ICSR (Soldering and Reliability) Conference Proceedings

Low Temperature Sinter-Able Silver Paste For High Power Die Attach Application

Authors: Sihai Chen, Christine LaBarbera and Ning-Cheng Lee
Company: Indium Corporation
Date Published: 6/6/2017   Conference: ICSR (Soldering and Reliability)

Abstract: Pressure-less silver sintering paste, which can be sintered at temperature as low as 173C under air atmosphere to form robust silver joints, has been developed. The sintering time can be shortened from two hours (at 173C) to one hour or even to half hour if heating under 192oC or 206oC, respectively. The thermal conductivity of the bulk silver materials reaches 210W/mK. The low temperature sinter-able paste provides new possibilities for applications such as in LED or high power die attach fields that require reducing the thermal stress, protecting the components and using substrates that cannot survive high temperatures.

Ag joints having no or less than 1% voids have been generated on 3mm x 3mm and 10mm x 10mm Ag-die/Au-Si3N4 AMB combination under optimized sintering conditions. Methods including changing the die surface finish and varying aging temperature (250 or 300C) and time (1 to 6 hours) have been tried to improve the joint quality. It is found that the aging indeed increase the joint shear strength. The increased inter-diffusion of elements between substrate and sintered silver is the reason for bonding enhancement. Inter-diffusion between surface Au finish and sintered Ag promotes the formation of an Ag dense layer at interface.

Long term stability of the sintered joints was studied by aging at 250oC with different dies including Au and Ag finished silicon dies and Cu dies. Experiments show that both Au and Ag finished dies show an enhancement in shear strength, with Ag showing a steady shear strength increase. In contrast, Cu dies result in a quick decrease in shear strength even after one day of aging, the failure points are located between the sintered Ag and Cu surface, presumably due to the formation of copper oxide on top of the Cu die surface.

Key Words: 

Silver sintering paste, pressure-less, die-attach, porosity, high temperature aging, surface finish

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