Effects of thermoplastic resin content of anisotropic conductive films on the pressure cooker test reliability of anisotropic conductive film flip-chip assembly

The flip-chip technology using anisotropic conductive films (ACFs) is gaining growing interest due to its technical advantages such as environmentally friendly, simpler, and lower cost processes. Electrical performances and reliability of ACF flip-chip assembly depend on thermomechanical properties of ACF polymer resins. In this paper, the changes in ACF resin morphology due to the phase separation of thermoplastics, and subsequent changes of physical and mechanical properties were investigated as a function of thermoplastic contents of ACF formulation. Furthermore, the pressure cooker test (PCT) reliability of ACF flip-chip assemblies with various thermoplastic contents was also investigated. As thermoplastic contents increased, coefficient of thermal expansion (CTE) of ACFs increased, and elastic modulus (E′) of ACFs decreased. In contrast, water absorption rate decreased as thermoplastic content increased. As a result, PCT reliability of ACF flip-chip assembly was improved adding up to 50 wt.% content of thermoplastic. An erratum to this article is available at .     

环境对各向异性导电胶膜性能参数的影响

各向异性导电胶膜(ACF)的玻璃转化温度Tg是它的一个重要性能参数,用差示扫描热示计(DSC)分别测定商用各向异性导电胶膜固化前和固化后的玻璃转化温度,并确定改变固化时间对它的玻璃转化温度的影响,以及高温高湿(85℃,85%RH)环境对它的玻璃转化温度的影响,得到各向异性导电胶玻璃转化温度下降是其粘接强度下降的原因之一。     

Highly reliable flip-chip-on-flex package using multilayered anisotropic conductive film

Anisotropic conductive film (ACF) has been used as interconnect material for flat-panel display module packages, such as liquid crystal displays (LCDs) in the technologies of tape automated bonding (TAB), chip-on-glass (COG), chip-on-film (COF), and chip-on-board (COB). Among them, COF is a relatively new technology after TAB and COG bonding, and its requirement for ACF becomes more stringent because of the need of high adhesion and fine-pitch interconnection. To meet these demands, strong interfacial adhesion between the ACF, substrate, and chip is a major issue. We have developed a multilayered ACF that has functional layers on both sides of a conventional ACF layer to improve the wetting properties of the resin on two-layer flex for better interface adhesion and to control the flow of conductive particles during thermocompression bonding and the resulting reliability of the interconnection using ACF. To investigate the enhancement of electrical properties and reliability of multilayered ACF in COF assemblies, we evaluated the performance in contact resistance and adhesion strength of a multilayered ACF and single-layered ACF under various environmental tests, such as a thermal cycling test (−55°C/+160°C, 1,000 cycles), a high-temperature humidity test (85°C/85% RH, 1,000 h), and a high-temperature storage test (150°C, 1,000 h). The contact resistance of the multilayered ACF joint was in an acceptable range of around a 10% increase of the initial value during the 85°C/85% RH test compared with the single-layered ACF because of the stronger moisture resistance of the multilayered ACF and flex substrate. The multilayered ACF has better adhesion properties compared with the conventional single-layered ACF during the 85°C/85% RH test because of the enhancement of the wetting to the surface of the polymide (PI) flex substrate with an adhesion-promoting nonconductive film (NCF) layer of multilayered ACF. The new ACF of the multilayered structure was successfully demonstrated in a fine-pitch COF module with a two-layer flex substrate.     

Microwave preheating of anisotropic conductive adhesive films for high-speed flip chip on flex bonding

