Anisotropic conductive adhesives with enhanced thermal conductivity for flip chip applications

This paper presents the development of new anisotropic conductive adhesives (ACAs) with enhanced thermal conductivity for improved reliability of adhesive flip chip assembly under high current density condition. As the bump size in the flip chip assembly is reduced, the current density through the bump also increases. This increased current density causes new failure mechanisms, such as interface degradation due to intermetallic compound formation and adhesive swelling resulting from high current stressing. This process is found especially in high current density interconnection in which the high junction temperature enhances such failure mechanisms. Therefore, it is necessary for the ACA to become a thermal transfer medium that allows the board to act as a new heat sink for the flip chip package and improve the lifetime of the ACA flip chip joint. We developed the thermally conductive ACA of 0.63 W/m·K thermal conductivity by using a formulation incorporating the 5-μm Ni-filled and 0.2-μm SiC-filled epoxy-based binder system. The current carrying capability and the electrical reliability under the current stressing condition for the thermally conductive ACA flip chip joints were improved in comparison to conventional ACA. This improvement was attributed to the effective heat dissipation from Au stud bumps/ACA/PCB pad structure by the thermally conductive ACA.     

Conductivity enhancement of nano silver-filled conductive adhesives by particle surface functionalization

Five different types of surfactants were employed for nanoparticle functionalization and the effects of the surfactants on electrical properties of nano silver (Ag)-filled conductive adhesives were investigated. The Ag nanoparticles pretreated with the surfactants were incorporated into isotropic conductive adhesives (ICA) formulations as conductive fillers. By using the surfactants (S3, S4, and S5), the reduced resistivity of the nano Ag-filled adhesives could be achieved with 2×10⁻⁴ Θ-cm. The morphology studies showed that the low resistivity resulted from the sintering of nanoparticles.     

Onset of electrical conduction in isotropic conductive adhesives: a general theory

A new theory is introduced for the onset of electrical conduction in isotropic conductive adhesives, based on the observation that conduction is a result of the creation of conducting contacts in metal–insulator composite adhesives. The present theory resolves several prevalently contradicting issues including the onset dependency of electrical conduction on the volume fraction of filler particles, the particle size, the pressure effect, and the type of insulator matrix of an adhesive. The theory also predicts the condition for the occurrence of two percolation thresholds.     

Monolayer-protected silver nano-particle-based anisotropic conductive adhesives: Enhancement of electrical and thermal properties

Two types of self-assembled monolayers (SAMs), dicarboxylic acid and dithiol, were used to treat the silver nanoparticles. Thermogravimetric analyzer (TGA), differential scanning calorimeter (DSC), and contact angle results indicated that the SAMs were well coated on the silver nanoparticles and thermally stable below 150°C. By introducing the monolayer-coated silver nanoparticles into the anisotropic conductive adhesives (ACAs), the electrical properties and thermal conductivity of ACAs were significantly improved. The joint resistance of the ACA decreased from 10⁻³ Ohm to 10⁻⁵ Ohm with SAMs-coated silver fillers, and the current carrying capability of ACAs was also obviously improved. The improved electrical properties are due to the stronger bonding between nanofillers and the SAM coating materials; consequently, this improved the ACA interfacial properties. The enhanced interfacial properties with the SAM-protected nanofillers also attributed to the improved thermal conductivity of ACAs.     

导电胶粘剂的研究进展

随着微电子技术的发展以及环保意识的增强,作为Pb/Sn焊料替代物的导电胶粘剂,已成为当前研究的热点和重点。介绍了导电胶粘剂的导电机理及其组成、功能和分类。重点概述了近年来国内外导电胶粘剂的研究现状,并展望了未来导电胶粘剂的研究方向。     

Fracture behavior of isotropically conductive adhesives

The fracture behavior of several commercial, silver-filled epoxies was studied using a combination of fracture mechanics, surface science, and microscopy. Three-point bend tests revealed that the bulk fracture toughness of the silver-filled epoxies fell within a narrow range 1.1-1.3 MPa-m^0.5. Both electron and optical microscopy studies indicated that crack path deflection due to the silver-particles was the primary micromechanical deformation mechanism. Surprisingly, the interfacial fracture energies between the epoxies and a copper surface ranged from 50 to 900 J/m². Contact angle measurements on the cured epoxies indicated that some epoxy surfaces are more active than others. However, the correlation between thermodynamic work of adhesion and fracture energy is rather weak and suggests only a modest trend. In summary, although the use of contact angles/surface energies to predict adhesion is promising, much more effort is required to make it a reliable screening tool. Fortunately, the use of interfacial fracture mechanics can detect differences in adhesive strength, and should allow packaging engineers to select die attach adhesives with improved adhesion     

