Rapid Formation of Intermetallic Joints with Sn/Metal Composite Alloys by Ultrasonic-assisted Soldering for High-temperature Chip Attachment

摘要

With the dramatic requirements of high power and high integrity in field of power devices, the study of novel interconnection materials and packaging methods has been paid an extremely attention. Presently, the most candidates for the chip bonding are Zn-based, Bi-based, noble-metal-based (such as Au-Sn, Au-Ge, Au-Si), nano-metal particles (Ag or Cu nanoparticles) and transient-liquid-phase (TLP) bonding. However, a high bonding temperature and poor ductility for Zn-based solder (more than 420°C) causing irreversible damages or accelerated aging affects to the electronic devices; Bi-based solder alloys are still under way mainly due to their inferior thermal and electrical conductivity as well as poor workability; It's worth noting that nano-silver sintering and TLP bonding technology have been proven to be the potential solutions to the power electronic packaging, because their processing temperatures are relatively low and the produced joints are of high re-melting temperatures. However, Ag is expensive and easy to electromigration, which will influence the stability/reliability of the joints and hinder to universal industry applications. For the TLP bonding, the process to produce full intermetallic compounds (IMCs) is time-consuming, commonly more than 60 min, due to relatively low reaction velocity. To solve the problem of time-consuming of the TLP, an enhanced and low-cost auxiliary energy is necessary. Ultrasonic-assisted soldering has been proven able to solder dissimilar metals at low temperatures within a very short time. In our previous work, a full Cu6Sn5/Cu3Sn IMCs joint through TLP assisted by ultrasound was obtained. Because of the acoustic cavitation and streaming effect created by ultrasonic waves propagating in molten alloys, the atom diffusion and physical-chemical interactions were accelerated at the liquid/solid interfaces. However, a sole IMC joint with single phase has not been fabricated so far. High re-melting temperature joint consisted of nearly sole Ni_3Sn_4 with a high shear strength of 43.4 MPa was achieved with Sn-24 wt.%Ni by ultrasonic-assisted soldering for 10 s. This rapidly formed intermetallic joint was phase-stable as isothermally aged at 300 °C for 72 h in air and its shear strength was 33.4 MPa due to the Ni_3Sn_4 grains coarsening. These results demonstrate that the less time-consuming ultrasonic-assisted soldering has wide potentials for high-temperature power electronic packaging.