| Isotropic electrically conductive adhesive (ICA) is a new type of environmental friendly materials. As a supplement to traditional solders, ICAs have been widely used in microelectronic packaging such as IC packaging and LED encapsulation. Compared with traditional solders, the operation process is simpler and the bonding temperature is lower for ICAs. However, the currently available ICAs still have some drawbacks, such as relatively low thermal conductivity, high volume resistivity, not enough bonding strength, and instability for storage and transportation. In this paper, the optimization of the formula of ICAs, mechanism of the functional additives on improving the performance of ICAs, factors that affect the thermal conductivity of ICAs, and the application and reliability of ICAs were studied.Considering storage performance, shear strength, thermal conductivity and electrical conductivity after solidification, a new ICAs with dicyandiamide as curing agent were prepared. An optimized formulation with15.8wt.%of resin system,1.2wt.%of dicyandiamide curing agent,78wt.%of silver powder and5wt.%of solvent and other additives was obtained. This ICAs could be stored at room temperature for three months. After curing at170℃in60min, this ICAs exhibited a low volume resistivity of1.3×10-4Ω·cm, a high thermal conductivity of10.41W/(m-K) and a high shear strength of19.1MPa.The influence of the types and contents of metal fillers and functional additives, curing conditions and other factors on the properties, especially electrical and thermal conductivity of ICAs were investigated. It was found that the thermal conductivity and electrical conductivity increased with silver content increasing. The electrical conductive percolation threshold of ICAs was found to be75wt.%of silver while no percolation phenomenon was related to thermal conductivity. SEM-EDS analysis revealed that addition of low melting point SnBi alloy into ICAs resulted in the formation of metallurgical connection among silver powders, thus enhanced the thermal conductivity and electrical conductivity of ICAs. It was also found that the addition of the adipic acid could significantly improve the properties of ICAs. When adipic acid’s content was0.3wt.%, the bulk resistivity and thermal conductivity reached the best values. The effects of the curing process on thermal properties of ICAs were also studied. It was observed that:1) with the same curing time, the higher the curing temperature, the better the thermal conductivity of ICAs;2) when the curing temperature was higher than Glass transition temperature of the epoxy resin, the longer the curing time, the better the thermal conductivity.The influence of adipic acid as functional additives on the properties of ICAs, as well as the mechanism of the interaction of adipic acid with silver powder and resin system were studied by using all kinds of analytical tools. The DSC-TG curves, Laman spectra and XPS spectra of silvers before and after treated with ethanol and adipic acid in ethanol were recorded and the results showed that adipic acid could effectively remove and replace the non-conductive lubricant absorbed on the surface of the silver. Comparing the DSC curves and infrared spectra of dicyandiamide, adipic acid and the reaction mixture of them, it was found that an amidation reaction occurred between the adipic acid and the dicyandiamide. Mechanism study above revealed that two reactions occurred during the curing process contribute to the performance enhancement:one is one of the acidic functional groups in adipic acid reacted with the silver, which effectively removes and replaces the non-conductive lubricant and thus improves the conductivity of ICAs; the other is an amidation reaction occurred between the other acidic functional group in adipic acid and the dicyandiamide, adipic acid acting as a bridge and as a result improves the binding strength between the silver and the resin and ultimately enhances the mechanical properties of ICAs.Research has been conducted on the reliability of the ICAs, the chip and LED modules encapsulated by the ICAs. Results showed that the volume resistivity, contact resistance, and shear strength of the self-developed ICAs after aging for500h under85℃/85%RH conditions were changed by8.2%,18.5%and5.8%, respectively. The luminous flux of the self-developed ICAs encapsulated LED modules changed by29%,11%and15%after aging for6000h at high temperature and high humidity, room temperature and high temperature conditions, respectively. For comparison, America-made ICAs encapsulated LED modules were aged at same conditions, and their luminous flux changed by42%,23%and17%, respectively. This result indicated that the self-developed ICAs had better anti-aging performances.Due to its water absorptivity, both the mechanical properties and contact resistivity of the ICAs packaging chip module deteriorated when exposed to85℃/85%RH environment. Water invasion induced plasticization of the resin and thus decreased the mechanical properties of ICAs. In addition, water invasion resulted in the formation of a micro cell at the interface between the adhesive and the Sn/Cu substrate, which induced the electrochemical corrosion reaction between metals (Ag and Sn) with different electrochemical potentials. A layer of low conductive metallic oxide formed on the interface when the electrochemical corrosion reaction occured, resulting in an increase of the contact resistivity.
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