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2006 Progress Report: Environmentally Benign Lead-Free Electrically Conductive Adhesive for Electronic Packaging Manufacturing
EPA Grant Number: R831489Title: Environmentally Benign Lead-Free Electrically Conductive Adhesive for Electronic Packaging Manufacturing
Investigators: Wong, C. P.
Institution: Georgia Institute of Technology
EPA Project Officer: Bauer, Diana
Project Period: December 22, 2003 through December 21, 2008
Project Period Covered by this Report: December 22, 2005 through December 21, 2006
Project Amount: $325,000
RFA: Technology for a Sustainable Environment (2003)
Research Category: Pollution Prevention/Sustainable Development
Description:
Objective:Migration behaviors and electrical properties of the nano-scale electrically conductive adhesives (ECAs) for microelectronic packaging applications were investigated. Silver migration has long been one of the most critical issues in the semiconductor electronics industry, but no effective approaches have been developed to control silver migration and maintain its excellent electrical and thermal properties. Under our U.S. Environmental Protection Agency (EPA) project, we discovered a novel approach of using molecular self-assembled monolayers (SAMs) to dramatically reduce silver migration in the nano-Ag conductive adhesives. The protection of silver nano particles with molecular monolayers reduced the silver migration dramatically, and no migration was observed upon application of high voltages (up to 500 V) due to the formation of surface chelating compounds between the SAM and nano silver fillers. The migration behavior of SAM passivated nano-Ag conductive adhesives was investigated by analyzing the results with a migration model. In addition to a controlled migration, the SAM-passivated nano-Ag fillers also significantly enhanced the electrical conductivity and current carrying capability of anisotropic conductive adhesive (ACA) joints due to the improved interfacial properties and high current density of those molecular monolayers. Unlike typical ACA joints, which showed high joint resistance and limited current carrying capability, the joint resistance of the SAM-incorporated nano-Ag conductive adhesive could be achieved as low as 10-5 Ohm (the contact area is 100 × 100 μm2) and the maximum allowable current was higher than 3500 mA. Furthermore, the improved electrical performance of SAM-treated nano-Ag ACAs was also achieved with the increased thermal conductivity. As such, fine pitch, high performance, non-migration, and high reliability adhesives are developed for potential solder replacement in high voltage, high power device applications. The novel approach for silver migration control and electrical properties enhancement can also be applied in next generation high performance semiconductor devices to replace aluminum and copper metallization with the better performing silver.
Progress Summary:Introduction
Tin-lead solder alloys have been widely used in the electronics industry. They serve as interconnects that provide the conductive path required to achieve the connection from one circuit element to another. However, there are increasing concerns with the use of tin-lead alloy solders in recognition of the hazards of using lead, because lead, a major component in solder, has long been recognized as a health threat to human beings. In response to concerns for the environment and human health, some legislation and restrictions banning lead from electronic products have been introduced in many countries around the world. Japan has banned the use of lead in all their new electronic products in January of 2005, and the European Union plans to ban all imports of lead-containing electronics by July 2006 (RoHS Regulations (UK) government, 2005; Cannis, 2001). In the United States, legislation limiting the use of lead has been introduced in both the Senate and the House of Representatives. In response to the new legislation, most major electronics manufacturers have stepped up their search for alternatives to lead-containing solders.
