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SPECIAL SECTION PAPERS

Lead-Free Alternatives for Interconnects in High-Temperature Electronics

[+] Author and Article Information
Sandeep Mallampati, Junghyun Cho

Department of Mechanical Engineering;
Department of Materials Science and Engineering,
Binghamton University (SUNY),
Binghamton, NY 13902

Liang Yin, David Shaddock

GE Global Research,
Niskayuna, NY 12309

Harry Schoeller

Germanna Community College,
Fredericksburg, VA 22408

1Present address: GLOBALFOUNDRIES, Malta, NY 12020.

Contributed by the Electronic and Photonic Packaging Division of ASME for publication in the JOURNAL OF ELECTRONIC PACKAGING. Manuscript received September 28, 2017; final manuscript received January 4, 2018; published online March 2, 2018. Assoc. Editor: Sreekant Narumanchi.

J. Electron. Packag 140(1), 010906 (Mar 02, 2018) (7 pages) Paper No: EP-17-1101; doi: 10.1115/1.4039027 History: Received September 28, 2017; Revised January 04, 2018

Predominant high melting point solders for high-temperature and harsh environment electronics (operating temperatures from 200 to 250 °C) are Pb-based systems, which are being subjected to RoHS regulations because of their toxic nature. In this study, high bismuth (Bi) alloy compositions with Bi-XSb-10Cu (X from 10 wt % to 20 wt %) were designed and developed to evaluate their potential as high-temperature, Pb-free replacements. Reflow processes were developed to make die-attach samples made from the cast Bi alloys. Die-attach joints made from Bi-15Sb-10Cu alloy exhibited an average shear strength of 24 MPa, which is comparable to that of commercially available high Pb solders. These alloy compositions also retained original shear strength even after thermal shock (TS) between −55 °C and +200 °C and high-temperature storage (HTS) at 200 °C. Brittle interfacial fracture sometimes occurred along the interfacial NiSb layer formed between Bi(Sb) matrix and Ni metallized surface. In addition, heat dissipation capabilities, using flash diffusivity, were measured on the die-attach assembly and were compared to the corresponding bulk alloys. The thermal conductivity of all the Bi–Sb alloys was higher than that of pure Bi. By creating high volume fraction of precipitates in a die-attach joint microstructure, it was feasible to further increase thermal conductivity of this joint to 24 W/m·K, which is three times higher than that of pure Bi (8 W/m·K). Bi–15Sb–10Cu alloy has so far shown the most promising performance as a die-attach material for high-temperature applications (operated over 200 °C). Hence, this alloy was further studied to evaluate its potential for plastic deformation. Bi–15Sb–10Cu alloy has shown limited plastic deformation in room temperature tensile testing in which premature fracture occurred via the cracks propagated on the (111) cleavage planes of rhombohedral crystal structure of the Bi(Sb) matrix. The same alloy has, however, shown up to 7% plastic strain under tension when tested at 175 °C. The cleavage planes, which became oriented at smaller angles to the tensile stress, contributed to improved plasticity in the high-temperature test.

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Figures

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Fig. 1

Vertical section of Bi–Sb–Cu phase diagram at Cu = 10 wt %. η-Cu2Sb and δ-Cu4Sb

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Fig. 2

Rhombohedral crystal structure of Bi

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Fig. 3

Bulk microstructures of (a) Bi–20Sb–10Cu, (b) Bi–15Sb–10Cu, and (c) Bi–10Sb–10Cu

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Fig. 4

Melting behavior of Bi–XSb–10Cu alloys, X = 10, 15, and 20 wt %

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Fig. 5

Interfacial intermetallic layers on the die side (left) and substrate side (right)

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Fig. 6

Die-attach microstructures of (a) Bi–20Sb–10Cu, (b) Bi–15Sb–10Cu, and (c) Bi–10Sb–10Cu reflowed on Cu and Ni metallization

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Fig. 7

Die-shear strength of Bi–XSb–10Cu alloys at room temperature (as reflowed)

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Fig. 8

Representative failure surfaces in die-shear testing of Bi–XSb–10Cu alloys

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Fig. 9

Summary of die shear test post accelerated testing

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Fig. 10

Backscatter detector image of die-attach microstructure after 500 cycles of TS

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Fig. 11

Thermal conductivity comparison for microstructures tested

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Fig. 12

Engineering stress–strain curves from tensile test on Bi–15Sb–10Cu

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Fig. 13

Fracture surfaces in tensile test: (a) room temperature and (b) 175 °C

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