In the era of high power electronics, voices asking for high performance miniaturized electronic devices are full of our minds. One of the keys in responding to these strong needs is the use of highly electrical conductive die bonding materials. Those with a strong ability to conduct heat generated during applications are of special interest as overheating of devices is avoided. Currently, tin-lead solders and electrically conductive adhesives are two commonly used die bonding materials in interconnecting components in circuit board. Although solder has a relatively higher conductivity, it has adverse environmental impacts upon disposal, and stringent regulations from European Union Waste Electrical and Electronic Equipment Directive require it to be progressively faded out. Developing...[ Read more ]

In the era of high power electronics, voices asking for high performance miniaturized electronic devices are full of our minds. One of the keys in responding to these strong needs is the use of highly electrical conductive die bonding materials. Those with a strong ability to conduct heat generated during applications are of special interest as overheating of devices is avoided. Currently, tin-lead solders and electrically conductive adhesives are two commonly used die bonding materials in interconnecting components in circuit board. Although solder has a relatively higher conductivity, it has adverse environmental impacts upon disposal, and stringent regulations from European Union Waste Electrical and Electronic Equipment Directive require it to be progressively faded out. Developing high performance die bonding alternatives is an urge and is of great importance.

Conductive adhesives especially silver epoxy has been used for decades as die bonding materials since its first application in 1951. It is considered as one of the best ways to replace the leading role of solder in electronic packaging industry due to its ability to devices miniaturization. Its wide applications are limited by its comparably lower conductivities. Enhancement of silver epoxy by incorporation of conductive nano fillers such as carbon nanotubes (CNTs) and nano-silver particles has been found promising recently. However, the addition of nano particles leads to drastic increase in the viscosity of the adhesive or decrease in processability. To ensure an effective use of these nano fillers without changing the process abilities during practical applications, understanding of the microrheological properties of the nano filler/silver epoxy adhesives becomes paramount.

In this study, we systematically investigated the effect of matrix viscoelasticity on CNTs dispersion using optical microscopy (OM), transmission electron microscopy (TEM) and dynamic rheometry. We discovered that the best dispersion system is a ternary mixture of silver epoxy containing nano Ag@CNTs and acid treated CNTs. This is the first report so far which has not been reported in literature before. Systematic investigations were also carried out on the effect of CNTs concentration and CNTs structure on the rheological properties of silver epoxy composites. The CNT structure effects refer to as-received CNTs, nano Ag@CNTs and acid treated CNTs, respectively. There is a clear correlation between flow-induced CNTs aggregation and steady shear viscosity of the composite. The ternary systems exhibit highest critical concentration for CNTs aggregation. Up to 7wt% of CNTs can be homogeneously dispersed inside the silver epoxy composite without any notable formation of aggregates. The highest concentration of homogenous dispersion of CNTs in literatures is less than 2 wt %.