The use of conventional tin-lead (SnPb) in circuit board manufacturing is under ever-increasing political scrutiny due to increasing regulations concerning lead. The Restriction of Hazardous Substances (RoHS) directive enacted by the European Union (EU) and a pact between the United States National Electronics Manufacturing Initiative (NEMI), Europe‘s Soldertec at Tin Technology Ltd. and the Japan Electronics and Information Technology Industries Association (JEITA) are just two examples where worldwide legislative actions and partnerships/agreements are affecting the electronics industry.
As a result, many global commercial-grade electronic component suppliers are initiating efforts to transition to lead-free (Pb-free) in order to retain their worldwide market. Pb-free components are likely to find their way into the inventory of aerospace or military assembly processes under current government acquisition reform initiatives. Inventories contaminated by Pb-free result in increased risks associated with the manufacturing, product reliability, and subsequent repair of aerospace and military electronic systems.
Although electronics for military and aerospace applications are not included in the RoHS legislation, engineers are beginning to find that the commercial industry’s move towards RoHS compliance has affected their supply chain and changed their parts. Most parts suppliers plan to phase out their non-compliant, leaded production and many have already done so. As a result, the ability to find leaded components is getting harder and harder. Some buyers are now attempting to acquire the remaining SnPb inventory, if it’s not already obsolete.
Original Equipment Manufacturers (OEMs), depots, and support contractors have to be prepared to deal with an electronics supply chain that increasingly provides more and more parts with Pb-free finishes—some labeled no differently than their Pb counterparts—while at the same time providing the traditional Pb parts. The longer the transition period, the greater the likelihood of Pb-free parts inadvertently being mixed with Pb parts and ending up on what are supposed to be Pb systems. As a result, OEMs, depots, and support contractors need to take action now to either abate the influx of Pb-free parts, or accept it and deal with the likely interim consequences of reduced reliability due to a wide variety of matters, such as Pb contamination, high temperature incompatibility, and tin whiskering.
Allowance of Pb-free components produces one of the greatest risks to the reliability of a weapon system. This is due to new and poorly understood failure mechanisms, as well as unknown long-term reliability. When the decision is made to consciously allow Pb-free solder and component finishes into SnPb electronics, additional effort (and cost) is required to make the significant number of changes to drawings and task order procedures.
The NASA-DoD Lead-Free Electronics Project compared the reliability of various lead-free and mixed tin-lead/lead-free assemblies to the benchmark tin-lead assemblies. The results show that the reliability of lead-free solders is dependent on the component type and the environment. The tin-lead controls were, in general for most component types, more reliable in high stress, military environments. For some component types in certain low stress environments, lead-free solders proved to be as reliable as the SnPb controls. Seven component styles were evaluated using two different board finishes, three solders, and seven component finishes, resulting in more than 30 “as-manufactured” solder joint chemistries over the 6755 components evaluated. Of these as-manufactured components, more than 800 lead-free components were reworked by the 2M Project Office at NSWC Crane using SnPb solder. The reliability of assemblies was evaluated by thermal cycling, vibration, drop shock, copper dissolution, and combined environments testing. The NASA-DoD Lead-Free Electronics Project consortium consisted of experts from academia, the aerospace industry, defense contractors, NASA, and the Department of Defense. This project is a follow-on effort to the Joint Council on Aging Aircraft/Joint Group on Pollution Prevention (JCAA/JG-PP) Pb-free Solder Project, which was the first group to test the reliability of Pb-free solder joints against the requirements of the aerospace and military community.
The following statements summarize the data and findings contained within this document. Additional statements can be found in the Joint Test Report.
- SnPb/SnPb or Pb-free/Pb-free systems are more reliable than mixed metallurgy.
- Mixed metallurgy solder joints containing a higher percentage of SnPb are more reliable than solder joints that contain a higher percentage of Pb-free solders.
- Rework using SnPb resulted in a solder joint as reliable as the as-manufactured solder joints.
- For some of the tests, reworked BGA-225 and CSP-100 components were not as robust as the as-manufactured.
- Despite rigid rework procedures, there were issues with successfully reworking the Pb-free BGA-225 components, primarily with the flux only option.
- The reliability of reworked BGA-225 components degrades under a vibration environment.
- Laminate selection is an important factor in lead-free solder assembly integrity, as evidence by pad cratering defects.
- Under high-stress mechanical and thermal conditions, SnPb generally outperforms Pb-free. For low stress conditions, Pb-free generally outperforms SnPb. One exception to this trend is the mechanical shock test results. These results are similar to the JCAA/JGPP Lead-Free Solder Project results.
- The results of this study suggest that for some component types and environments, Pb-free solders are as reliable as the currently used eutectic SnPb solder. This study also demonstrates that with other component types and environments, the Pb-free solders fail before the SnPb control.
