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Pressure-Less Silver Sintering Pastes for Low Porosity Joint and Large Die
Pressure-Less Silver Sintering Pastes for Low Porosity Joint and Large Die
Under pressure-less sintering conditions silver sintering pastes for different sized die attachment to form robust silver joints have been developed.
Materials Tech

Authored By:
Sihai Chen, Ph.D., Christine LaBarbera and Ning-Cheng Lee, Ph.D.
Indium Corporation
Clinton, NY, USA
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Summary
Under pressure-less sintering conditions silver sintering pastes for different sized die attachment to form robust silver joints have been developed. The sintering profiles of pastes developed in this work are compatible with conventional infrared reflow oven, resulting in a higher through-put as compared to that of box heating oven; the sintering work can be done under air and a highly reliable joint has been obtained as judged from the shear strength or thermal measurement results from aging tests.

Ag joints generated on 3mm x 3mm Ag-die/Au-DBC combination displayed a "center-dense-edge-porous" structure. As the 250 Celsius aging continued, joints with ~10% total porosity have been observed; in the center area, porosity even reaching as low as 2 - 3%. The shear strength reached as high as ~70Mpa after aging, demonstrating a robust bonding. Two routes for Ag diffusion are revealed: (1) in the bulk phase Ag continuously densifies through grain boundary and lattice diffusion, resulting in the formation of larger silver domains. (2) Ag diffuses to the sintered Ag/DBC interface to alloy with and form a dense Ag layer containing Au, Ni and Cu. The dense layer formation strengthened the bonding by eliminating the depletion layer.

Ag joints formed on 10mm x 10mm Ag-Die/Ag-Si3N4 AMB substrate have shear strength of at least > or = 10Mpa. They cannot be broken due to the shear tester's limited maximum shear forces. The joint was very stable after 300 Celsius thermal aging for at least 841h examined by laser flash thermal conductivity measurement. The shear strength measured at 841h aging time reached at least 16Mpa before Si die cracked.
Conclusions
The silver sintering pastes used for bonding different sized Si dies including 3mm x 3mm and 10mm x 10mm are demonstrated. The joints formed are reliable according to thermal aging data.

Ag joints generated on 3mm x 3mm Ag-die/Au-DBC combination displayed a "center-dense-edge-porous" structure. As the 250 Celsius aging continued, joints with ~10% total porosity have been observed; in the center area, porosity even reaching as low as 2 - 3%. The shear strength reached as high as ~70Mpa after aging, demonstrating a robust bonding. Two routes for Ag diffusion are revealed: (1) in the bulk phase Ag continuously densifies through grain boundary and lattice diffusion, resulting in the formation of larger silver domains. (2) Ag diffuses to the sintered Ag/DBC interface to alloy with and form a dense Ag layer containing Au, Ni and Cu. The dense layer formation strengthened the bonding by eliminating the depletion layer.

Ag joints formed on 10mm x 10mm Ag-Die/Ag-Si3N4 AMB substrate have shear strength of at least > or = 10Mpa. They cannot be broken due to the shear tester's limited maximum shear forces. The joint was very stable after 300 Celsius thermal aging for at least 841h examined by laser flash thermal conductivity measurement. The shear strength measured at 841h aging time reached at least 16Mpa before Si die cracked.
Initially Published in the SMTA Proceedings
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