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Wong,
C. P.
Wong,
C. P.
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ItemEnhanced Electrical Properties of Anisotropic Conductive Adhesive With $pi$ -Conjugated Self-Assembled Molecular Wire Junctions(Georgia Institute of Technology, 2009-09) Zhang, Rongwei ; Li, Yi ; Yim, Myung Jin ; Moon, Kyoung-Sik ; Lu, Daoqiang ; Wong, C. P.We have investigated the electrical properties of anisotropic conductive adhesive (ACA) joint using submicrometer- sized ( 500 nm in diameter) silver (Ag) particle as conductive filler with the effect of -conjugated self-assembled molecular wires. The ACAs with submicrometer-sized Ag particles have higher current carrying capability ( 3400 mA) than those with micro-sized Au-coated polymer particles ( 2000 mA) and Ag nanoparticles ( 2500 mA). More importantly, by construction of -conjugated self-assembled molecular wire junctions between conductive particles and integrated circuit (IC)/substrate, the electrical conductivity has increased by one order of magnitude and the current carrying capability of ACAs has improved by 600 mA. The crucial factors that govern the improved electrical properties are discussed based on the study of alignments and thermal stability of molecules on the submicrometer-sized Ag particle surface with surface-enhanced Raman spectroscopy (SERS), providing a fundamental understanding of conduction mechanism in ACA joints and guidelines for the formulation of high-performance ACAs in electronic packaging industry.
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ItemDevelopment of Conductive Adhesives for Solder Replacement(Georgia Institute of Technology, 2000-12) Wong, C. P. ; Lu, DaoqiangWith the phasing out of lead-bearing solders, electrically conductive adhesives (ECAs) have been identified as an environmentally friendly alternative to tin/lead (Sn/Pb) solders in electronics packaging applications. Compared to Sn/Pb solders, conductive adhesive technology offers numerous advantages. However, this new technology still has reliability limitations. Two critical limitations are unstable contact resistance on non- noble metals and poor impact performance. Our previous study proved that galvanic corrosion is the dominant mechanism for the unstable contact resistance during elevated temperature and humidity aging. The ultimate goal of this study is to develop conductive adhesives with stable contact resistance and desirable impact performance. In this study, 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 based on this study. 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 , 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 rubbermodified 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.
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ItemA Study of Contact Resistance of Conductive Adhesives Based on Anhydride-Cured Epoxy Systems(Georgia Institute of Technology, 2000-09) Wong, C. P. ; Lu, DaoqiangElectrically conductive adhesives (ECAs) are an environmentally friendly alternative to tin/lead (Sn/Pb) solders in electronics packaging applications. However, current conductive technology is still in its infancy and limitations do exist. One of the critical reliability issues is that contact resistance of silver flake-filled ECAs on nonnoble metals increases in elevated temperature and humidity environments. The main objective of this study is to investigate the contact resistance behaviors of a class of conductive adhesives, which are based on anhydride-cured epoxy systems. Curing profiles, moisture pickup, and shifts of contact resistance of the ECAs on a nonnoble metal, tin/lead (Sn/Pb), during aging are investigated. Also, two corrosion inhibitors are employed to stabilize the contact resistance. The effects of these corrosion inhibitors on contact resistance are compared. It is found that: 1) this class of ECAs shows low moisture absorption, 2) the contact resistance of the ECAs on Sn/Pb decreases first and then increases slowly during 85 C/85% relative humidity (RH) aging, 3) one of corrosion inhibitors is very effective to stabilize contact resistance of these ECAs on Sn/Pb, and 4) the corrosion inhibitor stabilizes contact resistance through adsorption on Sn/Pb surfaces. From this study, it can be concluded that ECAs based on anhydride cured epoxy systems are promising formulations for electronics packaging applications.
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ItemA Study of Lubricants on Silver Flakes for Microelectronics Conductive Adhesives(Georgia Institute of Technology, 1999-09) Wong, C. P. ; Lu, DaoqiangConductive adhesives are composites of polymer matrixes and metal fillers (conductive elements). Silver (Ag) flakes are widely used as fillers for electrically conductive adhesives (ECA’s). Generally, there is a thin layer of organic lubricant coated on the commercial Ag flake surface. This lubricant layer is needed for eliminating the Ag particle agglomeration while dispersing the Ag filler into the polymeric resin. Therefore the lubricant influences rheology, conductivity, and other properties of ECA’s. The nature of the lubricant on a Ag flake and the interaction between the lubricant and the Ag flake surface were studied by diffuse reflectance infrared spectroscopy (DRIR). Thermal decomposition of the lubricant was studied by differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA). In addition, the effects of some chemical compounds on lubricant removal and the enhancement of conductivity of the ECA were also investigated. It was found that 1) a chemical bonding was formed on the Ag flake surface between the lubricant and Ag; 2) the short chain acids replaced the long chain lubricants; 3) an ether and a poly(ethylene glycol) enhanced electrical conductivity by partially removing the Ag flake lubricants.
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ItemMechanisms Underlying the Unstable Contact Resistance of Conductive Adhesives(Georgia Institute of Technology, 1999-07) Wong, C. P. ; Lu, Daoqiang ; Tong, Quinn K.One critical obstacle of current conductive adhesives is their unstable contact resistance with nonnoble metal finished components during high temperature and humidity aging. It is commonly accepted that metal oxide formation at the interface between the conductive adhesive and the nonnoble metal surface is responsible for the contact resistance shift. Two different mechanisms, simple oxidation and galvanic corrosion, both can cause metal oxide formation, but no prior work has been conducted to confirm which mechanism is the dominant one. Therefore, this study is aimed at identifying the main mechanism for the metal oxide formation and the unstable contact resistance phenomenon of current conductive adhesives. A contact resistance test device, which consists of metal wire segments and conductive adhesive dots, is specially designed for this study. Adhesives and metal wires are carefully selected and experiments are systematically designed. Based on the results of this systematic study, galvanic corrosion has been identified as the underlying mechanism for the metal oxide formation and for the observed unstable contact resistance phenomenon of conductive adhesives.
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ItemConductivity Mechanisms of Isotropic Conductive Adhesives (ICA’s)(Georgia Institute of Technology, 1999-07) Wong, C. P. ; Lu, Daoqiang ; Tong, Quinn K.Isotropic conductive adhesives (ICA’s) 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 ℃) 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.