Influence of Pad Surface Finish on the Microstructure Evolution and Intermetallic Compound Growth in Homogeneous Sn-Bi and Sn-Bi-Ag Solder Interconnects

Low reflow temperature solder interconnect technology based on Sn-Bi alloys is currently being considered as an alternative for Sn-Ag-Cu solder alloys to form solder interconnects at significantly lower melting temperatures than required for Sn-Ag-Cu alloys. Microstructural evolution after reflow an...

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Bibliographic Details
Published in:	Journal of electronic materials Vol. 50; no. 12; pp. 6615 - 6628
Main Authors:	Fan, Yaohui, Wu, Yifan, Dale, Travis F., Lakshminarayana, Sukshitha Achar Puttur, Greene, Colin V., Badwe, Nilesh U., Aspandiar, Raiyo F., Blendell, John E., Subbarayan, Ganesh, Handwerker, Carol A.
Format:	Journal Article
Language:	English
Published:	New York Springer US 01-12-2021 Springer Nature B.V
Subjects:	Aging (metallurgy) Alloying Bismuth Characterization and Evaluation of Materials Chemistry and Materials Science Copper Copper base alloys Electronic Packaging and Interconnections 2021 Electronics and Microelectronics Eutectic alloys Evolution Fatigue failure Fatigue life Fatigue tests Finishes Gold Grain boundaries Instrumentation Interconnections Intermetallic compounds Materials Science Metallizing Microstructure Nickel Optical and Electronic Materials Silver base alloys Soldered joints Solders Solid State Physics Surface finish Tin base alloys Tin compounds Wetting electroless nickel immersion gold Tin-bismuth solder gold embrittlement intermetallic growth
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Summary:	Low reflow temperature solder interconnect technology based on Sn-Bi alloys is currently being considered as an alternative for Sn-Ag-Cu solder alloys to form solder interconnects at significantly lower melting temperatures than required for Sn-Ag-Cu alloys. Microstructural evolution after reflow and aging, especially of intermetallic compound (IMC) growth at solder/pad surface finish interfaces, is important to understanding fatigue life and crack paths in the solder joints. This study describes intermetallic growth in homogeneous solder joints of Sn-Bi eutectic alloy and Sn-Bi-Ag alloys formed with electroless nickel-immersion gold (ENIG) and Cu-organic surface protection (Cu-OSP) surface finishes. Experimental observations revealed that, during solid state annealing following reflow, the 50nm Au from the ENIG surface finish catalyzed rapid (Ni,Au)Sn 4 intermetallic growth at the Ni-solder interface in both Sn-Bi and Sn-Bi-Ag homogeneous joints, which led to significant solder joint embrittlement during fatigue testing. Intermetallic growth of (Ni,Au)Sn 4 was decreased by Ag alloying of eutectic Sn-Bi solder and was completely eliminated by changing the metallization from ENIG to Cu-OSP on the board side of the assembly. The reduction in (Ni,Au)Sn 4 growth rate with Ag additions is attributed to changes in grain boundary wetting of the IMC by Bi with Ag alloying.
ISSN:	0361-5235 1543-186X
DOI:	10.1007/s11664-021-09256-1