capacities. While these add a significant contribution to the overall Pb-free electronics knowledge base, it is still very limited, sparse, and often disconnected when addressing the reliability requirements of the DoD and A&D applications.
- Field Failures
- Field failure information regarding Pb-free electronics assemblies at this juncture is very sporadic. Equally critical, and found to be missing, is the information on the root causes in identifying mechanisms which are responsible for failure when it occurs, or how to mitigate them.
- Process Control
- Data to assess reliability of one specific Pb-free electronics manufacturing process over others can be difficult to interpret since many of the manufacturing variables are not consistent from one process to another. The Lead Free Electronics Manhattan Project has identified the best manufacturing current baseline practices that mitigate the effects of Pb-free electronics. However, more work needs to be done to address the various manufacturing technology gaps in order to produce greater process efficiency and controls. Manufacturing research and development will be required to reduce cost and increase the assurances of reliability control for A&D applications.
- Reliability Models
- There is a major shortcoming in the validation of existing Pb-free electronics reliability models to predict potential field failure and product lifetimes. The objective for Phase II of the Lead Free Electronics Manhattan Project is to develop a roadmap that integrates and prioritizes the research needed to properly evaluate the lifetime reliability modeling of Pb-free electronic assemblies.
