【公開日：2025.02.10】【最終更新日：2025.02.10】
課題データ / Project Data
課題番号 / Project Issue Number
23KU0031
利用課題名 / Title
In-situ observation of the deformation of Sn-Bi low temperature solder alloy under tensile loading
利用した実施機関 / Support Institute
九州大学 / Kyushu Univ.
機関外・機関内の利用 / External or Internal Use
外部利用/External Use
技術領域 / Technology Area
【横断技術領域 / Cross-Technology Area】（主 / Main）計測・分析/Advanced Characterization（副 / Sub）-
【重要技術領域 / Important Technology Area】（主 / Main）次世代ナノスケールマテリアル/Next-generation nanoscale materials（副 / Sub）-
キーワード / Keywords
Low temperature solder, Sn-Bi alloys, transmission electron microscopy, in-situ mechanical testing,電子顕微鏡/ Electronic microscope
利用者と利用形態 / User and Support Type
利用者名（課題申請者）/ User Name (Project Applicant)
NOGITA Kazuhiro
所属名 / Affiliation
The University of Queensland Mechanical and Mining Engineering
共同利用者氏名 / Names of Collaborators in Other Institutes Than Hub and Spoke Institutes
TAN Xin Fu
ARIM実施機関支援担当者 / Names of Collaborators in The Hub and Spoke Institutes
MAENO Hiroshi
利用形態 / Support Type
（主 / Main）技術補助/Technical Assistance（副 / Sub）-
利用した主な設備 / Equipment Used in This Project
報告書データ / Report
概要（目的・用途・実施内容）/ Abstract (Aim, Use Applications and Contents)
With low liquidus temperatures, low raw material costs, and non-toxicity, Sn-Bi low temperature solders are promising candidates for the replacement of the currently widely-used lead-free solders in situations in which process temperatures have to be reduced. The project elucidates the mechanical properties of the Sn-Bi alloys at microscales through in-situ electron microscopy. In particular, the failure mechanism of the material under a tensile load was studied. It was found that cracks initiate preferential at phase/grain boundaries perpendicular to the load direction. This is followed by sliding of the Sn/Bi and Sn/Sn boundaries, while the Bi/Bi boundaries are relatively brittle.
実験 / Experimental
Sample preparation:
The
near-eutectic Sn-57wt%Bi (Sn57Bi hereafter) alloy was prepared by Sn and Bi
ingots (99.9 %, supplied by Northern Smelters Pty Ltd., Australia) of
appropriate quantities were melted at 450 °C for 1 h to prepare the Sn-57wt%Bi
alloy, and casted into an ingot mould preheated to 150°C. The ingot was
remelted and casted into a slightly oversized aluminium mould preheated to
150°C, with planar dimensions shown in the Fig. 1. The thick cast sample was
then filed and polished to the dimensions shown in Fig. 1. A notch was filed
into the sample to ensure that the stresses are concentrated in this region.
Following this, the sample was sliced into pieces and polished to a thickness
of less than 0.1 mm with a final finish of 1 μm. An electron transparent region
was prepared at the tip of the notch by an improvised cryogenic focused ion
beam (cryo-FIB) technique, shown in Fig. 2.
TEM Experiment: The
JEOL JEM-1300NEF HV-TEM used for this experiment was set to an accelerating
voltage of 1,250 kV, and is equipped with an omega-type energy filter and a
Gatan Single Tilt Heating Straining Holder (Model 672). Conventional TEM
requires a sample thickness of less than 200 nm, while the HV-TEM enables the
use of a 500 nm thick lamellae for the experiment to minimise plane strain and
sample damage during FIB. The samples were mounted on the holder bolted down by
two screws and inserted in the HV-TEM for in-situ elongation observation.
An energy slit of 80.0 eV and an energy
shift of 100.0 eV was applied for the best image contrast. The sample was
elongated intermittently through displacement of the crosshead with a direct
current (DC) motor at a rate of 1.0 μm/s. The real-time tensile load was
recorded and shown in Fig. 3c. Elongation was stopped when significant changes
in the microstructure were observed, and images were taken for the whole
electron transparent region at each of these points, until the crack propagates
through the electron transparent region.
結果と考察 / Results and Discussion
Fig. 3 shows the energy-filtered images of the pristine sample stitched
together to observe the whole electron transparent region. Under this
condition, Bi which diffracts more electrons has a darker contrast, while the
brighter material is Sn. The Bi grain at the top left corner of the electron
transparent area showed signs of FIB damage including dislocations and
nanosized defects, while other grains had no significant damage.
As seen in
Fig. 3, crack initiated at 3.8 N at a Sn/Sn grain boundary perpendicular to the
load direction. The initiation of cracks perpendicular to the load direction is
expected as the stress is greatest in this direction. At 5.2 N, cracks
continued to form at Sn/Sn, Sn/Bi and Bi/Bi boundaries near-perpendicular to
the load direction. This is followed by sliding of the Sn/Sn and Sn/Bi
boundaries near-parallel to the load direction at 5.6 N, while the Bi/Bi
boundaries are more brittle than the aforementioned. Grain boundary sliding
often occur in polycrystalline materials at homologous temperatures above 0.4
at low strain rates. At 5.9 N, the Sn/Sn detached while the Sn/Bi boundaries
continued to slide until at 7.0 N when the crack propagated through the
electron thin region. The fracture mode is intergranular. The sample did not
break completely as the maximum drive range was 2.0 mm and thus the strain was
small.
Repeat
experiments with intermittent elongation (similar to the experiment above) and
continuous elongation confirm that crack initiations are often along boundaries
perpendicular to the load direction, with no significant preference of the
Sn/Sn, Sn/Bi or Bi/Bi boundary, and followed by sliding of Sn/Sn and Sn/Bi
boundaries parallel to the load direction.
図・表・数式 / Figures, Tables and Equations
Fig. 1. Planar geometry of the TEM lamella.
Fig. 2. FIB processing of the tip of the notch into electron transparent region.
Fig. 3. HV-TEM energy-filtered images of the pristine sample and the sample under various tensile loads, showing the crack initiation sites and crack propagation route.
その他・特記事項（参考文献・謝辞等） / Remarks(References and Acknowledgements)
成果発表・成果利用 / Publication and Patents
論文・プロシーディング（DOIのあるもの） / DOI (Publication and Proceedings)
口頭発表、ポスター発表および、その他の論文 / Oral Presentations etc.
特許 / Patents
特許出願件数 / Number of Patent Applications：0件
特許登録件数 / Number of Registered Patents：0件