The Principal Investigator (PI) has investigated the benefits of an ECA, an organic polymer doped with metal particles, as an alternative to tin-lead solder for circuit board production (Li, et al., 2005a; Li and Wong, 2006a; Moon, et al., 2003; Lu, et al., 1999; Li, et al., 2003; Moon, et al., 2005a; Li, et al., 2005b; Li, et al., 2006b; Moon, et al., 2005b). Polymer-based conductive adhesives have been widely used in LCD (liquid crystal display) and smart card applications as an interconnect material and in flip-chip assembly, CSP (chip scale package), and BGA (ball grid array) applications in replacement of solder due to the numerous advantages, such as environmental friendliness, mild processing conditions (enabling the use of heat-sensitive and low-cost components and substrates), fewer processing steps (reducing processing cost), low stress on the substrates, and fine pitch interconnect capability (enabling the miniaturization of electronic devices). Silver (Ag) filler is the most attractive choice among all the conductive fillers because of its excellent balanced properties and relatively low cost. Silver has the highest room temperature electrical and thermal conductivity among all metals. Furthermore, silver is also unique among all the cost-effective metals by nature of its conductive oxide (Ag2O). In addition, malleable silver particles are easily formed into different sizes (from a few nanometers to several microns) and shapes (such as spheres, flakes, rods, wires, disks, etc.) and are well dispersed in a variety of matrix materials. Therefore, silver is widely accepted as a promising material in the electronics industry, for uses such as conductive adhesives in electronic interconnects and high k (dielectric constant) composites in embedded passives (Rao and Wong, 2002). However, a major problem that impedes the wide application of silver, in particular for fine pitch application, is the electrochemical migration of silver in the presence of moisture and applied bias (Lin and Chan, 1996). In microelectronic devices, silver migration usually occurs between adjacent conductors/electrodes, which leads to the formation of dendrites growing from the cathode to the anode and finally accumulating with an electrical short-circuit failure (Boden, 1994; Rorgren and Liu, 1995). To avoid short-circuit, self-passivated aluminum (Al) has long been used in semiconductor devices, while copper (Cu) has been used in most electronic devices in recent years, due to the better electrical conductance. However, copper is easily oxidized, and a lot of efforts are needed to prevent the oxidation problem. Silver is the most promising metal for next-generation semiconductor devices because of its excellent electrical and thermal properties. By solving the migration issue for silver, it will have great potential to be used for next-generation high performance advanced semiconductor devices.
Metal migration is an electrochemical process, which requires chemical interaction between the surroundings and the metal (generating metal ions), a polar transport electrolyte in aqueous conditions through which ionic migration occurs, and an electric field. In fact, this situation prevails in most electronic packages where three conditions are present. When a potential is applied across the electrodes, a chemical reaction takes place at the positively biased electrode (anode) where positive metal ions are formed (Ag → Ag+ + e-). These ions, through ionic conduction, migrate toward the negatively charged electrode (cathode), where over time, they accumulate to form metallic dendrites (Ag+ + e-→ Ag). As the dendrite growth increases, a reduction of electrical spacing occurs. Eventually, the silver growth reaches the anode and creates a metallic bridge between the electrodes, resulting in an electrical short-circuit. In particular, in metal-polymer composites such as conductive adhesives or high k (dielectric constant) composites, polymers tend to absorb water and other ionic pollutants from the environment, such as H2S, HNO3, CO2, NO2, and tend to migrate between electrodes under the driving force of an electric field. As such, the insulation resistance of the polymer is reduced. Water not only can act as the solvent and vehicle for ionic transport, but also can participate in the conduction through electrolysis, especially under high voltages. Although other metals may also migrate under specific environments, silver has been the most prominent in terms of metal migration, mainly due to the high solubility of silver ions in water, the low activation energy for silver migration, the high tendency to form dendrites and the low propensity/possibility to form a stable passivation oxide layer such as aluminum (DiGiacomo, 1992; DiGiacomo, 1997; Manepalli, et al., 1999). Metal migration can be considered as a two-step process involving ionization and diffusion of the migrating species. DiGiacomo (1992) proposed a semi-empirical model of metal migration based on the electrochemistry of solutions, the theory of adsorption and condensation, and the transport through continuous water films and polymers characterized by the products of four functions (B [M, P], F [T], G [RH, T] and H [E, T]; see Equation 1). Since migration can be characterized by the current density, the model relates the current density to the four functions.
where B (M, P) is a function of materials (M) and process parameters (P); F (T) is the integrated form of the Arrhenius equation as a function of activation energy and temperature (T); G (RH, T) is the relative humidity (RH) function (which also depends on temperature T); and H (E, T) is the electric field (E) or voltage (V) function, which also depends on temperature. By considering different factors contributing to migration, an approximation relating these factors to the current density is shown as Equation 2 (DiGiacomo, 1992).
