Interrogation of system state for damage assessment in lead-free electronics subjected to thermo-mechanical loads

Requirements for system availability for ultra-high reliability electronic systems such as airborne and space electronic systems are driving the need for advanced heath monitoring techniques for early detection of the onset of damage. Aerospace-electronic systems usually face a very harsh environment, requiring them to survive the high strain rates, e.g. during launch and re-entry and thermal environments including extreme low and high temperatures. Traditional health monitoring methodologies have relied on reactive methods of failure detection often providing little on no insight into the remaining useful life of the system. Detection of system-state significantly prior to catastrophic failure can significantly impact the reliability and availability of electronic systems. Previously, Lall, et. al. [2004, 2005, 2006, 2007] have developed methodologies for health management and interrogation of system state of electronic systems based on leading indicators. Examples of damage pre-cursors include micro-structural evolution, intermetallics, stress and stress gradients. Pre-cursors have been developed for both eutectic 63Sn37Pb and Sn4Ag0.5Cu alloy systems on a variety of area-array architectures. In this paper, a mathematical approach for interrogation of system state under cyclic thermo-mechanical stresses has been developed for 6-different leadfree solder alloy systems. Thermal cycles may be experienced by electronics due to power cycling or environmental cycling. Data has been collected for leading indicators of failure for alloy systems including, Sn3Ag0.5Cu, Sn3Ag0.7Cu, SnlAg0.5Cu, Sn0.3Ag0.5Cu0.1Bi, Sn0.2Ag0.5Cu0.1Bi0.1Ni, 96.5Sn3.5Ag second-level interconnects under the application of cyclic thermo-mechanical loads. Methodology presented resides in the pre-failure space of the system in which no macro-indicators such as cracks or delamination exist. Systems subjected to thermo-mechanical damage have been interrogated for system state and the computed damage state correlated with known imposed damage. The approach involves the use of condition monitoring devices which can be interrogated for damage proxies at finite time-intervals. Interrogation techniques are based on derivation of damage proxies, and system prior damage based non-linear least-squares methods including the Levenberg-Marquardt Algorithm. The system's residual life is computed based on residual-life computation algorithms.