Enhanced Polymer Passivation Layer for Wafer Level Chip Scale ...

5.1.2 Material Properties
Many packaging materials have temperature and time dependent behavior, which is one of
the most important considerations in the simulation. Although it sounds reasonable to input as
much information about a material as possible in a model, the calculating time required for actual
analysis puts a limitation on what should be included. Therefore, prior to implementing a
constitutive model for a specific material, several important aspects need to be carefully
considered. First is the analysis objective. The main focus should be put on the most critical
response the analyst is interested in rather than other materials that may seriously slow down the
analysis time without significant accuracy improvement. Second, the simulation requires
modeling the material behavior, not the actual material. For example, the creep behavior of
eutectic Sn/Pb solder can occur at room temperature, but not under the circumstance with high
loading and high rate of strain. As a result, in order to avoid inefficiencies in the analysis, there is
no need to predict solder deformation caused by the rapid mechanical cycling at room
temperature if the rate of loading is high enough to cause no deformation, while it is essential to
include the creep behavior when simulating fatigue due to thermal cycling [83, 86].
5.1.3 Constitutive Models for SnAgCu Solder
A constitutive model is an approach to mathematically describe the relationship between
strain, stress and other state variables. The material constitutive model plays a very important
role in the development of thermomechanical models for microelectronic packaging assembly.
Two types of deformation (elastic and inelastic) are formed inside the solder alloy when it is
under thermomechanical loading. Since the inelastic deformation consists of time-dependent
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