无铅银浆烧结工艺与导电性能研究

制备了无铅低温玻璃粉,将其与银粉和有机载体混合配制成无铅导电银浆并烧结。通过SEM和EDX观察浆料烧结银膜的形貌并进行成分分析,用四探针测试仪测量烧结银膜的电阻率,讨论了浆料成分配比、烧结时间、烧结温度等方面对银膜导电性能的影响。确定了无铅导电银浆的最佳配比为:质量分数w(银粉)72%,w(玻璃粉)3%和w(有机载体)25%,最佳烧结温度为580℃,最佳保温时间为5min。     

Development of conductive adhesives for solder replacement

Develops conductive adhesives with stable contact resistance and desirable impact performance. Effects of purity of the resins and moisture absorption on contact resistance are investigated. Several different additives (oxygen scavengers and corrosion inhibitors) on contact resistance stability during elevated temperature and humidity aging are studied, and effective additives are identified. Then, several rubber-modified epoxy resins and two synthesized epoxide-terminated polyurethane resins are introduced into ECA formulations to determine their effects on impact strength. The loss factor, tan δ, of each formulation is measured using a dynamic mechanical analyzer (DMA) and impact strength is evaluated using the National Center for Manufacturing Science (NCMS) standard drop test procedure. Finally, high performance conductive adhesives are formulated by combining the modified resins and the effective additives. It is found that 1) purity of the resins and moisture absorption of the formulation affect the contact resistance stability of an ECA; 2) the oxygen scavengers and corrosion inhibitors can delay contact resistance shift; 3) one of the corrosion inhibitors is very effective in stabilizing the contact resistance; 4) some rubber-modified epoxy resins and the epoxide-terminated polyurethane resins can provide the conductive adhesives with superior impact performance; and 5) conductive adhesives with stable contact resistance and desirable impact performance are developed     

Effect of heating on the electrical resistivity of conductive adhesive and soldered joints

Thermal cycling from room temperature to 60°C was found to cause the contact resistivity of a silver-epoxy conductive adhesive joint to decrease irreversibly, due to an irreversible decrease of the thickness of the joint. This effect was much smaller for a soldered joint cycled to 40°C. An extended period of current on-off cycling caused a slight irreversible increase in the contact resistivity of the adhesive and soldered joints, but thermal cycling using a heater did not. Within each thermal cycle, the contact resistivity increased reversibly with increasing temperature, due to the increase in volume resistivity of the solder or adhesive. Temperature variation caused fractional changes in contact resistivity up to 48% and 6% for adhesive and soldered joints, respectively.     

Conductivity mechanisms of isotropic conductive adhesives (ICAs)

Isotropic conductive adhesives (ICAs) are usually composites of adhesive resins with conductive fillers (mainly silver flakes). The adhesive pastes before cure usually have low electrical conductivity. The conductive adhesives become highly conductive only after the adhesives are cured and solidified. The mechanisms of conductivity achievement in conductive adhesives were discussed. Experiments were carefully designed in order to determine the roles of adhesive shrinkage and silver (Ag) flake lubricant removal on adhesive conductivity achievement during cure. The conductivity establishment of the selected adhesive pastes and the cure shrinkage of the corresponding adhesive resins during cure were studied. Then conductivity developments of some metallic fillers and ICA pastes with external pressures were studied by using a specially designed test device. In addition, conductivity, resin cure shrinkage, and Ag flake lubricant behavior of an ICA which was cured at room temperature (25°C) were investigated. Based on the results, it was found that cure shrinkage of the resin, rather than lubricant removal, was the prerequisite for conductivity development of conductive adhesives. In addition, an explanation of how cure shrinkage could cause conductivity achievement of conductive adhesives during cure was proposed in this paper     

叠孔导通用导电胶研究进展

随着电子产品的小型化、高密度化以及人类环保意识的增强,导电胶取代传统的填孔电镀已成为国内外的研究热点。文章简述了叠孔填充用导电胶的组成及机理,总结了导电胶的优点及现存问题,并阐述了国内外研究现状。     

Spatial distribution of metal fillers in isotropically conductive adhesives

The dc electric conductance of isotropically conductive adhesive (ICA) was studied. Assuming completely random distribution of the metal fillers in the ICA, it was demonstrated that the percolation volume percentage of the metal fillers can be significantly reduced by adding nano-size fillers (nanofillers) into the system originally containing only micro-size fillers (microfillers). However experiments show that, due to the surface tensions, nanofillers tend to gather around microfillers as well as to form clusters themselves so the resistivity of the ICA increases following the increase of the nanofillers' volume percentage. In the present work, these cluster effects are investigated by simulating the detailed random walks of the nanofillers and microfillers in the system It is concluded that the cluster effects of the nanofillers deteriorate the electric conductivity of the ICA because the microfillers separate from each other so that it is more difficult to form the electrical conduction path in the ICA.     

