Effects of thermal cycling and room-temperature ageing on bismuth precipitates in Sn-Ag-Cu-Bi solder joints
Author(s)
Hsieh, CL
Coyle, RJ
Xian, JW
Gourlay, CM
Type
Journal Article
Abstract
The long-term stability of Bi-strengthened Sn–Ag–Cu solders is an important but insufficiently understood factor for the reliability of stored electronic assemblies.
In this study, (Bi) precipitates in Sn-2.25Ag-0.5Cu-6Bi (wt.%) solder joints were examined in two ball grid array (BGA) packages after long-term (> 6 years) room
temperature ageing following reflow soldering and after harsh thermal cycling between − 55 and 125 °C. Significant differences are observed between bulk and
surface (Bi) precipitates in their distribution, morphology, crystallographic orientation relationships (ORs) with β-Sn, and the formation of interfaces and ORs with
Cu₆Sn₅. Three distinct ORs between (Bi) and β-Sn are identified, each with eight crystallographic variants in good agreement with calculated variants. The pre
cipitates are plate-shaped, with the largest habit planes corresponding to the most coherent interfaces ( {100}𝛽Sn ∥{01̄ 12}(Bi) for OR1; {011}𝛽Sn ∥{01̄ 12}(Bi) for OR2),
resulting in multiple precipitate alignments within the matrix. Notably, OR2 is preferentially observed for bulk precipitates. Two additional ORs are identified
between (Bi) and Cu₆Sn₅. After prolonged room-temperature storage, however, the bulk (Bi) precipitates become coarsened and widely spaced (> 1 μm in size
and > 9 μm spacing), and (Bi) is observed to accumulate adjacent to Cu₆Sn₅ and Ag₃Sn particles. These observations indicate over-ageing and significant micro
structural evolution even at room temperature, suggesting a potential long-term reliability concern for Bi-strengthened solder joints in stored electronics.
In this study, (Bi) precipitates in Sn-2.25Ag-0.5Cu-6Bi (wt.%) solder joints were examined in two ball grid array (BGA) packages after long-term (> 6 years) room
temperature ageing following reflow soldering and after harsh thermal cycling between − 55 and 125 °C. Significant differences are observed between bulk and
surface (Bi) precipitates in their distribution, morphology, crystallographic orientation relationships (ORs) with β-Sn, and the formation of interfaces and ORs with
Cu₆Sn₅. Three distinct ORs between (Bi) and β-Sn are identified, each with eight crystallographic variants in good agreement with calculated variants. The pre
cipitates are plate-shaped, with the largest habit planes corresponding to the most coherent interfaces ( {100}𝛽Sn ∥{01̄ 12}(Bi) for OR1; {011}𝛽Sn ∥{01̄ 12}(Bi) for OR2),
resulting in multiple precipitate alignments within the matrix. Notably, OR2 is preferentially observed for bulk precipitates. Two additional ORs are identified
between (Bi) and Cu₆Sn₅. After prolonged room-temperature storage, however, the bulk (Bi) precipitates become coarsened and widely spaced (> 1 μm in size
and > 9 μm spacing), and (Bi) is observed to accumulate adjacent to Cu₆Sn₅ and Ag₃Sn particles. These observations indicate over-ageing and significant micro
structural evolution even at room temperature, suggesting a potential long-term reliability concern for Bi-strengthened solder joints in stored electronics.
Date Issued
2026-06-01
Date Acceptance
2026-04-02
Citation
Journal of Materials Science, 2026, 61 (23), pp.16674-16704
ISSN
0022-2461
Publisher
Springer Science and Business Media LLC
Start Page
16674
End Page
16704
Journal / Book Title
Journal of Materials Science
Volume
61
Issue
23
Copyright Statement
© The Author(s), 2026 Open Access This article is licensed under a Creative Commons Attribution 4.0 International License, which permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons licence, and indicate if changes were made. The images or other third party material in this article are included in the article's Creative Commons licence, unless indicated otherwise in a credit line to the material. If material is not included in the article's Creative Commons licence and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder. To view a copy of this licence, visit http://creativecommons.org/licenses/by/4.0/.
License URL
Identifier
https://www.ncbi.nlm.nih.gov/pubmed/42111494
PII: 12718
Publication Status
Published
Coverage Spatial
United States
Date Publish Online
2026-04-28