| As the requirements for interconnection and joining materials in microelectronics packaging applications have moved toward ultra-fine-pitch bonding and environmental friendliness, anisotropic conductive films (ACFs) have attracted considerable attention as an alternative to tin/lead (Sn/Pb) solders. ACFs not only allow products to be thinner, smaller and lighter, but also offer good electrical performance and stability. ACFs mainly consist of polymeric binder matrices and micrometre-scale conductive fillers. Only with applied heat and pressure during bonding process, the electric connections will be established in the Z-direction by a relatively low loading of conductive fillers. The cured polymer adhesive holds the two components together and provides pressure to maintain the interconnection area between bumps/pads and conductive fillers.Polymer/metal conductive microspheres are a kind of composite materials with a core/shell structure. Owing to the combination of round-shape, monodispersion, elasticity and lightweight of polymer microspheres and the excellent conductivity of metal, polymer/metal composite microspheres have been widely added into insulating adhesive matrix to prepare ACFs. Monodispersed polystyrene (PS)/metal composite microspheres were prepared by dispersion polymerization of styrene followed by electroless plating. The influences on the formation mechanism and morphology of composite microspheres were investigated, and their electrical properties were also characterized. To improve the storage stability of adhesives at ambient, modified epoxy (EP) and cationic hexafluoroantimonate were used as matrix and latent curing agent, respectively, to develop a new epoxy based adhesive. Such adhesives displayed an excellent storage stability at ambient, good adhesive strength and fast curing speed at relatively low temperature.Monodispersed PS microspheres were prepared by dispersion polymerization with controlled diameters. The effects of polymerization time, monomer concentration, dispersing agent concentration, initiator concentration, initiator type, dispersion medium and polymerization temperature on the monodispersion and morphology of PS microspheres were investigated. The results revealed that the sensitive nucleation period played the most important role on the preparation of monodispersed PS microspheres. Better monodispersion and morphology of PS microspheres can be obtained by one-step procedure. The influences of monomer concentration, dispersion medium and polymerization temperature on the diameters of PS were stronger than that of dispersing agent concentration and initiator concentration. However, they determined the monodispersion of PS to a large extent. In a word, monodispersed PS microspheres with diameters of1～3μm can be obtained by dispersion polymerization with controlled polymerization conditions.Nickel (Ni) has good conductivity, corrosion resistance and low cost. PS/Ni conductive composite microspheres with Ni coating on PS microspheres were prepared by electroless plating technology. The effects of decoration with active centers on PS microspheres surface, the condition of electroless plating, reductant concentration, complexant concentration and the loading of PS on the dispersion, Ni coating state and the electrical properties of PS/Ni composites were discussed. Furthermore, to improve the surface roughness, subsidiary fracture of the plated layer and high density, lanthanum (La) was added into electroless Ni plating bath. With the La content ranging from0to50mg L-1, the average grain size of the Ni based alloy coating reduced from300nm to70nin, the average shell thickness of the composites reduced from240nm to95nm, and the density of the composites decreased gradually from4.068g cm-3to1.515g cm-3Copper (Cu) has high electrical, thermal conductivity, moderate hardness, low cost and low electromigration. The formation of Cu shell on PS surface was obtained by electroless plating technology. The effects of pH of plating solution, reductant concentration, mode of feeding reductant and complexing system on morphology. Cu coating state and dispersion of PS/Cu composites were investigated with the assistant of linear sweep voltammetry (LSV) method. Tailoring of density, shell thickness and conductivity can be achieved by varying the loading of PS microspheres in plating bath. In addition, the existence of2,2-dipyridyl or La2O3as a stabilizer had a different influence on the deposition rate of Cu by electrochemical measurements, and thus led to a various morphology of PS/Cu composites. A more compact copper shell on PS surface can be obtained in the presence of La2O3Silver (Ag) has excellent conductivity, flexibility and malleability’. A modified electroless silver-plating process was developed for the preparation of monodispersed PS/Ag composite microspheres. PS surfaces were decorated with colloidal platinum of approximately10nm in size, much smaller than that of clusters fabricated by a conventional activation method, which contains Sn and Pd species of70nm. The effects of temperature, time of plating and loading of PS in plating bath on morphology. Ag coating state, dispersion and conductivity of PS/Ag composites were investigated. The results showed that a dense, stable and uniform Ag nanoshell was formed on the surface of PS microspheres with the assistant of poly (vinylpyrrolidone).A new epoxy based adhesive was prepared by using composite resin of diglycidylether of bisphenol A (DGEBA) and dimmer acid modified DGEBA as matrix and2-methylimidazole or N-phenethylpyrazinium hexafluoroantimonate (PPH) as curing agent. The curing behavior, storage stability at ambient of adhesive and thermostability of cured products were studied. The results showed that the pot life of PPH/EP adhesive exceeded to60days at25～30℃, but the glass transition temperature (Tg) of its cured product was only100℃. To improve the thermostability of cured products, bisphenol A novalac epoxy was added into the adhesive matrix to prepare PPH/MEP system. The results showed that Tg of cured PPH/MEP product increased to120℃, and PPH/MEP system maintained an excellent storage stability at ambient.ACFs with good conductivity and peel strength were prepared, which electrically connect ITO glass and flex print circuit (FPC) with conducting patterns of500μm in pitch. PS/Ag composite microspheres with diameter of3μm and PPH/MEP system were used as conductive fillers and adhesive resin, respectively. According to the theoretical analysis, contact resistance can be strongly influenced by applied pressure, filler size and number of filler between electrodes. The experimental results showed that increasing the filler content, bonding pressure, temperature and time played positive role on the conductivity of ACF. However, the adhesive property of ACF largely depended on the curing degree of resin. The critical content of conductive fillers was35wt.%in the flex on glass (FOG) process using ACF. When the filler content was20wt.%, ACF possessed good conductivity and peel strength at the same time. The optimal bonding pressure, temperature and time for reliability of interconnection using ACF were0.3MPa,170℃and15s, respectively. |
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