Effect of elastic recovery on the electrical contact resistance in anisotropic conductive adhesive assemblies

The successful design of anisotropic conductive adhesive (ACA) assemblies depends mainly on the accurate prediction of their electrical contact resistance. Among the parameters that influence this resistance, the bonding force used to compress the conductive particles against the conductive tracks during the assembly process is very important. This paper investigates how the contact resistance changes as the bonding force is removed at the end of the assembly process when the epoxy resin used to bond the surfaces has cured. The final contact resistance is determined by examining, through theoretical, experimental and numerical analyzes, the evolution of the residual stress as the elastic recovery of the compressed conductive particles and tracks takes place when the bonding force is removed. An iterative algorithm derived from methods found in fracture mechanics analysis is used to determine the relationship between the contact resistance, the adhesive strength and the stiffness of the cured resin. It is shown that smaller values of adhesive strength yield higher contact resistance values; and similarly, smaller values of modulus of elasticity of the resin lead to higher contact resistance values.     

Effect of particle size on optical and electrical properties in mixed CdS and NiS nanoparticles synthesis by ultrasonic wave irradiation method

The mixed phase CdS and NiS nanoparticles are prepared by adopting ultrasonic wave irradiation method under different doping concentration of Ni in CdS. The well defined nano spheres are obtained during this synthesis process. The predicted particle sizes from X-ray diffraction (XRD) analysis are found to lie in the range between 37 and 49 nm. The effective doping of Ni lead to form the mixture of two phases such as CdS and NiS. The respective change due to the formation of mixture of CdS and NiS is reflected well in the band gap energy which is measured in Diffused Reflectance Spectra (DRS). It is predicted in the range of 2.41–2.23 eV respectively. Consistency of particle size with XRD are confirmed from Transmission Electron Microscope (TEM) images and also identified the presence of Nickel Sulfide and Cadmium Sulfide in nanostate with average particle size as 54 nm. The Energy Dispersive X-ray (EDAX) analysis confirmed the existence of Ni, Cd and S the doping levels. The optical absorption analysis of samples are performed in UV–vis range 400–600 nm. The synthesized samples are further characterized Fourier Transform Infrared (FT-IR) spectroscopy, Thermogravimetric (TGA) analysis, I-V characteristic and conductivity measurements.     

Effect of silver additive on electrical conductivity and methane sensitivity of SnO2

A composite powder of tin oxide (SnO₂) and silver (Ag) clusters was prepared by a simple and cost effective method of reducing their aqueous mixture with sodium borohydride (NaBH₄). Gas sensors based on the composite were made by powder pressing procedure and characterized by X-ray diffraction (XRD) and scanning electron microscopy (SEM). The electrical conductivity and gas sensing behavior of the sensors for methane (CH₄) gas were studied as a function of Ag concentration (0.3, 0.5, 0.8 and 1.5 wt%). The Ag additive is found to improve sensor response and widen its working temperature range with notable sensor response. The best sensor response was achieved by the sensor with 0.5 wt% Ag. The enhanced response was proved to be due to both electrical and chemical mechanisms.     

Effect of heat treatment on physical and electrical characteristics of conductive circuits printed on Si substrate

Printed electronics is expected to increase its market share significantly in near future on account of its high material usage efficiency, environmental friendliness, the avoidness of expensive process steps, and the ability to rapidly change circuit designs in real time in a highly cost effective fashion. Herein, we tried to examine the physical characteristics of the two types of selected Ag nanopastes and electrical characteristics of screen printed Ag patterns on a Si substrate sintered at 200 °C for varying sintering times. The first type of the paste had no binder material and the second one was formulated with an addition of a polymeric binder. They had different thermal behaviors, i.e., the paste with a binder showed a continuous small reaction well beyond the temperature for the main reaction of the paste without any binder. The microstructure of the Ag tracks after sintering was also different between the two cases, which verified the poor electrical characteristics of the paste containing a binder than the one without any binder. We also measured the insertion losses of the Ag tracks with a design for the high frequency transmission, and the observed electrical behaviors were discussed with the physical properties of the pastes measured in this study.     

Effect of deviations from stoichiometry on electrical conductivity and photoconductivity of CuInSe2 crystals

The results of studying the photoelectric properties of bulk CuInSe₂ crystals grown at various deviations from the stoichiometry are reported. The crystals were grown by the Bridgman method. The concentration and mobility of charge carriers were measured. The dependence of the properties of CuInSe₂ crystals with n-and p-type conductivities on the deviation of composition from the stoichiometry is discussed.