Solidification - microstructure - performance relationships in sn-ag and sn-ag-cu solders
File(s)
Author(s)
Sun, Sihan
Type
Thesis
Abstract
Electronic solder joints are sub-millimetre tin-based alloys whose microstructure is created by solidification in an undercooled melt. This thesis studies the effect of undercooling on the microstructure and mechanical performance of Sn-Ag and Sn-Ag-Cu solders, and develops new insights into their solidification process.
The microstructures of 500 m diameter Sn-Ag balls with different Ag contents and β-Sn nucleation undercoolings are studied and developed into a solidification microstructure selection map. Both deeper undercooling and larger Ag content are found to increase the chance of β-Sn cyclic twinning. Besides the classical single-grain, beach ball, partially interlaced and fully interlaced microstructures, a microstructure with large fully eutectic regions is observed in hypereutectic Sn-5Ag balls, which is explained by competitive nucleation between -Sn and Ag3Sn and coupled zone theory. An orientation study on Sn-Ag balls suggested that solid-state processes happened after solidification and resulted in microstructural features like subgrains and cores.
An immersed thermocouple experiment is performed to study the solidification of SAC305 balls. The microstructures of SAC305/Cu joints with different undercoolings are studied. Deeper undercooling is found to promote β-Sn cyclic twinning, interlacing and refine the joint microstructure. The <110> dendrite growth direction is confirmed for β-Sn, and a dendrite tip splitting phenomenon is observed to help β-Sn grow out of its {001} plane.
Isothermal shear fatigue, shear creep and microhardness test are conducted on SAC305/Cu joints of known undercooling. Deeper undercooling benefits the joint performance under all the tests. Finer eutectic microstructure and the β-Sn interlaced microstructure are suggested to be the major contributors to the improved performance in joints that solidified at deeper undercooling.
This thesis builds new understanding of the solidification – microstructure – performance relationships of Sn-based solder balls and joints, and gives new insight on how to improve the solder performance by controlling the solidification and microstructure.
The microstructures of 500 m diameter Sn-Ag balls with different Ag contents and β-Sn nucleation undercoolings are studied and developed into a solidification microstructure selection map. Both deeper undercooling and larger Ag content are found to increase the chance of β-Sn cyclic twinning. Besides the classical single-grain, beach ball, partially interlaced and fully interlaced microstructures, a microstructure with large fully eutectic regions is observed in hypereutectic Sn-5Ag balls, which is explained by competitive nucleation between -Sn and Ag3Sn and coupled zone theory. An orientation study on Sn-Ag balls suggested that solid-state processes happened after solidification and resulted in microstructural features like subgrains and cores.
An immersed thermocouple experiment is performed to study the solidification of SAC305 balls. The microstructures of SAC305/Cu joints with different undercoolings are studied. Deeper undercooling is found to promote β-Sn cyclic twinning, interlacing and refine the joint microstructure. The <110> dendrite growth direction is confirmed for β-Sn, and a dendrite tip splitting phenomenon is observed to help β-Sn grow out of its {001} plane.
Isothermal shear fatigue, shear creep and microhardness test are conducted on SAC305/Cu joints of known undercooling. Deeper undercooling benefits the joint performance under all the tests. Finer eutectic microstructure and the β-Sn interlaced microstructure are suggested to be the major contributors to the improved performance in joints that solidified at deeper undercooling.
This thesis builds new understanding of the solidification – microstructure – performance relationships of Sn-based solder balls and joints, and gives new insight on how to improve the solder performance by controlling the solidification and microstructure.
Version
Open Access
Date Issued
2024-04-23
Date Awarded
01/02/2025
License URL
Advisor
Gourlay, Christopher
Publisher Department
Department of Materials
Publisher Institution
Imperial College London
Qualification Level
Doctoral
Qualification Name
Doctor of Philosophy (PhD)