Thesis-2005-Huang.pdf (36.14 MB)
Materials issues in the transition to lead-free solder alloys and joint miniaturization
thesis
posted on 2017-06-02, 11:39 authored by Zhiheng HuangWithin the context of the imminent implementation of the Pb-free soldering in
Europe in 2006, this thesis addresses the gap in understanding that has emerged in the
fundamental materials issues between well-understood and mature lead-containing
solders and a plethora of new, Pb-free solders for which there are neither long term
reliability data nor understanding of the materials behaviour and how these might be
influenced by manufacture and in-service conditions. In addition, this thesis also
addresses the question as to whether the solder joint size and geometry could become
a reliability issue and therefore affect the implementation of the Pb-free solders in
ultrafine micro joints.
Thermodynamic calculations using MTDATA (developed by the National
Physical Laboratory, NPL, UK) together with a thermodynamic database for solders
under either equilibrium or Scheil conditions, have shown their usefulness in Pb-free
solder design and processing, generating a wealth of information in respect of the
temperature dependence of phase formation and composition. The predictions from
MTDATA on a number of selected systems is generally in good agreement with the
results from experimental work, and has assisted in the understanding of the
microstructure and mechanical properties of the Pb-free solders and the implications
of their interactions with a tin-lead solder. However, further critical assessment and
the addition of new elements into the solder database, such as Ni and P, are required
to make MTDA TA a more effective computational tool to assist the optimization of
processing parameters and cost-effective production in using Pb-free solders.
Molten solder can interact with the under bump metallizations (UBM) and/or
board level metallizations on either side of the solder bump to form intermetallic
compounds (IMCs) during solder reflow. In the modelling of the kinetics of the
dissolution process of UBM into the liquid solder, the commonly used NernstBrunner
(N-B) equation is found to have poor validity for these calculations for micro
joints at 100 µm in diameter or less. Three bumping techniques, i.e. solder dipping
(SD), solder paste stencil printing followed by reflow (SPR) and electroplating of
solders and subsequent reflow (EPR), are used to investigate the interfacial
interactions of molten Sn/Sn-rich solders, i.e. pure Sn, Sn-3.5Ag, and Sn-3.8AgO.7Cu, on electroless nickel immersion gold (ENIG) and copper pads at 240°C. The
resultant bulk and interfacial microstructures from a variety of pad sizes, ranging from
1 mm down to 25 µm, suggest that in general the small bumps contain smaller β-Sn
dendrites and Ag₃Sn IMC particles, nevertheless the interfacial IMC is thicker in the
smalI bumps than in the large bumps. In addition, one and two-dimensional
combined thermodynamic and kinetic models have been developed to assist the
understanding of the kinetics of interdiffusion and the formation of interfacial
intermetallic compounds during reflow. Both the experimental results and theoretical
predictions suggest that the solder bump size and geometry can influence the as-soldered
microstructure, and therefore this factor should be taken into consideration
for the design of future reliable ultrafine Ph-free solder joints.
Funding
EPSRC
History
School
- Mechanical, Electrical and Manufacturing Engineering
Publisher
© Zhiheng HuangPublisher statement
This work is made available according to the conditions of the Creative Commons Attribution-NonCommercial-NoDerivatives 4.0 International (CC BY-NC-ND 4.0) licence. Full details of this licence are available at: https://creativecommons.org/licenses/by-nc-nd/4.0/Publication date
2005Notes
A Doctoral Thesis. Submitted in partial fulfilment of the requirements for the award of Doctor of Philosophy of Loughborough University.Language
- en