Characterization of Low-Temperature Sintered Nanoscale Silver Paste for Attaching Semiconductor Devices

Attachment of semiconductor devices to a package substrate is essential for providing electrical and structural connections as well as a heat dissipation path. The die-attach materials play a vital role in ensuring the system performance and reliability. As the electronics industry continues to integrate more functions in smaller packages, the electrical, thermal and mechanical properties of the existing die-attach materials such as solders and conductive epoxies fail to meet more demanding requirements for performance and reliability. To address this problem, we developed low-temperature sintered nanosilver as a new die-attaching material. Experimental results of the electrical, thermal, and mechanical properties of the sintered silver die-attachment are presented in this paper. The nanoscale silver paste was made by dispersing 30-nm silver powder under ultrasonic agitation in an organic binder system. The electrical resistivity, obtained from screen printed resistor patterns on an insulate substrate that were sintered at 280degC for around 10 minutes in air, was found to be 2.6 times 10 ⁵ (Omegamiddotcm)^-1. The thermal conductivity was obtained by the laser flash method and was found to be 240 W/K-m. Both values are lower than those of bulk silver because the sintered silver had a density around 80%. The coefficient of thermal expansion (CTE) of the sintered silver was measured by dilatometry and was found to be 19 times 10^-6/degC, nearly identical to that of bulk silver. The apparent elastic modulus of the sintered silver was found to be 9 GPa while the yield strength was around 43 MPa. Furthermore, die-shear tests on devices bonded by the sintered silver gave strength of around 21 MPa for a gold-metallized substrate and strength of 38 MPa for a silver-metallized substrate. These results demonstrate that the nanoscale silver paste sintered at low temperature is an excellent alternative to solders or epoxies for die attachment