In the equation, Z = valence; F = Faraday’s electrochemical equivalent; D is the diffusivity of the migration component, D = D0exp (-ΔHD/kT), where ΔHD is activation energy of the diffusion component; C0 is the ionic concentration of the migration component; E = V/d is the electric field, where V is the applied voltage, d is the distance between electrodes; t is time; k and R are Boltzmann constant and molar gas constant, respectively.
There have been some efforts worldwide to control the silver migration, and the approaches include: (1) alloying the silver with an anodically stable metal such as palladium (Harsanyi and Ripka, 1985), platinum (Wassink, 1987), or tin (Shirai, et al., 2001); (2) using hydrophobic coating over the printed wiring board (PWB) to shield its surface from humidity and ionic contamination (Der Marderosian, 1978), since water and contaminates can act as a transport medium (electrolyte) and increase the rate of migration; (3) plating of silver with metals such as tin, nickel, or gold to protect the silver fillers and reduce migration; (4) coating the substrate with polymer (Schonhorn and Sharpe, 1983); (5) applying benzotriazole (BTA) and its derivatives in the environment (Brusic, et al., 1995); and (6) employing siloxane epoxy polymers as diffusion barriers due to the excellent adhesion of siloxane epoxy polymers to conductive metals (Wang, et al., 2005). Although these approaches can more or less reduce the silver migration, they may need to compromise the electrical properties of silver or increase the cost and processing complexity.
In this study, a novel approach of using molecular SAM-passivated nano-Ag fillers is employed in conductive adhesives for high performance, fine pitch interconnects. The migration behavior is evaluated by measuring the leakage current-voltage (I-V) relationship and observing the morphology of the conductive adhesives. The different migration behaviors of monolayer passivated nano-Ag conductive adhesives are investigated by analyzing the results with DiGiacomo’s model. In addition, effects of SAMs on the electrical and thermal properties of nano-Ag conductive adhesives are also studied. The introduction of molecular monolayer treated nano-Ag fillers also significantly enhances the electrical conductivity and current carrying capability of ACA joints.
Experimental
Materials. Diglycidyl ether of bisphenol-F (DGEBF) EPON 862 epoxy resin and an anhydride-type hardener of methylhexa-hydrophthalic anhydride (MHHPA) were employed as matrix resin and crosslinker, respectively. The ratio of epoxy to hardener was 1:0.85 based on the epoxide equivalent weight (EEW) of the epoxy resin and the hydroxyl equivalent weight (HEW) of the hardener. The catalyst is 1-cyanoethyl-2-ethyl-4-ethylimidazole (2E4MZCN), and the concentration of the catalyst was 1 part per 100 parts resin (phr). The silver nanoparticles, synthesized by combustion chemical vapor deposition (CCVD), were used as fillers, and the TEM picture of the nano-silver particles is shown in Figure 1.
Figure 1. TEM Picture of the Nano-Silver Fillers Used in This Study
The loading levels of silver nano fillers in the composite are 15 vol% and 5 vol% for migration and electrical properties tests, respectively. Di-carboxylic acid, mono-carboxylic acid and dithiol were incorporated in the nano-Ag conductive adhesives, respectively.
Characterizations.
Migration Study of Nano-Ag Conductive Adhesives With SAMs. The formulated nano-Ag conductive adhesives were stencil printed on the patterned FR-4 (Flame Resistant 4) printed circuit boards with a spacing of 1.5 mm between electrodes (Figure 2). The samples were cured at 150°C for 1 hour. A drop of water was placed between the electrodes. The leakage I-V relationship of cured nano-Ag conductive adhesives was measured with an LCR multimeter (Keithley 6517A). The morphology of silver dendrites on the test boards in the vicinity of nano-Ag conductive adhesives after high voltage tests was observed with a microscope.
Figure 2. Schematic Illustration of Migration Test Vehicle for Nano-Ag Conductive Adhesives
Weight loss of untreated and SAM-treated nano-Ag particles during heating was monitored using a thermogravimetric analyzer (TGA) from TA Instruments, model 2050. The temperature was raised from 25°C to 600°C at a heating rate of 10°C/minute in a nitrogen environment. The weight loss versus temperature of the TGA was recorded.
