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SPECIAL SECTION PAPERS

Prediction and Mitigation of Vertical Cracking in High-Temperature Transient Liquid Phase Sintered Joints by Thermomechanical Simulation

[+] Author and Article Information
Hannes Greve, S. Ali Moeini, Patrick McCluskey

Department of Mechanical Engineering,
Center for Advanced Life Cycle
Engineering (CALCE),
University of Maryland,
College Park, MD 20742

Shailesh Joshi

Toyota Research Institute of North America,
Toyota Technical Center,
1555 Woodridge Avenue,
Ann Arbor, MI 48105
e-mail: hgreve@umd.edu

Contributed by the Electronic and Photonic Packaging Division of ASME for publication in the JOURNAL OF ELECTRONIC PACKAGING. Manuscript received September 27, 2017; final manuscript received January 13, 2018; published online May 9, 2018. Assoc. Editor: Sreekant Narumanchi.

J. Electron. Packag 140(2), 020903 (May 09, 2018) (9 pages) Paper No: EP-17-1100; doi: 10.1115/1.4039265 History: Received September 27, 2017; Revised January 13, 2018

Transient liquid phase sintering (TLPS) is a novel high-temperature attach technology. It is of particular interest for application as die attach in power electronic systems because of its high-melting temperature and high thermal conductivity. TLPS joints formed from sinter pastes consist of metallic particles embedded in matrices of intermetallic compounds (IMCs). Compared to conventional solder attach, TLPS joints consist to a considerably higher percentage of brittle IMCs. This raises the concern that TLPS joints are susceptible to brittle failure. In this paper, we describe and analyze the cooling-induced formation of vertical cracks as a newly detected failure mechanism unique to TLPS joints. In a power module structure with a TLPS joint as interconnect between a power device and a direct bond copper (DBC) substrate, cracks can form between the interface of the DBC and the TLPS joint when large voids are located in the proximity of the DBC. These cracks do not appear in regions with smaller voids. A method has been developed for the three-dimensional (3D) modeling of paste-based TLPS sinter joints, which possess complex microstructures with heterogeneous distributions of metal particles and voids in IMC matrices. Thermomechanical simulations of the postsintering cooling process have been performed and the influence of microstructure on the stress-response within the joint and at the joint interfaces have been characterized for three different material systems (Cu + Cu6Sn5, Cu + Cu3Sn, Ni + Ni3Sn4). The maximum principal stress within the assembly was found to be a poor indicator for prediction of vertical crack formation. In contrast, stress levels at the interface between the TLPS joint and the power substrate metallization are good indicators for this failure mechanism. Small voids lead to higher joint maximum principal stresses, but large voids induce higher interfacial stresses, which explain why the vertical cracking failure was only observed in joints with large voids.

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Figures

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Fig. 1

Cross section of a TLPS joint formed from Cu–Sn sinter pastes with small Cu-particles

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Fig. 2

Cross section of a TLPS joint formed from Cu–Sn sinter pastes with large Cu-particles

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Fig. 3

Cross section of a Ni–Sn TLPS joint

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Fig. 4

Examples of crack formation in Ni–Sn TLPS joints

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Fig. 5

Process flow of the simulation approach for TLPS joint simulations

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Fig. 6

Example structure of a sinter joint with objects of three different sizes

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Fig. 7

Side view on the meshed quarter-model

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Fig. 8

Horizontal cross section through the TLPS joint of the quarter-model with large voids

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Fig. 9

Stress versus plastic strain for Cu and Ni at 20 °C, compare [29,30]

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Fig. 10

Temperature dependence of the stress versus plastic strain for Cu, compare [30]

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Fig. 11

Dependence of the number of nodes in the simulation model on the maximum mesh size in the IMC layer

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Fig. 12

Difference of the principal stresses at the interface between the IMC and the DBC substrate compared to the finest body size (1.00 × 10−5 m)

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Fig. 13

Dependence of the maximum principal stress of the diode and the IMCs on the void size

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Fig. 14:

Cross section through a TLPS joint with large (30 μm radius) voids. Two distinct types of stress concentrations exist: (1) between voids and Cu or the die and (2) between adjacent voids in close proximity.

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Fig. 15

Cross section through a TLPS joint with small (12 μm radius) voids. The higher number of voids in the joint increases the probability of voids in close proximity.

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Fig. 16

Maximum principal stress distribution at the DBC_Cu-to-TLPS joint interfaces for large (30 μm radius) voids

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Fig. 17

Maximum principal stress distribution at the DBC_Cu-to-TLPS joint interfaces for small (12 μm radius) voids

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Fig. 18

Maximum principal stresses in the IMC, diode, IMC-to-Diode and IMC-to-DBC_Cu interfaces for the Cu + Cu6Sn5 system with large and small voids

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