Die-Attachment on Copper by Nanosilver Sintering: Processing, Characterization and Reliability
Die-attachment, as the first level of electronics packaging, plays a key role for the overall performance of the power electronics packages. Nanosilver sintering has becoming an emerging solder-free, environmental friendly die-attach technology. Researchers have demonstrated the feasibility of die-attachment on silver (Ag) or gold (Au) surfaces by pressure-less or low-pressure (< 5 MPa) nanosilver sintering. This study extended the application of nanosilver sintering die-attach technique to copper (Cu) surface. The main challenge of nanosilver sintering on Cu is the formation of thick Cu oxide during processing, which may lead to weak joints. In this study, different processes were developed based on the die size: for small-area dice (< 5 * 5 mm2), different sintering atmospheres (e.g. forming gas) were applied to protect Cu surface from oxidation; for large-area dice (> 5 * 5 mm2), a double-print, low-pressure (< 5 MPa) assisted sintering process was developed. For both processes, die-shear tests demonstrated die-shear strength can reach 40 MPa.
The effects of different sintering parameters of the processing were analyzed by different material characterization techniques. With forming gas as sintering atmosphere, not only Cu surface was protected from oxidation, but also the organics in the paste were degraded with nanosilver particles as catalyst. External pressure applied in the processing not only increased the density of sintered Ag, but also enhanced the contact area of sintered-Ag/Cu interface. Microstructure of Ag/Cu interface were characterized by transmission electron microscopy (TEM). Characterization results indicate that Ag/Cu metallic bonds formed at the interface, which verified the high die-shear strength of the die-attachment.
Thermal performance of nanosilver sintered die-attachment on Cu was evaluated. A system was designed and constructed for measuring both transient thermal impedance (Zth) and steady-state thermal resistance (Rth) of insulated gate bipolar transistor (IGBT) packages. The coefficient of variation (CV) of Zth measurement by the system was lower than 0.5%. Lead-free solder (SAC305) was applied in comparison of thermal performance with nanosilver paste. With same sample geometry and heating power level, nanosilver sintered joints on Cu showed in average 12.6% lower Zth and 20.1% lower Rth than SAC305 soldered joints. Great thermal performances of nanosilver sintering die-attachment on Cu were mainly due to the low thermal resistivity of sintered-Ag and the good bonding quality.
Both passive temperature cycling and active power cycling tests were conducted to evaluate the reliability of nanosilver sintered joints on Cu. For passive temperature cycling tests (-40 - 125 C), the die-shear strengths of mechanical samples had no significant drop over 1000 cycles, and nanosilver sintered IGBT on Cu packages showed almost no change on Zth after 800 cycles. For active power cycling test (Tj = 45 - 175 C), nanosilver sintered IGBT on Cu assembly had a lifetime over 48,000 cycles. The failure point of the assembly was the detachment of the wirebonds. Great reliability performances of nanosilver sintered die-attachment on Cu were mainly due to the low mismatch of coefficient of thermal expansion (CTE) between sintered-Ag and Cu. Meanwhile, low inter-diffusion rate between Ag and Cu prevented the interface from the reliability issue related to Kirkendall voids, which often took place in tin (Sn) -based solder joints.