| Electrically conductive adhesives as alternative of tin-lead solder with the advantages of environmental friendliness, mild processing conditions, fewer processing steps and fine pitch interconnect, have attracted people’s interesting. However, some issues still exist for the currently available ECAs, including lower electrical conductivity, unstable contact resistance and poor impact strength. To solve the emention-above problems, the relationship between curing conditions and bulk resistivity of isotropic conductive adhesives （ICAs） were first investigated, showing the dependence of resistivity on temperatures. Then, stablizing contact resistance of ICAs on non-noble metal surfaces was carried out by the addition of multi-functional additives; and improving the conductivity of ICAs was studied by the addition of multi-functional additives and sintering of silver nanoparticles （Ag NPs）. Effects of multi-functional additives on shear strength of ICAs were also investigated.Effects of different curing and post-heating treatment procedures on electrical properties of ICAs were investigated. The results showed that the bulk resistivity of ICAs cured by multi-step method was lower than that cured by the one-step. It was also found that the electrical resistance of ICAs continued to decrease during the following post-heating treatment processes. In addition,in-situ monitoring the variations in electrical resistance was studied during curing and post-heating treatment process, showing the cooling process continued to descend the electrical resistivity of ICAs. These results suggested that the internal stress of ICAs, which depended on the curing and post-heating treatment temperatures, had a significant effect on the electrical resistivity. Meanwhile, it was proved that the bulk resistivity of ICAs increased as the internal stress of ICAs weakened by the addition of PEG.Effects of two kinds of water-soluble silver salts （silver nitrate or silver acetate） on thermal behavior, bulk resistivity and contact resistance of ICAs on tin surfaces under aging condition were investigated. The DSC results showed silver salts did not influence curing behavior of ICAs. The bulk resistivity of ICAs with 2wt% silver nitrate or 1.5wt% silver acetate decreased 48.5% or 47.4% after damp heat aging 600h, respectively. But the control samples only decreased 27.3%. In addition, the contact resistance of ICAs with more than or equal to 1wt% silver nitrate or 0.5wt% silver acetate was stable on tin finish after damp heat age. XRD, XPS and SEM results indicated that under the aging condition, silver salts had plated on tin surfaces by in-situ replacement reaction, forming a compact silver layer on the interface between ICA/tin, which eliminated the electrochemical potential difference with silver fillers, and prevented the occurrence of galvanic corrosion. Thus, the stabilization of the contact resistance of ICAs was obtained.Different types of aminoaldehydes, N, N-dimethyl-4- aminobenzaldehyde （DABA） and formamide （FA） as multifunctional additives, were introduced into ICAs, the conductivity and contact resistance of ICAs were studied, and benzaldehyde （BA） as a comparison. The results showed that when FA, BA and DABA were individually added into ICAs, the maximum decrease of resistivity of ICAs were 34.8%, 67.2% and 41.7 % compared to the control sample, respectively. This is because, during curing process, aldehydes act as reducing agents and reduce the oxidized silver flakes. Furthermore, ICAs with FA or DABA had better contact resistance stability than that of the control sample under the condition of 85oC/85%RH, but ICAs with BA could not stabilize the contact resistance on tin finish, The results indicated that FA and DABA could be adsorbed onto tin surface and inhibited the occurrence of galvanic corrosion on the ICA/tin interface. The rheological results showed the processability of ICAs with FA, BA and DABA were better than that of the control sample. However, the shear strength of ECAs with aldehydes slightly declined. From the view of the comprehensive performance, ICAs with DABA are the best. Subsequently, The lower resistivity (7.5×10-5Ω·cm) of nano-isotropic conductive adhesives （nano-ICAs） with in-situ formed and sintered Ag NPs by DABA as a reducing agent was developed. At room temperature, no reaction between silver nitrate and DABA occurred. However, during curing process, silver nanoparticles were immediately generated through reducing silver nitrate by DABA in absence of stabilizing agents. At the same time, the increased viscosity of epoxy due to the cure could prevent the agglomerates of Ag NPs. Morphology studies showed that most Ag NPs have been attached onto the surfaces of silver flakes due to well affinity between them, resulting in more effectively interconnecting with silver flakes by sintered Ag NPs. Thus, the lower bulk resistivity was obtained. On the other hand, DABA, containing a tertiary amine, could stabilize contact resistance of nano-ICAs by effectively preventing galvanic corrosion at the interface between nano-ICAs and Sn surfaces due to the fact that amines can strongly bond to a Sn surface.For in situ preparation of nano-ICAs, silver-imidazole complexes were synthesized by silver acetate and 2-ethyl-4–methy limidazole 2E4MZ （or 2-ethyl-4-methyl-1H- imidazole- 1-propanenitrile 2E4MZ-CN）, and were charactered by NMR, EA, MS and XRD etc. The results showed that [Ag（2E4MZ）2]Ac complex was formed by the 1:2 ratio of silver acetate to 2E4MZ. Preparation of epoxy resin/silver nanocomposites were successfully carried out : the thermal decomposition of silver-imidazole complex generated Ag NPs and the released imidazole could cure the epoxy resin. XRD results showed well crystal structure of zero-valent silver, TEM results showed Ag NPs, the average sizes of which were in the range of 10-20nm, uniformly dispersed in epoxy matrix. The results of DSC, FTIR and 1H NMR proves that the released 2E4MZ from [Ag（2E4MZ）2]Ac can cure epoxy resin, however ,only the pyrrole-type N of [2E4MZ]+Ac- involve in the cure of epoxy resin. ICAs were prepared by epoxy resin/silver-imidazole complex with silver flakes, and the bulk resistivity of ICAs was as low as 5×10-5Ω·cm. SEM results showed Ag NPs sintered clearly among silver flakes, and formed the conducive paths by interconnecting silver flakes. |
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