Electrical and Thermal Properties of Nano-Ag Conductive Adhesives With SAMs. The electrical resistance of the ACA joints (contact area: 100 x 100 μm2) was measured by a four-point probe method. The applied four-point probe currents were varied from 500mA to approximately 4000mA by a power supplier (HP model 6553A) and the voltage of the interconnect joints was measured by a Keithley 2000 multimeter.
The thermal conductivity of ACAs was studied by using an LFA 447 NanoFlash apparatus. The cured ACA samples were cut into a square of 10 x 10 mm with a thickness of approximately 1.0 mm. A Xenon flash lamp fired a pulse at the sample’s lower surface, while the infrared detector measured the temperature rise of the sample’s top surface. A computer software program then determined the sample’s thermal diffusivity (α). Thermal conductivity (k) of the sample was obtained from Equation 3, where Cp was specific heat measured by differential scanning calorimetry (DSC), and ρ was the sample density.
Results and Discussion
Migration Behaviors of Nano-Ag Conductive Adhesives at Low Voltages. In order to study the migration behavior of nano-Ag conductive adhesives, the leakage I-V relationship was evaluated at 0–5 V, and the results are shown in Figure 3. For all the nano-Ag conductive adhesive samples, an obvious threshold voltage for migration was observed at 0.5 V. Below the threshold voltage, no migration occurred, and the leakage currents were stabilized at a near-zero value. However, when the voltage increased over 0.5 V, the leakage current occurred. The untreated nano-Ag conductive adhesives showed a dramatic increase in leakage current with increasing voltage due to the high tendency for silver migration. On the other hand, the nano-Ag conductive adhesives incorporated with SAMs (carboxylic acids) showed a much slower increase in leakage current. This observation indicated that carboxylic acids could reduce the silver migration in the nano-Ag conductive adhesives. In comparison of two types of carboxylic acids, di-acid showed a more significant improvement than mono-acid. The effects of carboxylic acids on Ag migration control could be attributed to the high affinity of the carboxylic anion toward the Ag ion, which forms a strong and stable Ag complex at the interfaces. The stronger and more carboxylic acid functional group, the stronger the complex formation, and hence, Ag migration was reduced.
Figure 3. Leakage I-V Relationship of Nano-Ag Conductive Adhesives at 0–5 V
Characterization of SAM-treated Nano-Ag Fillers. To characterize the SAM compound’s coating on the nano-Ag surfaces, the weight changes of untreated and different SAM-treated nano-Ag particles was measured by a TGA, and the result is shown in Figure 4. For untreated nano-Ag particles, there was no obvious weight loss with increasing temperatures, indicating that there were no organic compounds (i.e., SAMs) present on the untreated nano-Ag particles. For di-acid and mono-acid treated nano-Ag fillers, an obvious weight loss of approximately 18 wt% was observed at 250°C and 300°C, respectively. The dramatic weight loss of SAM-treated nano-Ag particles suggested that SAM organic compounds were well coated on the nano-Ag particles due to the strong affinity of carboxylic groups toward Ag particles. As such, the nano-Ag fillers are protected by the molecular SAMs.
Figure 4. Weight Changes of Untreated and SAM-Treated Nano-Ag Particles
The obvious stabilized leakage current and subsequently the well controlled electrochemical migration are due to the protection of silver ions with carboxylic acids, which form the strong and stable chelating compounds. The adsorption of carboxylic acids on silver has been studied (Ulman, 1996; Moskovits and Suh, 1985; Samant, et al., 1993; Joo, et al., 2000; Tao, 1993). The reaction is considered an acid-base reaction, and the driving force is the formation of a chelating bond/complex between the carboxylate anion and a surface silver ion (Equation 4).
