Ag Sintering Die and Passive Components Attach for High Temperature Applications

Abstract

With an increasing demand for SiC and GaN high power devices and high temperature electronics operating in extreme environments, traditional solder materials are reaching their limitations in performance. Compatible, high temperature materials are required for attachment of semiconductor die and passives to the thick film metallized substrate pads. In addition, there is a strong desire to eliminate high lead containing solders in Si power device packaging for use over conventional temperature ranges. Low temperature Ag sintering technology is a promising method for high performance lead-free die attachment. Due to its high thermal and electrical conductivity and high melting point, sintered Ag die attach has received much attention for assembly of power modules and for high temperature (300oC)applications. Previous work with Ag sintering has required pressure during the sintering process or been limited to small area die. In chapter 3, the pressureless sintering of micro-scale silver paste is examined for larger (8mm x 8mm) area die. Experimental combinations included: Si die metallization (Ag and Au); thick film substrate metallization (Au, Ag and PdAg); and sintering temperature. For Au metallization (die and/or substrate), the initial shear strength results were good with 8mm x 8mm die sintered at lower temperatures (200 °C). The shear strength was beyond the limit of the shear test machine (100 kg), corresponding to >15.3 MPa. However, after aging for 24 hours at 300oC, the shear strength dropped significantly. An SEM was used to characterize cross sections of as-built and aged sample. The decrease in die shear strength with high temperature sintering (250oC and 300oC) or high temperature aging is attributed to surface diffusion of Ag along the Au surface resulting in a dense Ag layer adjacent to the Au surface and a depletion layer within the die attach adjacent to the dense Ag layer. Shear failures occurred through the depleted region. For Ag metallization (die and substrate), no decrease in shear strength was observed with 300oC aging. Shear strength of 8x8cm2 dies was >100 kg ( >15.3 MPa) as-built. The shear strength remained >15.3MPa after 8000 hours of 300oC aging. The pressureless sintering process for large die (8 mm x 8 mm) and suitable metallization were demonstrated to provide high reliability die attach by using micro-size Ag sintering. The resulting die attach layer had approximately 30% porosity. In chapter 4, passive component (chip resistors) attachment with pressureless Ag sintering was explored. Due to termination geometry differences between resistors and dies, different processing procedures and parameters were developed. For PtAu terminated resistors, the mean shear force of as-built samples on thick film Ag metallized substrates was 90 N, but dropped to 18.6 N after 1500 hours at 300oC. Formation of a dense Ag layer near the PtAu resistor termination and a void region near the thick film metallization was seen in cross-sections after 1000 hours at 300oC For PdAg terminated resistors with a plated Ni/Au finish, the initial shear force results were low due to Ag diffusion along Au metallization surface. For PdAg terminated resistors with Ag thick film substrates, the initial shear force was approximately 60 N and remained in the range of 50-70 N during aging at 300oC for 1500 hours. A new thick film metallization (Au+Ag) was developed to enable the use of thick film Au interconnect metallization. In chapter 5, a low temperature pressure-assisted rapid sintering process was developed, which include a 150°C, one-minute pre-dry and a 300°C one-minute sintering process. The reduction in the sintering time allowed the use of a flip chip thermo-compression bonder for automated die attach instead of hydraulic hot press. The porosity was decreased from 30% to 15% with application of a low pressure (7.6 MPa) during a one minute sintering process. The shear strength for a 3 mm x 3 mm die was 70 MPa and the 8 mm x 8 mm die could not be sheared off due to a 100 kg shear module force limit. Both the Ag and Au metallization (die and substrate) were studied. Furthermore, a new substrate metallization combination was found that allows the use of Au thick film metallized substrates. High temperature (300oC) storage tests for up to 2000 hours aging were conducted and results were presented.