Reinforcement of Ag nanoparticle paste with nanowires for low temperature pressureless bonding

Abstract

Low temperature interconnection processes for lead-free packaging and flexible electronics are currently of great interest. Several studies have focused on bonding using silver nanoparticles (Ag NPs) or copper nanoparticles (Cu NPs). However, pressure assistance is generally necessary for bonding with nanomaterial pastes, which limits its industrial applications. Here, a unique method for bonding of copper wires using Ag NP and nanowire binary pastes is examined, in which joining is accomplished from 60 to 200°C and yet without the application of pressure. Bonding is facilitated by solid state sintering of Ag nanomaterials and metallic bonding between Cu and Ag interfaces. The effects of different additions of Ag nanowires in bonded joints are studied, in which addition of 20 vol% Ag nanowires improves bonding strength after low temperature sintering by 50-80 % compared with Ag nanoparticle paste. A mechanical reinforcement effect due to introduction of Ag nanowires has been confirmed by observation of the fracture path propagation, where necking, breakage and pullout of nanowires occur on loading. This low temperature pressureless bonding technology has the potential for wide use for interconnection in lead-free microcircuits and flexible electronic packaging.

Ag Nanowire and Nanoplate Composite Paste for Low Temperature Bonding

“…The regions reveal the role of the nanowire in the fracture of the bonding material because the primary crack cannot be readily characterized after failure. 19) Crack propagation is suppressed by bridging of the nanowires as shown in Fig. 5(a), (b) in a similar way to Ag NP/NW paste bonding at 150°C.…”

“…2(c) and (d) show the top view and crosssectional view of nanocomposite paste. Interestingly, different from the isotropy of Ag NP/NW paste, 19) this Ag NPL/ NW paste shows an anisotropic stacked structure, in which Ag nanoplates act as matrix while Ag nanowires being uniform dispersed in Ag nanoplates serving as fillers.…”

“…3(b)(d). The sintering between Ag nanowire/nanoplate at room temperature is probable because the thickness of Ag nanoplates is 520 nm, which is only ³10%20% of the Ag nanoparticles reported in Peng et al 14,19) This dimensional decrease enhances the Ag atom diffusion between Ag nanoplate edges and Ag nanowire sides and further enables sintering. The sintered Ag nanoplates and nanoplates lead to the neck growth and direct metallic bonding.…”

“…12) This can probably be due to the tendency of nanowires to form reticular structures to a lesser extent than nanoparticles. 19) Ongoing efforts focus on further improving the bonding strength using composite materials, similar to a ferroconcrete structure material while having a low processing temperature and a low bonding process pressure.…”

“…19) Although a maximum bonding strength of 16.8 MPa has been achieved at 200°C, it is still relatively low (3 MPa at 60°C) with a processing temperature below 100°C. 19) Obviously, a further improvement of bonding strength is desired.…”