It has been reported that on Ag surfaces, the two oxygen atoms in carboxylate tend to delocalize the negative charge and bind to the Ag surface symmetrically. The bonding can change the surface properties of silver and control the properties of the metal-organic interfaces. Although the adsorption and alignment configurations of molecular monolayers on metal surfaces have been studied for over a decade, there were no reports on the applications of those SAMs until recently. SAMs such as thiolates and carboxylic acids are used to control the interface chemistry as well as electrical and adhesion properties (Cho, et al., 2005; Parthasarathy, et al., 1996; Li, et al., 2005c; Hirose, et al., 1996; Piva, et al., 2005; Li, et al., 2004; de Boer, et al., 2005). Studies have been conducted by surface analyses approaches such as contact angle, X-ray photoelectron spectroscopy (XPS), infrared (IR) characterization, and surface-enhanced Raman scattering (SERS) for the understanding of bonding energy and alignment configurations between various carboxylic acids and silver. The accepted conformations of mono- and di-carboxylic acid adsorption on silver surfaces are shown in Figure 5. To reduce the energy, the molecules of mono-carboxylic acids tend to have an all-trans conformation on bonding to the silver surface (Figure 5a). By adopting the all-trans forms, the molecule may lay close to the surface with a hydrophobic tail (CH3), protecting the silver clusters. For di-carboxylic acids, it was found that both carboxylate groups were chelating to silver surface sites. The molecule gains sufficient stability by bonding two carboxylate groups to the surface that it is able to adopt a less favorable chain conformation. The polymethylene chain from the di-acid was therefore arranged in the most favorable conformation to accomplish both carboxylate groups chelating. The gauche or gauche-like conformations were most possible in order to stand on the surface of its two carboxylic groups due to the repulsion between methylene groups and the particle surfaces (Figure 5b). With the incorporation of carboxylic acid and the subsequent interaction between carboxylic acids and silver nano fillers, the interface properties of the nano-composites could be modified. Unlike typical nano-Ag composites in which Ag+ is the major migration species, for the carboxylic acid-incorporated nano-Ag composite, the major diffusion component becomes the Ag+…COO- complex. The Ag+…COO- complex has a lower solubility in water and higher activation energy (lower driving force) for migration towards the cathode than that of free Ag+, due to the neutral charge of the Ag+…COO- complex. The lower solubility decreased the C0 value, while the higher activation energy decreased the D value in Equation 2. As such, migration became a kinetically unfavorable reaction and the leakage current of carboxylic acid-incorporated nano-Ag conductive adhesives was much lower than that of untreated composites. Comparing mono- and di-acids, di-acid performed better in terms of migration control, due to more coverage on Ag surfaces (Figure 5). Consequently, it provided a better and more complete protection of silver metal from migration.
Figure 5. Alignment Configurations of Carboxylic Acids on Ag Particles: (a) mono-acid on Ag particle and (b) di-acid on Ag particle.
Migration Behaviors of Nano-Ag Conductive Adhesives at High Voltages. When the nano-Ag conductive adhesives were tested at higher voltages (0–500 V), the leakage current difference of conductive adhesives before and after SAM incorporation was more apparent (Figure 6). At high voltages, the untreated nano-Ag conductive adhesives showed a dramatically increased leakage current, in particular, at voltages higher than 200 V. For SAM-incorporated/passivated nano-Ag conductive adhesives, however, a much more stable leakage current value was observed and no migration behaviors were detected even at voltages up to 500 V, due to the formation of surface chelating compounds between SAMs and nano-silver fillers. Although di-acid showed better protection of nano-silver fillers at low voltages than mono-acid, the difference between the two types of SAMs at high voltages was not obvious. This is because at low voltage, most Ag+ concentrates in the vicinity of nano-Ag particles, and better coverage of di-acid than mono-acid, as shown in Figure 5, contributed to the better protection of silver from migration. At high voltages, however, Ag+ could be dispersed/separated from the particles and could exist in the water medium. The interaction between acid and Ag+ will not be limited by the coverage or configuration as shown in Figure 5. Therefore, the same amount of acids has a similar effect on the migration control.
