| dc.description.abstract | Lead-free solders typically consist of tin as the primary element, with small amounts
of other metals like silver, copper, bismuth, and antimony added to enhance the alloy's
properties. The higher melting point necessitates higher processing temperatures
during manufacturing, which can lead to increased stress on components and printed
circuit boards (PCBs). One of the primary challenges with lead-free soldering is the
formation and growth of intermetallic compounds (IMCs) at the interface between the
solder and the component/PCB pads. IMCs are necessary for forming a strong
metallurgical bond; however, excessive growth of these compounds can lead to brittle
joints that are more prone to cracking and failure, especially under thermal and
mechanical stress. Reflow soldering where solder joints are formed by heating solder
materials to a temperature that allows them to melt and bond with the substrate. This
process is often repeated multiple times to complete complex assemblies, rework, or
repair needs. Each additional reflow substantially affects the solder joint's
microstructure, especially in the formation and growth of IMCs.
The first part of the work examines the effects of repeated reflow numbers on
interfacial evolution and joint-level mechanical response. High-reliability lead-free
alloys, including Sn-3.0%Ag-0.5%Cu (SAC305) and Bi-modified SAC (Cyclomax)
solders, were assembled on organic solderability preservative (OSP), and electroless
nickel immersion gold (ENIG) surface finishes and subjected to controlled numbers of
reflow cycles. IMC thickness and morphology was quantified and related to shear
behavior across multiple strain rates and to board-level drop performance. The results
show that additional reflow cycles promote IMC thickening and microstructural
changes whose impact on strength and fracture mode depends strongly on alloy
chemistry and surface finish, with Bi-doped SAC on ENIG exhibiting an especially
pronounced tendency toward brittle interfacial fracture at high strain rates.
The second part investigates post-reflow aging effects, with particular emphasis on
Bi-containing low-temperature solder alloys. Solder joints were tested at different times
after assembly to capture room-temperature aging and microstructural stabilization
processes. Microstructural analysis, mechanical testing, and fracture surface
observations are integrated to establish quantitative links between processing history,
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joint structure, and reliability metrics. Microstructural characterization and mechanical
testing demonstrate that aging can stabilize as-reflowed microstructures, leading to
aging “windows” in which short-term room-temperature storage improves impact.
The final part focuses on the role of solder-joint geometry and stand-off height in
SAC305 drop reliability. Ball grid array (BGA) and land grid array (LGA) configurations
with different joint sizes and heights were assembled and evaluated under
standardized board-level drop conditions. Contrary to conventional expectations that
taller joints are inherently more impact resistant, the experiments show that shorter
SAC305 joints can match or exceed the drop performance of taller joints, and that the
relationship between stand-off height and lifetime is non-monotonic. Detailed failure
analysis reveals geometry dependent transitions in failure location and crack path,
highlighting the coupled influence of joint size, Sn grain morphology, and package
configuration on drop induced damage.
Overall, this dissertation provides a mechanistic framework for understanding how
multiple reflows, solder-joint geometry, and post-reflow aging govern the degradation
and failure modes of lead-free solder joints. | en_US |
