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|Title: ||Microstructural features and mechanical behaviour of lead free solders for microelectronic packaging|
|Authors: ||Gong, Jicheng|
|Keywords: ||Ph-free solder|
Inter- and intra- granular behaviour
|Issue Date: ||2007|
|Publisher: ||© Jicheng Gong|
|Abstract: ||The demands for high density, fine pitch interconnections in electronics systems has
seen solder-based approaches for such interconnections miniaturized to the scale of tens
of micro meters. At such a small scale, such 'micro joints' may contain only one or a
few grains and the resultant mechanical behaviour may not be that for a polycrystalline
aggregate, but rather for a single crystal. Since the ~-Sn matrix of SnAgCu solder has a
contracted body-centred tetragonal (BCT) structure, such a solder grain is expected to
demonstrate a considerably anisotropic behaviour. In such cases the reliability of a Phfree
solder is strongly dependent on the local microstructural features, such as the size
and orientation of the grains. This thesis presents the investigation of the evolution of
microstructure within a joint or at the interface and, the influence of such
microstructural features on the meso-scale mechanical behaviour of the Ph-free solder.
Evolution of the interface between a molten solder and the Cu substrate
To form a joint, the solder alloy is heated and molten, wetting a solid under-bump
metallization. After solidification, layers of brittle intermetallic compounds (IMCs) are
formed at the interface. In this project, facilities were set up to obtain interfacial
reactants at an arbitrary moment of the liquid/solid reaction. Formation and evolution
during reflow of SnCu IMCs at the interface between the molten SnAgCu alloy and the
Cu UBM was captured and presented for the first time.
Formation of phases and IMCs with the body of a liquid SnAgCu solder during
The formation behaviour of basic components for a SnAgCu grain (including Sn
dendrites, AIDSn and Cu6Sns IMCs) during solidification was investigated.
Relationships between the growth behaviour of these components and their internal
lattice orientation were studied. The characteristic growth and coupling of AIDSn IMCs
and the Sn matrix to form eutectics has been elaborated and presented in this study for
the first time. Based on the results, the forming process of a eutectic SnAgCu grain
under the non-equilibrioum solidification condition was illustrated; and major factors
that determine the lattice-orientation, size and substructure of the grain were discussed.
Meso- and Micro- scale mechanical behaviour of a SnAgCu solder joint
To study the size effect on the microstructure, and subsequently, the meso-scale ·
mechanical behaviour, solder joints were manufactured with varying geometries.
Shearing tests were performed·on these meso-scale joints. The results first demonstrated
that the anisotropic characteristics of a SnAgCu grain play an important role in the
mechanical behaviour of both a meso-scale solder joint and the adjacent interfacial
IMCs. To further investigate the micro-scale deformation and damage mechanisms,
micro-mechanical tests were preformed within a SnAgCu grain.
Constitutive equations for a SnAgCu grain
Based on the experimental results, a crystal model was established to describe the
local microstructure-dependent mechanical behaviour. The constitutive equation was
implemented by means of the finite element approach, and applied in solder joints of a
Flip Chip (FC) package by a multi-scale method. To describe the crystal behaviour at
the higher temperature, the model was improved to account for deformations due to
vacancy diffusion and thermal expansion. This model was integrated by an implicit
approach, and implemented in a full three dimension (3D) finite element (FE) model.|
|Description: ||A Doctoral Thesis. Submitted in partial fulfilment of the requirements for the award of Doctor of Philosophy of Loughborough University.|
|Appears in Collections:||PhD Theses (Mechanical, Electrical and Manufacturing Engineering)|
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