Figure 6. Leakage I-V Relationship of Nano-Ag Conductive Adhesives at 0–500 V
Morphology of Silver Dendrites Formed From Ag Migration. The morphology of silver dendrites on the test boards in the vicinity of nano-Ag conductive adhesives after migration tests are shown in Figure 7. For the untreated nano-Ag conductive adhesives, obvious silver dendrites with several dendritic branches were observed after high voltage tests, indicating severe Ag migration upon application of high voltage. For the di-acid incorporated nano-Ag conductive adhesives, however, no obvious dendrites were detected. The dark area around the edge of the nano-Ag conductive adhesives is considered to be from the typical interdiffusion between different materials rather than the ionic migration. The mono-acid incorporated nano-Ag conductive adhesive also showed a dendrite formation, but with shorter dendrite length and fewer dendritic branches.
(a) Untreated Nano-Ag Conductive Adhesive Shows Obvious Dendrite Formation
(b) Di-Acid Incorporated Nano-Ag Conductive Adhesive Shows No Obvious Dendrite Formation
(c) Mono-Acid Incorporated Nano-Ag Conductive Adhesive Shows Moderate Dendrite Formation
- Figure 7. Morphology of Ag Dendrites After High Voltage Migration Tests. (a) untreated nano-Ag conductive adhesives; (b) di-acid-incorporated nano-Ag conductive adhesives; and (c) mono-acid-incorporated nano-Ag conductive adhesives.
Electrical Properties of Nano-Ag ACAs with SAMs. Conventionally, metal-plated particles (e. g., gold-plated elastomers) measuring 3–5 μm are widely used as conductive fillers in anisotropically conductive adhesives/films (ACAs or ACFs). In recent years, however, in response to the fine pitch requirements for miniaturization of electronic devices, nano-scale conductive fillers are increasingly attracting attention. Among all the metal fillers, silver (Ag) nano filler is the most attractive choice because silver offers moderate cost, the highest electrical and thermal conductivity and high current carrying capability. In addition, silver nano-particles are relatively easy to form into different sizes (as mentioned before) and are well dispersed in a variety of matrix materials. However, problems of silver migration and subsequently the short-circuit issues have limited the application of silver for fine pitch ACAs. Since a novel and effective approach of silver migration control is discovered in this research, the electrical and thermal properties of nano-Ag ACA with SAMs will be discussed.
The current-resistance (I-R) curves of ACA joints with different fillers are shown in Figure 8 and are compared with typical lead-free solder joints (tin-silver-copper). For the conventional ACA with micron-sized Au-coated polymer fillers, the joint resistance was larger than 10-3 Ohm, and increasing the current higher than 2000 mA increased the joint resistance dramatically, due to the low current carrying capability. By using nano-Ag particles (20 nm) as conductive fillers, the joint resistance was stabilized at around 10-3 Ohm until the applied current increased over 2500 mA. Therefore, the highest applied current without damaging the samples (maximum allowable current) for nano-Ag ACA was 2500 mA. Over 2500 mA, the adhesive joints could not survive and the sample was burned and destroyed. With the introduction of SAM-treated nano-Ag fillers and substrates, the resistance of ACA joints decreased dramatically, indicating that a significantly improved conductivity was obtained. For the dithiol-treated nano-Ag ACAs, the resistance was reduced from 10-3 to 10-4 Ohm, but the current carrying capability was the same as untreated samples. This is because thiols tend to degrade the electrical performance of Ag due to the formation of less conductive Ag2S compounds. Increasing the current over 2500 mA led to the failure of adhesive joints. For the di-acid treated nano-Ag ACA samples, dramatically reduced joint resistance was achieved as low as 10-5 Ohm (the contact area was 100 × 100 μm2). The resistance value was comparable to or even lower than some of the metallic solder joints. As also shown in Figure 8, the joint resistance of a lead-free solder (tin-silver-copper alloy) was approximately 10-4 Ohm. Moreover, the maximum allowable current of di-acid treated samples also increased significantly, from 2500 mA to 3500 mA. For the conventional ACAs, when the applied current was higher than the maximum allowable current of ACAs, the adhesive joint burnt out due to the high joint resistance of the ACA materials at the higher current. For nano-Ag ACAs, the conduction lines got burnt at high currents due to the limited current carrying capability. For the SAM-treated nano-Ag ACA and solder joint, however, the failure occurred at a much higher current and the failure location was at the probe tips. This observation suggested that di-acid treated nano-Ag ACA joints have the potential to carry even higher currents, provided a suitable or optimized test vehicle could be designed and used in this study. The improved electrical properties of SAM-treated nano-Ag ACAs are considered to be due to the protection of the SAM compounds on the silver nanoparticles. The SAM compounds adhere to the conductive fillers, forming physi-chemical bondings and allowing electrons to freely flow/tunnel through the interfaces due to the high current density of SAM compounds and the capability of the SAM compounds to control the band gap of the materials. As such, it reduces electrical resistance and helps carry a high current flow for ACA joints.
