Low-pressure solid-state bonding technology using fine-grained silver foils for high-temperature electronics

Abstract

A solid-state bonding technique using fine-grained silver (Ag) foils is presented. The Ag foils are manufactured using many runs of cold rolling and subsequent annealing processes to achieve the favorable microstructure. X-ray diffraction and pole figure measurement are performed to examine the crystal structure and grain orientations. Si chips are bonded to bare Cu substrates using the Ag foil as the bonding medium at 300°C in 0.1 torr vacuum assisted by 6.9 MPa static pressure, which is much lower than that used in conventional thermal compression bonding. Cross sections prepared by focus ion beam show clear bonding interfaces with only a few voids smaller than 100 nm. The bonded structures do not crack after cooling down to room temperature, indicating that the ductile Ag layer is able to manage the strain induced by the large coefficient of thermal expansion mismatch between Si and Cu. The average shear strength of as-bonded samples is 29 MPa. High-temperature storage tests are conducted, and slight increase in strength can be observed after 300°C aging. Fracture analyses show that the breakage occurs within the Ag foil rather than on the bonding interface. Transmission electron microscopy and energydispersive spectroscopy (TEM/EDX) are conducted for Ag/Cu interface after 200-h aging, and the result shows that slight diffusion proceeds during the aging. Since Ag has the highest electrical and thermal conductivities among metals, therefore the bonded structures reported in this paper probably represent the best possible design for high-temperature and high-power electronic packaging applications.

Solid-state bonding of silicon chips to copper substrates with graded circular micro-trenches

“…The fracture initiated at the edge of the chip followed by cleavage. Since the shear tool applied force on the chip edge and the shear speed is quite high, the stress concentration there broke the chip, which has been reported elsewhere [25,26]. On Fig.…”

Low temperature SiC die-attach bonding technology by hillocks generation on Al sheet surface with stress self-generation and self-release

“…It slightly decreases from the as-bonded strength of 33.4 MPa to 31.2 MPa after 500 h, which suggests that the bonding technology has excellent high-temperature reliability. The high-temperature stability is comparable to that of Pb-5Sn solder 40 , joined sintered Ag nanoparticles 41,42 and Ag foil bonding technology 25 , as shown in Fig. 8.…”

“…The high purity Al sheet bonding technology introduced in this study is more cost-efficient and has a simpler bonding process compared with the Ag thin film bonding technology and Ag sinter paste joining. In addition, the melting point is Ag thin film 25,26 1 Table 1. Material properties of pure Al sheet die-attach compared with other materials.…”

“…Recently, a solid-state bonding technology that uses porous Ag [22][23][24] or Ag foils 25,26 has been reported for high-temperature applications. The bonding mechanism is attributed to solid interface inter-diffusion, which requires high-temperature or high-pressure conditions 24,26 , which contribute to high costs.…”

In-air sintering of copper nanoparticle paste with pressure-assistance for die attachment in high power electronics

“…These advantages have encouraged the recent utilization of Ag NP paste as a die-attach material in power electronics applications [11][12][13]. The sintering parameters and materials were tuned to increase the mechanical strength of the sintered joints [14][15][16]. There have also been studies on the sintering mechanisms of Ag pastes [17][18][19].…”