|During the recent two decades, considerable efforts have been made to explore new generations of interconnecting materials and printed lines to replace the traditionally used toxic lead-based solders in electronic industries. Electrical conductive adhesive (ECA) which consists of conductive metallic particles and a polymeric matrix has attracted a great deal of attention as one of the most promising alternative materials. The conventional ECAs are typically made of silver micro flakes and epoxy resin. The low electrical conductivity of these ECAs is their main drawback compared to traditional lead-based solders, which hinders their applicability in today’s blooming electronic industries. An enormous amount of research works have been conducted to enhance the electrical conductivity of the conventional ECAs, including increasing the polymer shrinkage, surface modification of silver flakes, introduction of low melting components to the ECA formulation, and the use of nano-sized conductive materials inside the formulation of the conventional ECAs. All of these approaches affect the quality of inter filler interaction inside the electrical network, in different ways.
The recent progress in nanotechnology helped material scientists to precisely design nanomaterials with different morphologies and surface chemistry. Owing to this capability, incorporation of nano-sized conductive fillers with different natures, morphologies, and surface properties inside the conventional formulation of ECAs has drawn considerable attention to overcome the common drawbacks of conventional ECAs, such as poor electrical and mechanical properties, reliability issues, and large filler content. It has been reported that the introduction of conductive nanomaterials into the conventional ECAs can improve the electrical conductivity of ECAs if their size, morphology, and the ratio between nanofiller and silver flakes is carefully taken under consideration. In this project, we developed new generations of hybrid electrical conductive adhesives (ECAs) by introducing conductive nanofillers (spherical silver nanoparticles (Ag NPs), high aspect-ratio silver nanobelts (Ag NBs), and graphene) into the conventional formulation of ECAs. To harness the characteristic properties of the nanofillers and to facilitate their homogeneous dispersion inside the epoxy, nanofillers were functionalized.
In the first step of this project, spherical Ag NPs were synthesized and simultaneously functionalized with thiocarboxilic acids, resulting in the formation of NPs less than 5 nm. Two thiocarboxylic acids with the same chemical structure but different chain lengths (3 and 11 carbons) were used to functionalize the NPs. We showed that the size and the electrical properties of the NPs can be controlled by varying the chain length of their covering organic layer. The diameter of the Ag NPs functionalized with the short-chain acid was two times smaller than those with the long-chain acid. We also found that the short-chain functionalized NPs were electrically conductive while the long-chain functionalized ones were nonconductive. The short-chain functionalized NPs were incorporated into the conventional ECAs. We found that at low NPs contents (< 20 wt %) the electrical conductivity of the hybrid ECAs increased due to the filling of NPs into the interstices of the micron-sized silver flakes, bridging of the NPs among separated flakes, and sintering of the NPs at relatively low curing temperature of 150 °C. However, higher NPs contents reduced the electrical conductivity because they may cluster and increase the gaps between the silver flakes. Furthermore, at higher NPs content, the number of contact points increases, which in turn decreases the electrical conductivity of the final ECAs.
The positive effect of the synthesized NPs on the electrical conductivity of the nanocomposite is basically attributed to the increased number of electrical pathways inside the electrical network due to the bridging of the NPs between separated silver flakes. However, a large amount of NPs are needed to form effective bridges inside the network, which increases the number of contact points inside the filler system and also increases the cost of the final ECAs. In the second step, we implement a novel type of high aspect-ratio silver nanostructure, silver nanobelts (Ag NBs), as co-filler inside the conventional formulation of the ECAs. The Ag NBs (10-40 nm thick, 100-400 nm wide and 1-10 µm long) were synthesized through self-assembly and room-temperature joining of hexagonal and triangular silver unit blocks which were synthesized by chemical reduction of silver nitride in the presence of poly(methacrylic acid). The incorporation of a small amount of the Ag NBs (2 wt%, NBs to flakes weight-ratio, K = 0.03) into a conventional ECA with 60 wt% silver flakes resulted in an electrical conductivity enhancement of 1550% in comparison to that of the conventional ECAs with the same total silver weight fraction, while addition of 2 wt% (K = 0.03) NBs into the conventional ECA with 80 wt% silver flakes enhanced the electrical conductivity of the hybrid ECA approximately 240%. These results imply high aspect-ratio NBs are more effective to improve the electrical conductivity of ECAs at concentrations close to percolation threshold.
Considering the importance of the aspect-ratio of the nanofillers, in the next step, we implemented graphene, which is known for its exceptional electrical, mechanical and thermal properties, to further reduce the amount of silver flakes while maintaining a high electrical conductivity. Graphene, possessing the highest aspect-ratio among all the nanostructures and also due to its 2D structure, can provide extremely high surface area for electron transformation inside the electrical network. However, to exploit the interesting properties of the graphene, their single layer structure must be preserved inside the polymeric matrix. To achieve this goal, we applied two types of surface modification to exfoliate and stabilize graphene layers. First, we decorated graphene surface with Ag NPs, functionalized with a short chain length thiocarboxylic acid, and introduced this 2D nanostructure into the conventional ECAs. The electrical conductivity measurements revealed that the decorated graphene significantly improves the electrical conductivity of the conventional ECAs only at low filler concentrations, while to achieve high electrical conductivity, elevated curing temperatures are needed. This situation is a result of the increased number of contact points because of Ag NPs on graphene surface. Second, we used a non-covalent approach to stabilize graphene using the surfactant; sodium dodecyl sulfate (SDS). Our results showed that the stabilization of graphene with SDS noticeably enhance the electrical conductivity of the ECAs, which is attributed to the role of SDS in exploiting the high aspect-ratio of graphene. In order to examine this hypothesis, we used a larger size graphene and applied the same SDS modification protocol. The electrical resistivity measurements showed that the electrical conductivity enhancement in the case of hybrid ECAs with large SDS–modified graphene was more significant than that with small SDS–modified graphene. The percolation threshold for the hybrid ECA with 1.5 wt% of both large and small graphene was reduced to an interestingly low value of 10 wt% while this value for conventional ECAs, and hybrid ECAs with non-modified graphenes was 40 wt%. Furthermore, adding 1.5 wt% of large SDS-modified graphene into the conventional ECA with 80 wt% silver flake content resulted in a very low electrical resistivity of 1.6 × 10-5 Ω.cm which is lower than that of eutectic lead-based solders.