Lead-free technology in mil/aero electronics
The European Union’s Restriction of the use of certain Hazardous Substances (RoHS) Waste Electrical and Electronic Equipment (WEEE) directives ban the use of lead and five other substances in many electronics applications. As the lead-free movement grows worldwide, the scarcity of lead-containing electronic components will increase. These components form the foundation of advanced technology Aerospace and Defense (A&D) applications.
Today, A&D applications, which are not explicitly named in RoHS and WEEE, can continue using components that contain lead. Even so, to ensure reliability in harsh environments, it is essential to understand the impact that lead-free COTS components might have on advanced technology military applications.
The “tin whiskers” phenomenon is a well-known issue related to the use of lead-free devices in A&D applications. But many other effects including pad cratering, PCB delamination, and solder joint cracking must be understood and addressed to ensure that the industry can continue producing reliable high-performance electronics.
The Pb-free Electronics Risk Management (PERM) Consortium, formerly known as Lead-Free Electronics in Aerospace Project (LEAP), has published several important standards addressing control and management of lead-free issues. An outgrowth of the PERM Consortium project, the Lead-Free Electronics Manhattan Project (LFEMP) group, has identified numerous technical risks associated with using lead-free electronics in A&D applications. So far, the LFEMP group has produced two reports titled Phase I and Phase II. Phase I identified best practices with lead-free and highlighted significant gaps that require investigation and resolution for A&D electronics. The Phase II draft report details these gaps and presents a comprehensive and coordinated three-year Pb-free Electronics Risk Reduction (PERR) project to address them. The next report, Phase III, is intended to execute the PERR project.
Lead-free reliability risk for A&D electronics
The Phase II draft report outlines five top-level R&D project areas to deal with technical risks and knowledge gaps for the use of lead-free electronics in A&D applications: tin whiskers, electronics assembly, solder joints, PCBs, and components. Once the PERR project is funded, its activities are expected to take three years, with the aim of resolving most lead-free technical issues for A&D electronics.
Meanwhile, three main approaches are available to COTS module suppliers for dealing with lead-free components:
- Reball area array components to tin-lead and solder with tin-lead solder
- Accept lead-free components and solder with lead-free solder (predominantly SAC305)
- Take lead-free components and solder with tin-lead solder
Approaches for lead-free electronics
The first approach can elevate lead-free electronic assemblies close to the baseline solder joint reliability of tin-lead. Concerns about tin whiskers can be mitigated with techniques such as solder dipping of non-BGA components and/or conformal coating. Concerns about reliability due to additional heat exposure and handling needed for component reprocessing can be mitigated through appropriate process controls. This first approach incurs the highest direct costs.
The second approach, popular outside of A&D and other high- reliability industries, is favored by some in the COTS industry because of lower direct costs and reduced lead-time impact. However, it might not address the reliability risks and knowledge gaps detailed in the LFEMP reports. To aid the customer, vendors should demonstrate how these risks and knowledge gaps are addressed. A large amount of data such as thermal cycling is available for some aspects of this approach, but details regarding applicability to specific products need to be assessed. COTS vendors should be able to produce lead-free test data such as extended temperature cycling, vibration in A&D environments on COTS components and assemblies, and compare data to tin-lead and mixed-solder approaches.
In the third approach, area array components such as BGAs with lead-free solder balls are soldered with tin-lead solder. This combines lower costs and reduced lead-time impact (no component reprocessing) with less perceived risk (tin-lead solder). Some studies have indicated acceptable thermal cycling reliability using commercial temperature ranges, while other studies have shown inconsistent reliability across several component packages using an extended temperature range. Overall, the details of this approach including solder microstructure, strength, fatigue, and acceleration factors are less understood than the lead-free approach.
Curtiss-Wright Controls Embedded Computing (CWCEC) has extracted a list of reliability risk factors from the LFEMP reports. Following extensive testing and analysis including the impacts of risk mitigations, CWCEC can guide customers in evaluating the reliability of lead-free approaches. For more information on the use of lead-free devices in COTS applications, see www.cwcembedded.com/lead_free.htm.