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Drying and Densification Kinetics of a Low-temperature Sinterable Nanosilver Die-attach Paste

Fig. 1. Schematic of the optical setup used for measuring shrinkage profiles of nanosilver paste samples placed in a cantilever assembly.
To provide fundamental understanding on the sintering behavior of the nanosilver paste for its die-attach application, the drying and densification kinetics of the die-attach material were studied. An optical setup was used to measure the shrinkage profiles of the paste in the die-attach assembly as the material underwent solvent evaporation, binder-burnout, and densification stages of the die-attach process. The measured shrinkage kinetics were found in good agreement with the weight-loss kinetics of the paste measured by thermogravitational analysis (TGA), and the comparison offered direct evidence of shrinkage contributed by the late-stage densification of silver nanoparticles. When observing the microstructure evolution of sintered joint, it is found that densification mostly happened after the removal of organics from since binders systems prevent the silver particles coming into contact. Drying of the paste tends to arise pushing up of the chip and further cracking of the sintered joint for large sintered joint in die-attached structure.

Fig. 2. Result of the TGA and thickness shrinkage measurements for 3mm 3mm sample size, plotted together to show a direct evidence of sample shrinkage as the result of densification of silver nanoparticles.
Fig. 3. SEM characterization of the fractured surfaces of the sandwiched nanosilver samples with 3mm 7mm die heated at 7.5°C/min to: (a) 260°C; (b) 275°C; (c) 275°C for 4 min; (d) 275°C for 15 min; and (e) 275°C for 20 min.
Fig. 4. Cross-sectional SEM images of the die-attach samples, 1 mm 1 mm, 3 mm 3 mm, and 5 mm 5 mm that were heated with the (a) constant-rate and (b) ramp-soak profile.
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