Elviz George, Deng Yun Chen, Michael Osterman, and Michael Pecht
CALCE, Center for Advanced Life Cycle Engineering, Department of Mechanical Engineering, University of Maryland, College Park, Maryland 20740, USA
Abstract:
Miner's rule assumes that damage in solder interconnects accumulates linearly
under cyclic loading and is independent of the load sequence. Under
these constraints, damage is equivalent to cycle ratio, defined as the ratio of
the applied cycles to the cycles to failure for the specific cyclic loading condition.
Due to these inherent assumptions, Miner's rule can inaccurately estimate
solder interconnect life under sequential loading conditions. A nonlinear
damage model with load-dependent damage exponents would take into account
the effect of loading sequence under sequential loading conditions. In
the nonlinear damage model, damage is related to cycle ratio using a power
law relationship with a load-dependent damage exponent. This paper presents
an experimental approach to determine the load-dependent exponents under
three load levels. Load drop in the specimens, as a result of cyclic loading
induced changes, is used as the criterion for the damage state. The tests
consisted of a series of constant amplitude (standalone) and blocks of
sequential variable amplitude cyclic shear tests in a thermo-mechanical micro-
scale analyzer. The load-dependent damage exponents were developed for
SAC305 (96.5%Sn + 3.0%Ag + 0.5Cu) solder. The result of the study can be
used for SAC305 damage accumulation model formulation. Further, the
experimental approach can be used to generate additional fatigue data under
variable amplitude loads.