Thermomechanical Stress
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Overview:
The control of thermomechanical stress levels in packaged components is
critical to ensure component reliability. Thermomechanical stresses result
from mismatches between the mechanical properties of component materials and
arise during assembly and processing of components as well as during
component operation. High levels of these stresses can cause failure of
components due to problems such as cracking, fatigue and excessive
deflection. ETG employ finite element modelling techniques to simulate
magnitude and distribution of component stresses. Validation of simulated
results is undertaken, whenever possible, through correlation with
reliability measurements. ETG works closely with experimental groups within
tyndall to provide complete reliability characterisation as well as
optimisation of packaging solutions for specific applications.
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Figure 1: 3D FE model of packaged component
Figure 2: Delamination in packaged components can give rise to package cracking
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Solder Joint Modelling
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Solder joint fatigue is a well recognised reliability concern in packaged
components. Continual thermal cycling of joints occurs during normal
operating conditions. The resulting cyclical stresses in the joints can
cause mechanical failures at stress levels lower than joint material yield
stresses.
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Figure 3: Simulation of Plastic Strain in Component Solder Joint
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Design Optimisation
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Figure 4: Model of Capacitance Pressure Sensor
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FEM techniques are used in parametric design studies to optimise packaging
geometries and materials. The structure of the lid, shown in fugure 4, was
optimised to ensure lid deflection was sufficiently large to enable
capacitative measurements, yet small enough to avoid reliability problems
caused by excessive deflections.
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Systems Modelling
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Systems on package technologies are currently being developed to address
current trends in packaging towards increased functionality coupled with
continuing miniaturisation. Figure 5 shows a model of one such system being
developed by Tyndall. 4 die are mounted on a flexible substrate and the
substrate si folded to produce a verticlly stacked system. FEM techniques
are used to identify the optimum assembly materials and investigate whether
system stress levels are likely to lead to any reliability concerns.
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Figure 5: Schematic of 4 die stack system on flexible substrate
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Figure 6: Influence of Die Stack Position on Stress
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for more information:
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Orla Slattery
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back to Electronics Theory page
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