Figure 8. Joint Resistance of Conventional ACA, Nano-Ag ACA, Lead-Free Metal Solder and SAM-Treated Nano
Thermal Conductivity of Nano Ag-filled ACA with SAMs. For the ACA interconnect joints to deliver high current, not only a low electrical resistance, but also a high thermal conductivity of the interconnect materials is required. It was reported that the addition of high thermal conductivity fillers into the ACA formulation rendered high current carrying capability (Yim, et al., 2005; Ekstrand, et al., 2005). In this study, the effects of the SAM treatment on the thermal conductivity of the ACA were also investigated. Thermal conductivity variation with response to temperature is important information, because the ACA joint material is heated up when applying current. Therefore, the thermal conductivity as a function of temperature was measured. The heat capacity and thermal conductivity of cured nano-Ag ACA with SAMs are shown in Figures 9 and 10, respectively. For the untreated nano-Ag ACA, the thermal conductivity changed from 0.2 to 0.24 W/m•K. With SAM-treated nano-Ag ACA, the thermal conductivity was significantly increased. Especially for di-acid treated samples, the thermal conductivity increased to 0.27 W/m•K at room temperature, about 35% higher than the untreated samples.
Figure 9. Heat Capacity of Nano-Ag Filled ACA with SAMs
Figure 10. Thermal Conductivity of Nano Ag-Filled ACAs with SAMs
The significantly improved thermal conductivity could be attributed to the improved interface properties (contributed by both better electron and phonon transports) between metal fillers and the epoxy matrix with SAM treatment. The higher thermal conductivity could help dissipate heat more efficiently from adhesive joints generated at high current by the high frequency devices, such as the high performance microprocessors. Therefore, higher thermal conductivity nano-Ag ACAs also contributed to the improved current carrying capability. With increasing temperatures, the thermal conductivity improvement of monolayer incorporated nano-Ag ACA was less significant, especially at temperatures higher than 170°C, probably due to the thermal degradation of the coated monolayer molecules. Therefore, to further improve the electrical and thermal behaviors of ACA with SAM-treated nano-particle fillers, both the strong interaction between monolayer and metal finishes and the high thermal stability are preferred.
Conclusions
In this study, a novel approach of using SAMs is designed to protect nano-silver particles and control the silver migration in the nano-Ag conductive adhesives. Formation of the surface complex between the carboxylate anion and a surface silver ion reduces the solubility and diffusivity of the silver metallization, which reduces the Ag migration components and therefore leads to effective migration control in nano-Ag conductive adhesives. Upon application of high voltages up to 500 V, no migration was observed for di-acid treated nano-Ag conductive adhesives. In addition to a controlled migration, the introduction of SAM-passivated/protected nano-Ag fillers also significantly enhanced the electrical conductivity and current carrying capability of ACA joints due to the improved interfacial properties and high current density of those molecular monolayers. Unlike typical ACA joints, which showed high joint resistance and limited current carrying capability, the joint resistance of the SAM incorporated nano-Ag conductive adhesive could be achieved as low as 10-5 Ohm and the maximum allowable current was higher than 3500 mA. The improved electrical performance of molecular monolayer-protected nano-Ag ACAs was also achieved with the increased thermal conductivity. As such, fine pitch, non-migration, high performance, high reliability adhesives are developed for potential solder replacement in high voltage, high power device applications. This novel approach for silver migration control and electrical properties enhancement can also be applied in next generation semiconductor devices to replace aluminum and copper metallization with silver.
