Fatigue Testing of Copper Nanoparticle-Based Joints and Bonds

Abstract

Fused or sintered Cu nanoparticle structures are potential alternatives to solder for ultra-fine pitch flip chip assembly and to sintered Ag for heat sink attach in high temperature microelectronics. Meaningful testing and interpretation of test results in terms of what to expect under realistic use conditions do, however, require a mechanistic picture of degradation and damage mechanisms. As far as fatigue goes, such a picture is starting to emerge. The porosity of sintered nano-particle structures significantly affects their behavior in cycling. The very different sensitivities to parameters, compared to solder, means new protocols will be required for the assessment of reliability. The present study focused on fatigue in both isothermal and thermal cycling. During the latter, all damage occurs at the low temperature extreme, so life is particularly sensitive to the minimum temperature and any dwell there. Variations in the maximum temperature up to 125 °C did not affect, but a maximum temperature of 200 °C led to much faster damage. Depending on particle size and sintering conditions deformation and damage properties may also degrade rapidly over time. Our picture allows for recommendations as to more relevant test protocols for vibration, thermal cycling, and combinations of these, including effects of aging, as well as for generalization of test results and comparisons in terms of anticipated behavior under realistic long-term use conditions. Also, the fatigue life seems to vary with the ultimate strength, meaning that simple strength testing becomes a convenient reference in materials and process optimization.

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.…”

“…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. ,, …”

“…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. 24,27,30 Another common disadvantage of nanoparticulate Cu pastes is the formation of copper oxides at low temperatures and low pressure. 24,25,30 Separately, work to attach Cu pillars to pads by sintering of a nanoparticulate Cu paste has also been ongoing in IBM.…”

“…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).…”

A study on the thermomechanical response of various die attach metallic materials of power electronics

“…In addition to TLP and silver sintering die attachments, sintering of nano copper (nano Cu) has been recently considered as a promising low-temperature bonding technology (Sivasubramony et al, 2022;Zürcher et al, 2015). The sintering process of nano copper particles is somewhat similar to the silver sintering procedure (Hu et al, 2023).…”