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Test Results of Sintered Nano-Paste Silver Die Attach for High Temperature Applications
Keywords: sintered silver, die attach, high temperature
The emergence of wide band gap devices has pushed the boundaries of power converter operations. For high power density applications, it is desirable to operate the power inverter at high switching frequencies to reduce passive filter weight and at high temperature to reduce the cooling system requirement. Operating power converters at higher junction temperatures necessitates the use of higher temperature capable materials as well as the careful design of material stack-ups to achieve desired reliability. Therefore, reliable materials and components for use at temperatures ranging from -55-200C, or higher, are needed in the power electronics industry. Reliable bonding of electronic components onto a substrate is of particular interest. Depending on the materials used for ceramic die components and substrates, potentially significant coefficient of thermal expansion mismatches can exist between dies and substrates. A CTE mismatch will produce a stress field within the die attach during changes of ambient temperature or heat generation of the device. Operation of a PE device produces cyclic thermal loading that leads to crack formation in the die attach and limits the useful life of the component. Sintered silver is receiving significant attention in the industry. Sintering of nano-silver particles allows the formation of a pure but porous silver bondline at a temperature of about 270C, far below bulk silvers melting point of 962C. The porous nature of sintered silver, with a typical porosity of about 10-30%, produces a reduced modulus of elasticity that has potential as a strain relief between the ceramic die component and substrate. Test results presented herein include tensile testing to rupture of sintered silver film, die shear testing of silicon dies bonded with sintered silver die attach, and thermal cycle testing square silicon dies sintered to copper substrates. The nano-silver paste used for all fabrication of test specimens was NanoTach X-Series from NBE Tech, Blacksburg, VA. Tensile testing of 80um thick sintered silver film was performed to obtain elastic and plastic stress-strain behavior of the material for future material modeling. Tensile rupture tests were performed at various strain and stress rates as well as various elevated temperatures. Typical ultimate strengths were 30-35MPa at ambient temperature and 10MPa at 200C. The die shear tests were performed prior to thermal cycling tests at various temperatures to obtain baseline shear strengths as a function of temperature and bondline thicknesses. Shear strengths for 50-120um thick bondlines were approximately 35-40MPa at ambient temperature with a sharp decrease to 20MPa for a 20um bondline. Accelerated life testing was performed by thermal cyclic testing on a number of highly CTE mismatched silicon-to-copper specimens. Variations of bondline thickness (50-120um), die size (2 and 4mm square), and temperature range (-40-150C and -50-200C) were investigated. Life was negatively impacted with increased die size and decreased bondline thickness. Crack formation and propagation were observed and documented through SEM imaging of samples at regular intervals.
Paul F. Croteau, Research Engineer
United Technologies Research Center
East Hartford, CT

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