Effective Constitutive Relations for Sintered Nano Copper Joints

Abstract

Sintered copper nano particles are being considered as alternatives to solder and/or sintered silver in different applications. Like for the alternatives, interpretation of accelerated fatigue test results does however require modeling, typically involving prediction of stresses and strains vs. time and temperature based on constitutive relations. This poses a challenge as the inelastic deformation properties depend strongly on both the initial particles and details of the processing, i.e. unlike for solder general constitutive relations are not possible. The present work provides a mechanistic description of the early transient creep of relevance in cycling, including effects of sintering parameters and subsequent oxidation. Inelastic deformation is dominated by diffusion, rather than dislocation motion. Generalized constitutive relations are provided to the extent that quantitative modeling of a specific structure only requires the measurement of a single creep curve for that.

Copper-Based Nanomaterials for Fine-Pitch Interconnects in Microelectronics

“…Work done by Thekkut et al showed densified Cu-on-Cu bonds of nanoparticulate paste (Showa Denko Materials Co. Ltd., Tokyo, Japan/former Hitachi Chemical Co Ltd.) after 1 h of sintering at 300 °C in forming gas (FG) environment under 20 MPa pressure. 27,30 A similar study was also done by Liu et al whereby Cu-on-Cu bonds were formed by sintering of Cu nanoparticles that were pretreated with formic acid. 31 As seen in Figure 2, the sintered joints feature porosity due to the evaporation and partial evacuation of organic binding material during sintering, that leads to inferior mechanical properties.…”

“…In recent years, nanomaterials have increasingly gained interest as next generation interconnects in electronic packaging. Nanomaterials have shown promising potential as the next generation solder materials for interconnection below 20 μm in ultrafine pitch assembly. , Sintering of nano-Cu pillars or nanoporous (np) Cu caps have shown potential to realize a direct Cu–Cu bonding. , It is known that nanostructured metals have high surface area-to-volume ratio which significantly lowers their melting temperature in comparison with their bulk counterparts . This makes them particularly attractive for low-temperature sintering.…”

“…As seen in Figure , the sintered joints feature porosity due to the evaporation and partial evacuation of organic binding material during sintering, that leads to inferior mechanical properties. Overall, said binders are used in the pastes to hold together the otherwise non-self-supported nanoparticles, yet not allowing them to agglomerate below the sintering temperature of 300 °C. ,, …”

“…Another common disadvantage of nanoparticulate Cu pastes is the formation of copper oxides at low temperatures and low pressure. ,, Separately, work to attach Cu pillars to pads by sintering of a nanoparticulate Cu paste has also been ongoing in IBM. To solve the reliability issues associated with void formation in the solder interconnects, IBM researchers considered Cu–Cu bonding as an alternative interconnect material with applicability in most chips and PCBs. , The physical limitation of all-Cu interconnects is the high melting temperature of (1085 °C). This challenge is avoided by using Cu nanoparticles supported on the Cu pillars, followed by 1 h sintering at 400 °C in FG environment under high pressure on a Cu pad.…”

“…25,26 Sintering of nano-Cu pillars or nanoporous (np) Cu caps have shown potential to realize a direct Cu−Cu bonding. 27,28 It is known that nanostructured metals have high surface area-to-volume ratio which significantly lowers their melting temperature in comparison with their bulk counterparts. 29 This makes them particularly attractive for lowtemperature sintering.…”