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 precipitates are plate-shaped, with the largest habit planes corresponding to the most coherent interfaces (<InlineEquation ID="IEq1"> <EquationSource Format="TEX">${\{100\}}_{\beta \text{Sn}}\parallel {\{01\bar{1}2\} }_{\left(\text{Bi}\right)}$</EquationSource> <EquationSource Format="MATHML"><math> <mrow> <msub> <mrow> <mo stretchy="false">{</mo> <mn>100</mn> <mo stretchy="false">}</mo> </mrow> <mrow> <mi>β</mi> <mtext>Sn</mtext> </mrow> </msub> <mo stretchy="false">‖</mo> <msub> <mrow> <mo stretchy="false">{</mo> <mn>01</mn> <mover accent="true"> <mrow> <mn>1</mn> </mrow> <mrow> <mo stretchy="false">¯</mo> </mrow> </mover> <mn>2</mn> <mo stretchy="false">}</mo> </mrow> <mfenced close=")" open="("> <mtext>Bi</mtext> </mfenced> </msub> </mrow> </math></EquationSource> </InlineEquation> for OR1; <InlineEquation ID="IEq2"> <EquationSource Format="TEX">${\{011\}}_{\beta \text{Sn}}\parallel {\{01\bar{1}2\} }_{\left(\text{Bi}\right)}$</EquationSource> <EquationSource Format="MATHML"><math> <mrow> <msub> <mrow> <mo stretchy="false">{</mo> <mn>011</mn> <mo stretchy="false">}</mo> </mrow> <mrow> <mi>β</mi> <mtext>Sn</mtext> </mrow> </msub> <mo stretchy="false">‖</mo> <msub> <mrow> <mo stretchy="false">{</mo> <mn>01</mn> <mover accent="true"> <mrow> <mn>1</mn> </mrow> <mrow> <mo stretchy="false">¯</mo> </mrow> </mover> <mn>2</mn> <mo stretchy="false">}</mo> </mrow> <mfenced close=")" open="("> <mtext>Bi</mtext> </mfenced> </msub> </mrow> </math></EquationSource> </InlineEquation> 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 microstructural evolution even at room temperature, suggesting a potential long-term reliability concern for Bi-strengthened solder joints in stored electronics. Graphical abstract

Effects of thermal cycling and room-temperature ageing on bismuth precipitates in Sn-Ag-Cu-Bi solder joints

期刊论文
出版模式：

开放获取
发表日期：

2026年4月28日

查看全文

C. L. Hsieh,
R. J. Coyle,
J. W. Xian,
C. M. Gourlay

摘要

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 precipitates are plate-shaped, with the largest habit planes corresponding to the most coherent interfaces ( ${\{100\}}_{\beta \text{Sn}}\parallel {\{01\bar{1}2\} }_{\left(\text{Bi}\right)}$ ${100}_{β Sn} ‖ {01 \bar{1} 2}_{(Bi)}$ for OR1; ${\{011\}}_{\beta \text{Sn}}\parallel {\{01\bar{1}2\} }_{\left(\text{Bi}\right)}$ ${011}_{β Sn} ‖ {01 \bar{1} 2}_{(Bi)}$ 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 microstructural evolution even at room temperature, suggesting a potential long-term reliability concern for Bi-strengthened solder joints in stored electronics.

Graphical abstract