Packaging Technology for Electronic Applications in Harsh High-Temperature Environments

Silicon-on-insulator (SOI) integrated circuits have been demonstrated for use at temperatures up to 300 degrees C. However, to build functional electronics, multiple devices must be interconnected to provide the desired functionality. A system-inpackage approach has been developed using thick-film technology on Si3N4 ceramic substrates. Si3N4 has a near coefficient-of-thermal-expansion match to Si and a higher flexural modulus than Al2O3, which is commonly used for thick-film applications. The conductor metallization is Au. For 300 degrees C operation, eutectic Au-Ge die attach was used with a Ti/Ti:W/Au backside die metallization. After 3000 h at 325 degrees C, the mean die shear strength decreased from 3.96 to 3.33 kg/mm(2), a decrease of only 16%. Formation of Au-Si-Ge ternary eutectic (melting point 326 degrees C) was observed and limits the use of Au-Ge die attach to 300 degrees C. SOI dies typically have Al wire bond pads that are not compatible with Au thermosonic wire bonding for high-temperature applications. Two plating processes have been examined: electroless Ni/immersion Au/electroless Au and electroless Ni/electroless Pd/immersion Au. The plating processes provide a barrier layer (Ni) and a wire-bondable finish (Au or Pd-Au) over the Al wire bond pads. After 10 000 h at 300 degrees C, the wire pull force for the Ni/Au samples decreased by similar to 30% due to annealing of the Au wire, while the ball shear force increased by similar to 35%. The daisy-chain electrical resistance remained relatively constant. For the Ni-Pd-Au samples, after 2000 h at 320 degrees C, the ball shear force remained constant or increased slightly, the wire pull force decreased by similar to 25% due to annealing of the Au wire, and the daisy-chain resistance remained relatively constant. After 3000 h, however, cratering of the Si wire bond pad was observed corresponding to some first bond pad lifts and increased daisy-chain resistance. Optimization of the wire-bonding parameters for bonding to the harder Ni/Pd/Au bond pad is required to eliminate cratering.