Research Papers

The Strength of High-Temperature Ag–In Joints Produced Between Copper by Fluxless Low-Temperature Processes

[+] Author and Article Information
Yuan-Yun Wu

Electrical Engineering and Computer Science,
Materials and Manufacturing Technology,
University of California,
2226 Engineering Gateway Building,
Irvine, CA 92697-2660
e-mail: yuanyunw@uci.edu

Chin C. Lee

Electrical Engineering and Computer Science,
Materials and Manufacturing Technology,
University of California,
2226 Engineering Gateway Building,
Irvine, CA 92697-2660

1Corresponding author.

Contributed by the Electronic and Photonic Packaging Division of ASME for publication in the JOURNAL OF ELECTRONIC PACKAGING. Manuscript received July 2, 2013; final manuscript received December 2, 2013; published online January 8, 2014. Assoc. Editor: Yi-Shao Lai.

J. Electron. Packag 136(1), 011006 (Jan 08, 2014) (6 pages) Paper No: EP-13-1063; doi: 10.1115/1.4026171 History: Received July 02, 2013; Revised December 02, 2013

Two copper (Cu) substrates were bonded using silver (Ag) and indium (In) and annealed at 200–250 °C to convert the joints into the solid solution (Ag) for enhanced strength and ductility. Cu–Cu pair was chosen so that the samples break in the joint during shear test. The upper Cu was electroplated with 15 μm Ag. The lower Cu was plated with 15 μm Ag, followed by In and 0.1 μm Ag to inhibit indium oxidation. Two designs were implemented, using 8 μm and 5 μm In, respectively. The Cu substrates were bonded at 180 °C in 100 mTorr vacuum without flux. Afterwards, samples were annealed at 200 °C for 1000 h (first design) and at 250 °C for 350 h (second design), respectively. Scanning electron microscope with energy dispersive X-ray analysis (SEM and EDX) results indicate that the joint of the first design is an alloy of mostly (Ag) with micron-size Ag2In and (ζ) regions, and that of second design has converted to a single (Ag) phase. Shear test results show that the samples are very strong. The breaking forces far exceed requirements in MIL-STD-883 H standards. Fracture incurs inside the joint and is a mix of brittle and ductile modes or only ductile mode. The joint solidus temperatures are 600 °C and 900 °C for the first and second designs, respectively.

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Grahic Jump Location
Fig. 1

Bonding structure design of Cu/Ag and Cu/Ag/In/Ag, not to scale. First design: indium thickness = 8 μm, second design: indium thickness = 5 μm.

Grahic Jump Location
Fig. 2

Silver–indium (Ag–In) binary phase diagram

Grahic Jump Location
Fig. 3

Cross section optical microscopy image and backscattered SEM image of sample bonded at 180 °C. (a) Unannealed, (b) EDX data of layers marked “1”–“7”, and the results are given in Table 1.

Grahic Jump Location
Fig. 4

Cross section OM images and backscattered SEM images of first-design samples bonded at 180 °C. (a) After annealing at 200 °C for 300 h, (b) EDX data of layers shown in (a) marked “1”–“7” and the results are given in Table 2, (c) after annealing at 200 °C for 500 h, (d) cross section backscattered SEM image of the sample shown in (c), (e) after annealing at 200 °C for 1000 h, and (f) EDX data of layers shown in (e) marked “1”–“7” and the results are given in Table 3.

Grahic Jump Location
Fig. 5

Cross section OM image and backscattered SEM image of second-design sample bonded at 180 °C. (a) After annealing at 250 °C for 350 h, (b) EDX data of layers marked “1”–“10”, and the results are given in Table 4.

Grahic Jump Location
Fig. 6

The bar chart and table illustrate the breaking forces of the six samples of the first design annealed at 200 °C for 1000 h and seven samples of the second design annealed at 250 °C for 350 h. The shear tester could not break samples 2 and 3 of first design.

Grahic Jump Location
Fig. 7

SEM image of the fracture surface of samples after the shear test. (a) First-design sample: EDX detects Ag2In, (ζ), and (Ag) and (b) second-design sample: EDX detects only the solid solution phase (Ag).




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