Characterization of Emerging Technologies in Military Environment
Dr. Pradeep Lall
BACKGROUND: There are two change happening in commercial parts including migration to copper-wirebonding and the emergence of leadfree solders for first, second-level interconnects. These changes have ramification on the part reliability in extreme environments. Risk mitigation methods are needed for defense systems, which increasingly rely on commercial electronic parts for design and sustainment.
RESEARCH NEEDED: Munitions electronics may be subjected to up to 10,000g in normal transportation and handling. This is in contrast to commercial electronics, which often function in office benign environments. Commercial off-the-shelf electronics parts are increasingly being used for design and sustainment of electronic systems in defense applications. However, commercial component architectures are evolving with the migration of the first level interconnect to copper wire, silver wire from the earlier technologies, which primarily focused on gold wire. Further, the solder joint alloys have migrated to tin-silver-copper based formulation from the earlier tin-lead based formulations. The migration of commercial architectures impacts the defense systems immensely, which increasingly rely on the use of commercial off the shelf components. The test standards for accelerated testing and reliability assurance have lagged behind the introduction of the new architectures. Risk mitigation methods are needed for the use of the new technologies in munitions electronics. The research objective is the development of risk mitigation methods for use of new copper-wirebonded parts and leadfree solders for applications, which require exposure to high strain rate and prolonged storage. The research will focus on commercial parts usage in extreme environments with emphasis on copper-wirebonding and leadfree solders.
Cu-Wirebonding: The migration of high-reliability applications requiring sustained operation in harsh environments needs a better understanding of the acceleration factors under the stresses of operation. Prolonged exposure of the copper wire to elevated temperatures results in growth of excessive intermetallics and degradation of the interface. Behavior of Copper wirebond under high current-temperature conditions is not yet fully understood. Cu tends to react more readily than Au, which makes Cu more susceptible to various environmental factors, in addition to physical and chemical attach of surrounding materials. The heightened propensity for corrosion of the Cu-wirebond system has limited the deployment of the Cu wirebonded devices in the extreme environment applications. Exposure to high current may induce Joule heating and electromigration, and thus significantly increase the degradation rate in comparison with low current operating conditions. Further, the accelerated test results of unbiased conditions cannot be used for life prediction of such high powered parts. EMCs used for encapsulation of the chip and the interconnects may vary widely in their formulation including pH, porosity, diffusion rates, levels and composition of the contaminants. Selection of different materials, such as EMC used in the molding process plays key role in defining lifetime for wirebond system. There is need for predictive models which can account for the exposure to environmental conditions, operating conditions and the EMC formulation in order to be realistically representative of the expected reliability.
Leadfree Solders: Electronic devices may be exposed to high temperatures and high strain loads in harsh environment applications such as defense systems. In such applications, electronics components are usually subjected to operating temperatures around 175-200 degrees C and may experience the 1-100 per sec strain rates. Lead free solder properties including elastic modulus and ultimate tensile strength in electronic assemblies evolve when exposed to high operating temperature or thermal aging environments. In absence of material properties and material models, which include the effects of thermal aging, the simulation of solder joint reliability often involves the use of material properties for pristine alloys for life prediction of aged assemblies. Variance of life prediction from the measured data can be significantly reduced with the quantification of the effects of aging and development of material models for inclusion in finite element framework.
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