Preparing for lead-free electronics
Commercial and domestic electronics products of all types have been heavily influenced by the European directives for Restriction of the use of certain Hazardous Substances (RoHS) and Waste Electrical and Electronic Equipment (WEEE). Across industry, the most significant change has been the rapid adoption of lead-free solder. The worldwide component supply chain is now fully geared up to provide lead-free terminations for all types of components as first choice, while some manufacturers are still offering traditional tin/lead as an alternative but at an increasingly higher cost. Military and aerospace equipment vendors, integrators, and end users are still apprehensive about using lead-free materials. This is because of lingering doubts about the critical long-term reliability of lead-free electronics in the more highly stressed temperature and mechanical environments of many military applications.
As a result of the worldwide lead-free initiatives, future military contracts will require lead-free project plans. For the U.S. DoD, these will need to be in accordance with GEIA-STD-0005-1 “Performance Standard for Aerospace and High Performance Electronic Systems Containing Lead-free Solder.” Such a plan will include reliability, configuration control, risks and limitations of use, and the effects of tin whiskers, plus the repair, rework, maintenance, and support of the equipment. However, the preparation of a standard or plan does not necessarily impose the use of lead-free electronics. Instead, it demonstrates an understanding of the impact of the component and materials supply situation in order to mitigate risks. Although the U.S. is likely to adopt similar legislation pertaining to RoHS in the near future – and despite possible exemptions for military and aerospace – some newer programs such as the Future Combat System (FCS) are already being selectively proactive in moving to lead free.
Using lead-free solder is the most obvious change, and it has a number of secondary effects on reliability. A leading contender for lead-free solder is SAC305 (Ag – silver 3.0 percent, Cu – copper 0.5 percent, the remainder being Sn – tin). This type of solder has some undesirable mechanical characteristics compared to tin/lead:
- Its higher melting point exacerbates manufacturing and rework stresses in components and the Printing Wiring Board (PWB).
- Its microstructure is less homogeneous, causing its properties to be more orientation dependent.
- The absence of lead results in stiffer solder joints. During extended thermal cycling, this increased stiffness can become a critical reliability issue for larger Ball Grid Arrays (BGAs) due to stresses imposed by differential rates of expansion between the device and PWB.
The long-term solution does not lie with expedients such as interposers or replacing solder balls with tin/lead but instead lies in additional material research. New, more stable PWB materials have been developed to reduce some of these effects but in turn have introduced the risk of pads tearing from the PWB, known as pad cratering, illustrated in Figure 1.
Tin whiskers is a phenomenon whereby thin conductive filaments grow from the surface of a tin-plated termination that can potentially short-circuit adjacent pins of devices. The cause of whiskers is not entirely understood but is believed to be related to compressive stresses in the plating. It can happen at any time during the equipment’s life and is therefore difficult to predict or mitigate. However, the most effective mitigations for tin whiskers include design rules that reduce the risk of shorts between solder pads, in addition to some types of conformal coatings that counter whisker growth.
Impact on COTS vendors
Suppliers of COTS embedded computing equipment are planning to introduce lead-free assemblies for military applications. But, as lead-free technology is introduced, it will require much additional test and qualification data to support its use in deployable projects. There will be a lengthy transition period during which COTS vendors will continue to manufacture and support both tin/lead and lead-free alternatives of the same product lines and provide the levels of configuration and materials management required to maintain each type of product over the life cycle. Curtiss-Wright Controls Embedded Computing (CWCEC) has adopted a proactive approach, having already amassed much test data, introduced new materials and test regimes such as Interconnect Stress Testing (IST), and developed the full manufacturing and life-cycle capability to introduce lead-free assemblies.
Lead-free products will require additional testing and tailoring during development to optimize long-term reliability, and COTS vendors will need to incorporate this data into their customers’ projects lead-free plans. The rate of adoption of lead-free electronics for use in harsh environments will be determined partly by legislation but primarily by both vendors’ and integrators’ accumulated knowledge base and continuing research and development efforts.