【公開日：2024.10.22】【最終更新日：2024.10.22】
課題データ / Project Data
課題番号 / Project Issue Number
22KU0020
利用課題名 / Title
In-situ high voltage transmission electron microscope study of the precipitation of Bi in primary Sn dendrites in Sn-Bi low temperature solder alloys
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, transmission electron microscopy
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)
Kazuhiro Nogita
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
Xin Fu Tan
ARIM実施機関支援担当者 / Names of Collaborators in The Hub and Spoke Institutes
Kazuhiro Yasuda,Hiroshi Maeno
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)
The development of low temperature solders are driven by environmental, costs and technical reasons to reduce the temperature during the electronic manufacturing processes. Lower soldering temperatures can reduce the process energy and costs, while also reduces thermal damage to temperature sensitive electronic components and thermally induced mechanical stresses [1-2]. Sn-Bi is identified and prioritized as the most promising low temperature solders [3-4]. However, due to the low melting point, the Sn-Bi solders are operating close to their melting point. As a result, solid state processes such as diffusion proceed at faster rates than in conventional solder alloys. Also, the solid solubility of constituent elements varies widely over that normal range of operating temperatures [5]. One consequence is that fine Bi precipitates in the Sn dendrite over time only to redissolve when the temperature increases during operation of the circuitry. Electronic manufacturers have found these issues compromise reliability in service. Therefore, the aims of this experiment are: To study the kinetics of fine Bi precipitation at temperatures relevant to low temperature soldering applications. To study the solubility of Bi in Sn at these temperatures.
実験 / Experimental
1.1 Sample fabrication:
A Sn-37wt%Bi alloy characterised by
significant quantities of both independent Sn phases and Sn-Bi eutectic phases
(Fig. 1.) is used for this experiment. The sample was prepared by alloying Sn
and Bi (99.9% purity) in a crucible at 400℃ for 1 hour, and casting the sample
into cylindrical moulds of 10 mm in diameter. The cast sample was cut into 20
mm lengths, mounted in epoxy resin and the surface was polished with standard
metallography procedures. Transmission electron microscopy (TEM) lamellae was
cut out from the polished sample with a focused-ion-beam (FIB) technique. The
sample was thinned to 500 nm to prevent excessive FIB damage to the temperature
sensitive Sn-Bi alloys.
1.2 In-situ heating high voltage
transmission electron microscopy (HV-TEM)The TEM experiment was conducted on the
High voltage TEM (HV-TEM) JEOL JEM-1300NEF with an accelerating voltage of
1,250 keV. The TEM lamellar was placed on a double-tilt JEOL EM-HSTH heating
TEM holder. The sample was heated stepwise to temperatures relevant to the
production and service temperatures of solder alloys, and time dependent
microstructural changes, in particular the dissolution of fine Bi in the
primary Sn dendrite during heating, was captured in a video. The following
experimental procedures was used: • Tilt to the zone axis of Sn
• Take image and diffraction pattern of ROI
• Start Video 1
• Heat sample to 80℃ at 20℃/minute,
hold 15 minutes and observe dissolution of Bi
• Heat sample to 100℃ at 20℃/minute,
hold 15 minutes and observe dissolution of Bi
• Heat sample to 120℃ at 20℃/minute,
hold 15 minutes and observe dissolution of Bi
• Heat sample to 130℃ at 20℃/minute,
hold 10 minutes and observe dissolution of Bi
• Stop Video 1, continue to hold at 130℃, take images, check diffraction
patterns of dissolved Bi areas
• Start Video 2
• Cool sample to 30℃ (cooling rate uncontrolled)
• Hold 30 minutes and observe precipitation of Bi
• Stop Video 2
結果と考察 / Results and Discussion
Fig. 2a shows the TEM lamellar before the
heating experiment. The lamellar is tilted to the zone axis of the matrix of
single crystal primary Sn. The diffraction pattern of the SADP area marked on
Fig. 2a is shown in Fig. 2b, consistent with the pattern of the Sn [111] axis.
Bi precipitates in the primary Sn are also visible in Fig. 2a. Fig. 3 shows the snapshots taken from Video
1 as the sample was heated. Dissolution of Bi precipitates was visible after 5
minutes at 80℃, resulting in the change in band contours. Bi continued to
dissolve after 15 minutes at 80℃. The dissolved Bi areas have the contrast of
the Sn phase but retained the FIB damaged features. At 130℃, holes at the
Sn/Bi interface began to enlarge. SADPs at 130℃ shows the Bi precipitates
have dissolved, sharing the same pattern as the Sn matrix (Fig. 4). The TEM
lamellar after the heating experiment is shown in Fig. 5a. The sample is still
on-zone of the single crystal primary Sn matrix (Fig. 5b).
図・表・数式 / Figures, Tables and Equations
Fig. 1. Sn-37wt%Bi alloy showing fine Bi precipitates (brighter phase) in the independent Sn phase (darker phase), and the eutectic Sn-Bi phases.
Fig. 2. (a) Image of the TEM lamellar before the heating experiment. (b) SADP of the area marked in (a).
Fig. 3. Snapshots of the ROI during the in-situ heating TEM experiment at different temperatures and times.
Fig. 4. SADPs taken at 130°C showing Bi had dissolved into Sn.
Fig. 5. (a) Image of the TEM lamellar after the heating experiment. (b) SADP of the area marked in (a).
その他・特記事項（参考文献・謝辞等） / Remarks(References and Acknowledgements)
Funding: This work was supported by The University of Queensland,
Australia [Knowledge Exchange & Translation fund 2021002690]; Nihon
Superior Co., Ltd, Japan [2016001895, 2021002341]; Australian Research Council,
Australia [DP200101949]; and ANSTO, Australia [AS211/PD/16842].
[1] Kang, H., S.H. Rajendran, and J.P.
Jung, Low Melting Temperature Sn-Bi Solder: Effect of Alloying and Nanoparticle
Addition on the Microstructural, Thermal, Interfacial Bonding, and Mechanical
Characteristics. Metals, 2021. 11(2): p. 364. DOI: 10.3390/met11020364.[2] Wang, F., et al., Recent progress on
the development of Sn–Bi based low-temperature Pb-free solders. Journal of
Materials Science: Materials in Electronics, 2019. 30(4): p. 3222-3243. DOI:
10.1007/s10854-019-00701-w.[3] Fu, H., et al. iNEMI project on process
development of BISN-based low temperature solder pastes — Part II:
Characterization of mixed alloy BGA solder joints. in 2018 Pan Pacific
Microelectronics Symposium (Pan Pacific). 2018. DOI: 10.23919/PanPacific.2018.8318989.[4] Scott Mokler, et al. The application of
Bi-based solders for low temperature reflow to reduce cost while improving SMT
yields in client computing systems. in SMTA International. 2016. Rosemont, IL,
USA.
[5] Hao, Q.C., et al., The Effects of
Temperature and Solute Diffusion on Volume Change in Sn-Bi Solder Alloys. JOM,
2022. 74(4): p. 1739-1750. DOI: 10.1007/s11837-021-05145-4.
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