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Evaluation of Thermal-Cycling Reliability of Sintered-Nanosilver Versus Soldered Joints by Curvature Measurement

Fig. 1. Curvatures decrease with thermal cycling for samples with different substrates and different die-attach layers
In this work, the thermo-mechanical reliability of sintered-silver joints was studied in compar-ison with the soldered joints of two lead-free solders: SN100C and SAC305. Die-attach samples were fabricated by bonding 10 mm 10 mm silicon "mechanical" chips to silver-metallized cop-per blocks and direct-bond-copper (DBC) substrates according to the respective heating pro-files of a nanosilver paste and the two solders. The die-attach samples were thermally cycled between -40°C and 125°C. Bonding reliability was evaluated by measuring the bending curva-tures of the cycled samples and examining the cross sections of the samples under an electron microscope. Bending of the bonded structures, which is the result of mismatched coefficients of thermal expansion between silicon and copper or DBC, offered a non-destructive method for monitoring the integrity of the bond line. The bending curvatures of all of the die-attach samples decreased rapidly after they were thermally cycled. Most of the drop in curvature can be at-tributed to stress relaxation in the bonding materials without bond-line cracking. However, in the samples on copper blocks, after 800 cycles, the curvatures of the soldered samples decreased to near zero m-1, whereas those of the silver-sintered samples retained approximately 30% of the original curvatures. Scanning electron microscopy images showed that the joints of the sol-dered samples with near zero m-1 curvature had been cracked almost all the way through, whereas the joints of the sintered samples were still intact. These results demonstrate that sin-tered-silver joints are more reliable than soldered joints.

Fig. 2. Cross sections of copper samples after 800 cycles for (a) cracked SAC305 soldered joints, (b) cracked SN100C sol-dered joints, and (c) intact sintered-silver joints, and (d) intact sintered-silver joints under low magnificatio)
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