Microstructure and property changes in Cu/Sn-58Bi/Cu solder joints during thermomigration

Yu An Shen, Shiqi Zhou, Jiahui Li, K. N. Tu, Hiroshi Nishikawa

Research output: Chapter in Book/Report/Conference proceedingConference contributionpeer-review

2 Scopus citations


Thermomigration has become a critical reliability issue in consumer electronic products because of Joule heating. To conduct heat away, it requires temperature gradient. Just 1°C difference across a microbump of 10 μm in diameter produces a temperature gradient of 1000 °C/cm, which can cause thermomigration, especially in low melting eutectic SnBi solder joints. However, the study of thermomigration in SnBi soler joints is hardly seen, not to mention the effect and the details. In this study, a Cu/Sn-58Bi/Cu joint with a 42 μm solder height, bonded by a reflow process, is examined for thermomigration with a thermal gradient of 1309 °C/cm. We report here that Bi atoms move from the hot end to the cold end, following the temperature gradient, it is the dominant diffusing species. The Sn atoms move from the cold end to the hot end. Under the assumption of constant volume, the Sn atoms are being squeezed by the Bi atoms at the cold end and have to make room for the Bi atoms, so they move to the hot end. Moreover, the formation of Cu-Sn intermetallic compound (IMC) layers at the cold and hot end site were symmetrical, unaffected by thermomigration. Additionally, finite-element-method (FEM) simulations showed that the phase separation of Bi and Sn has reduced current crowding regions, which affects the electromigration of the eutectic SnBi solder joints.

Original languageEnglish
Title of host publicationProceedings - IEEE 69th Electronic Components and Technology Conference, ECTC 2019
PublisherInstitute of Electrical and Electronics Engineers Inc.
Number of pages6
ISBN (Electronic)9781728114989
StatePublished - May 2019
Event69th IEEE Electronic Components and Technology Conference, ECTC 2019 - Las Vegas, United States
Duration: 28 May 201931 May 2019

Publication series

NameProceedings - Electronic Components and Technology Conference
ISSN (Print)0569-5503


Conference69th IEEE Electronic Components and Technology Conference, ECTC 2019
CountryUnited States
CityLas Vegas


  • Finite element method (FEM)
  • Phase segregation
  • Sn-Bi solder joint
  • Thermal stress
  • Thermomigration

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