Simulation-based Investigation of Interface Delamination in Plastic IC Packages under Temperature and Moisture Loading (Volume 4) (English shop)

Preview

Table of Contents, Datei (51 KB)
Extract, Datei (170 KB)

Interface delamination between dissimilar materials is one of the major threats to the structural
integrity and reliability of multi-layered structures. In many of these structures, such as micro-
electronic assemblies, one or more of the materials are made of polymers. In this case, moisture
poses a significant threat to the reliability of these products and can be regarded as one of the
principal causes of many premature failures.
Epoxy Molding Compounds (EMCs) are widely used as encapsulating materials in semiconduc-
tor packaging industry. The interface between EMC and the copper-based leadframe in plastic IC
packages has been found to be one of the weakest joints. Most of cracks in plastic encapsulated
microcircuits (PEMs) initiate and propagate in this interface. The crack propagation facilitates,
when the EMC materials absorb moisture from the environment. This research provides a com-
prehensive understanding of the primary mechanisms responsible for the interfacial delamination
due to the presence of moisture.
The experimental and numerical investigations of this research can be divided into four catego-
ries:
 The process-induced stresses due to cure shrinkage of the epoxy molding compound and
thermal mismatch between the materials in plastic IC packages were investigated. A numeri-
cal solution was proposed and the method was benchmarked by comparing the warpage of a
simple bimaterial beam with the results from Finite Element (FE) analysis.
 Moisture diffusion (absorption, desorption and re-sorption) experiments were carried out on
bulk EMC samples. Several sample geometry and aging conditions were used to understand
the anomalous behavior of these materials in terms of moisture uptake. The results of these
tests explain the complicated mechanism of moisture diffusion in epoxy molding compounds.
Two highlights of these experiments are identifying the dual-stage moisture absorption during
exposure to humid environments and residual moisture content upon baking the plastic parts.
Additionally, the moisture diffusion in the plastic package was modeled by FE analysis. Since
the standard FE tools can only solve the problem of conventional Fickian diffusion, novel si-
vi
mulation techniques were suggested and verified to account for the non-Fickian dual stage
moisture diffusion during moisture absorption and residual moisture content upon baking of
these products at elevated temperature.
 The hygroscopic swelling of the polymers upon moisture absorption is also a major threat to
the reliability of PEMs. The value of the coefficient of hygroscopic swelling was determined
experimentally by warpage analysis of bimaterial beams and implemented in the FE code.
 Fracture tests were carried out to measure the interfacial adhesion in terms of interfacial frac-
ture toughness. Effects of temperature, aging in dry and humid conditions and mode angle
were investigated. It was observed that the exposure of a Cu/EMC bimaterial beam to mois-
ture prior to fracture tests results in a degradation of the adhesion. This degradation is the re-
sult of the diffusion of water in the interface. For samples that were shortly exposed to humid
environment the degradation was partially reversible by applying an appropriate heat treat-
ment at mild annealing conditions. However, long-term aging in humid condition caused a
permanent adhesion loss, which was attributed to the effect of hydrogen bonding between wa-
ter molecules and polymer chains at the interface.
The investigation of the delamination risk in a plastic IC package by FE analysis showed that the
proposed method can be well used to evaluate the critical crack length in a package. Additional-
ly, the method may be used in order to reduce the delamination risks by design optimization.
Moreover the predictive model provides a useful tool for selecting proper material combinations.
This enables rapid virtual qualification technique by experimental and numerical fracture me-
chanics approach.