Research On Preparation Of Organic-coated Ag/Cu Nanoparticles And Their Joining Of Cu Substrates

The preparation of organic coated Ag/Cu nanoparticles and their low temperature sintering joining is of great importance for the Pb-free package in photoelectric applications.The organic coated Ag nanoparticles with large and middle size as well as Cu nanoparticles have been synthesized with liquid chemical reduction method,while the silver stearate-coated coated Ag nanoparticles with small size have been produced with liquid chemical–thermal decomposition method.Then,low temperature sintering joining of Cu substrates has been realized.The interfacial structure of joints has been characterized.The sintering behavior of powders at low temperature and the joining mechanism have been discussed.The PVP-coated Ag nanoparticles with large size?the average size of about 100 nm?and middle size?the average size of about 20 nm?as well as Cu nanoparticles?the average size of about 60 nm?have been synthesized with liquid chemical reduction method.The CH₃?CH₂?₁₆COOAg-coated Ag nanoparticles with small size?the average size of about 4.4nm?have been produced with liquid chemical–thermal decomposition method.In the case of preparation of Ag nanoparticles with small size,the concentration of AgNO₃ has little effect on the particle size of Ag powders;the type of organic acid has a certain influence on the size and distribution of Ag powders,and the thermal decomposition temperature has a great influence on the size and dispersibility of the nanoparticles.Low temperature joining of bare Cu and Ni/Ag-coated Cu base materials have been realized with the small-sized Ag paste.The shear strength of the joints was 20 MPa and 24MPa,respectively,indicating that coated Ni/Ag layers on the surface of bare copper is beneficial to the improvement of joint strength.The microstructure analysis of the joints shows that the small-size nano-Ag sintered layer is dense and the sound interfacial bonding is obtained,but a small amount of pores are observed at the interlayer and the interfaces.In the initial stage of sintering of Ag nanoparticles,the surface diffusion mainly takes place,while in the middle and later stages of sintering the diffusion of grain boundary and volume diffusion mainly occur.The shear strength of the joints was 29 MPa and 36 MPa,respectively,by using the composite Ag paste A?composed of medium-sized and small-sized nano Ag powders?and composite Ag paste B?composed of large-size,medium-size and small-size nano Ag powders?,showing that the composite Ag paste is beneficial to the joint strength.The microstructure characterization shows that many defects such as cracks and voids can be observed in the sintered Ag intermediate layer for composite Ag paste A.For the joint with the composite Ag paste B,the interfacial bonding is favorite,and the sintered Ag interlayer is also dense.The sintering behavior of composite Ag powders shows that the powders mainly tend to aggregate at low temperatures,and the grain growth occurs at the relatively high temperatures.The shear strength of the joint was 26 MPa for the composite Ag-Cu paste.Microstructure analysis shows that the interlayer consists mainly of Ag and a small amount of Cu.The distribution of Ag and Cu in the interlayer is relatively not uniform.