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Li Y, Moon K, Wong CP. Electrical property of anisotropically conductive adhesives joint modified by self-assembled monolayer. In: Proceedings of The 54th IEEE Electronic Components and Technology Conference, Las Vegas, Nevada, June 1-4, 2004, pp1968-1974.
de Boer B, Hadipour A, Mandoc MM, van Woudenbergh T, Blom PWM. Journal of Advanced Materials 2005;17:621.
Yim MJ, Kim H-J, Paik K-W. Journal of Electronic Materials 2005;34:1165-1171.
Ekstrand L, Krstiansen H, Liu J. In: Proceedings of The IEEE 28th Int. Spring Seminar on Electronics Technology, Wiener Neustadt, Austria, May 19-20, 2005, pp. 35-39.
Journal Articles on this Report: 10 Displayed | Download in RIS Format
| Other project views: | All 49 publications | 22 publications in selected types | All 22 journal articles |
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Dong H, Li Y, Yim MJ, Moon KS, Wong CP. Investigation of electrical contact resistance for nonconductive film functionalized with π-conjugated self-assembled molecules. Applied Physics Letters 2007;90(9):092102, 3 pp. |
R831489 (2006) |
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Jiang H, Moon K, Li Y, Wong CP. Surface functionalized silver nanoparticles for ultrahigh conductive polymer composites. Chemistry of Materials 2006;18(13):2969-2973. |
R831489 (2006) |
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Li Y, Xiao F, Moon KS, Wong CP. Novel curing agent for lead-free electronics: amino acid. Journal of Polymer Science Part A: Polymer Chemistry 2005;44(2):1020-1027. |
R831489 (2005) R831489 (2006) |
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Li Y, Moon KS, Wong CP. Electrical property improvement of electrically conductive adhesives through in-situ replacement by short-chain difunctional acids. IEEE Transactions on Components and Packaging Technologies 2006;29(1):173-178. |
R831489 (2005) R831489 (2006) |
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Li Y, Moon KS, Wong CP. Enhancement of electrical properties of anisotropically conductive adhesive joints via low temperature sintering. Journal of Applied Polymer Science 2006;99(4):1665-1673. |
R831489 (2005) R831489 (2006) |
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Li Y, Moon KS, Whitman A, Wong CP. Enhancement of electrical properties of electrically conductive adhesives (ECAs) by using novel aldehydes. IEEE Transactions on Components and Packaging Technologies 2006;29(4):758-763. |
R831489 (2006) |
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Li Y, Wong CP. High performance anisotropic conductive adhesives for lead-free interconnects. Soldering & Surface Mount Technology 2006;18(2)33-39. |
R831489 (2006) |
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Li Y, Wong CP. Monolayer protection for eletrochemical migration control in silver nanocomposite. Applied Physics Letters 2006;89(11):112112, 3 pp. |
R831489 (2006) |
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Li Y, Wong CP. Recent advances of conductive adhesives as a lead-free alternative in electronic packaging: materials, processing, reliability and applications. Materials Science and Engineering R: Reports 2006;51(1-3):1-35. |
R831489 (2005) R831489 (2006) |
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Li Y, Xiao F, Wong CP. Novel, environmentally friendly crosslinking system of an epoxy using an amino acid: tryptophan-cured diglycidyl ether of bisphenol A epoxy. Journal of Polymer Science Part A: Polymer Chemistry 2007;45(2):181-190. |
R831489 (2006) |
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, INTERNATIONAL COOPERATION, TREATMENT/CONTROL, Sustainable Industry/Business, Scientific Discipline, RFA, POLLUTION PREVENTION, Technology for Sustainable Environment, Sustainable Environment, Chemical Engineering, Technology, Energy, Environmental Chemistry, Economics and Business, energy conservation, cleaner production, waste reduction, clean technologies, lead reduction, environmentally benign adhesive, environmentally conscious manufacturing, energy efficiency, electronic packaging, corrosion resistant
Progress and Final Reports:
2004 Progress Report
2005 Progress Report
Original Abstract