Anisotropic conductive adhesive film (ACF) can be preheated by microwave (MW) radiation in order to reduce the bonding time for flip-chip technology. Due to sluggish and nonuniform curing kinetics at the beginning of the curing reaction, thermal curing of epoxy is more time consuming. Therefore, MW radiation may be more effective, due to its uniform heating rate during the cycle. In this paper, MW preheating (for 1–4 sec) of ACF prior to final bonding has been applied to determine the electrical and mechanical performance of the bond. Powers of 80 and 240 W MW were used to study the effect of the MW preheating. A final bonding time of 6–7 sec can be used for flip chip on flex bonding instead of 10–15 sec (standard time for flip chip bonding) for MW preheating time and power used in this study. The contact resistance (as low as 0.01) is low in these samples, whereas the standard resistance is 0.017 ohm (bonded at 180°C for 10 sec without prior MW preheating). The shear forces at breakage were satisfactory (0.167–0.183 KN) for the samples bonded for 6–7 sec with MW preheating. This is very close and even higher than the standard sample (0.173 KN). For MW preheating power of 80 W and sweeping time of 2 sec, final bonding at 6 sec can also be used because of its low contact resistance (0.019 ohm). Scanning electron microscope (SEM) investigation of microjoints and fracture surface shows uneven distribution of conductive particles and thick bond lines in samples bonded for 5 sec (with MW preheating). Samples treated with MW radiation (80 W and 2–3 sec time) serve as evidence that well-distributed particles along with thin bond lines cause low contact resistance and high joint strength.     

Curing kinetics of anisotropic conductive adhesive film

Polymer-based conductive-adhesive materials have become widely used in many electronic packaging interconnect applications, such as chip-on-glass, chip-on-flex, etc. Among all the conductive-adhesive materials, anisotropic conductive adhesive film (ACF) is an attractive interconnect material because of its fine pitch capability. Anisotropic conductive-adhesive film is a thermosetting, epoxy matrix impregnated with a small amount of electrically conductive particles. During component assembly, the epoxy resin is cured to provide mechanical connection, and the conducting medium provides electrical connection in the z direction. The thermal cure process is critical to develop the ultimate electrical and mechanical properties of the ACF. In this paper, the curing reaction of ACF was studied with a differential scanning calorimeter (DSC) under isothermal conditions in the range of 120–180°C. An autocatalyzed kinetic model was used to describe the curing reaction. The rate constant and the reaction orders were determined and used to predict the progress of the curing reaction. A good agreement is found between the proposed kinetic model and the experimental reaction-rate data. The reaction-rate constants were correlated with the isothermal temperature by the Arrhenius equation. The glass-transition temperature also has been studied as a function of cure degree and moisture absorption.     

A continuous contact resistance monitoring during the temperature ramp of anisotropic conductive adhesive film joint

In this study, a systematic experimental work was performed to evaluate the reliability of the anisotropic conductive adhesive film (ACF) joint at high temperature for flip-chip-on-flex (FCOF) assemblies. A four-point probe method was developed to measure the contact resistance at high temperature. Measurement was also conducted along the length of the chip. The correlation between the increased resistance and the failure mechanism was investigated using scanning electron microscopy (SEM). Initially, the contact resistance increased linearly with rising temperature, but later, it increased abruptly. This changeover was related to the glass-transition temperature (T_g) of the ACF matrix. The coefficient of thermal expansion (CTE) is very high at temperatures above T_g; thus, the ACF swells too much, reducing the mechanical contact of the particles with the bump and/or pad. Again, as the adhesive strength becomes weaker at temperatures above the glass transition, it is unable to resist the thermal stress of the flex. The cumulative thermal stress at the edges dislodges the particles from the interconnection. Even below T_g, the thermal stress at the edges is higher than the middle point. Thus, the contact resistance varied from the middle joint of the chip to that of the corner at the same high temperature. To reduce the contact resistance at the corner joint of the FCOF packages bonded by ACF, a square-shaped chip instead of a chip with a higher aspect ratio should be used. It was also suggested to use an adhesive with a higher glass-transition temperature and lower CTE.     

倒装芯片各向异性导电胶互连的剪切结合强度

采用冲击试验方法研究了各向异性导电胶膜(ACF)互连的玻璃和柔性基板上倒装芯片(COG和COF)的剪切结合强度.结果表明:COF比COG的剪切强度高.ACF的固化程度达85 %时有最大的结合强度.键合温度、导电颗粒状态、缺陷等因素对ACF互连的结合强度有较大影响,而键合压力的影响不大.     

Reliability analysis of the fine pitch connection using anisotropic conductive film (ACF)

A novel method (the V-shaped curve) is presented to predict the failure probability of anisotropic conductive film (ACF) in IC/substrate assemblies. The Poisson function is used to calculate the probability of opening failure in the vertical gap between the pads, while the box and modified box models are used to estimate the probability of bridging failure between the pads in the pitch direction. The opening and bridging probabilities are combined using probability theory to establish four different failure prediction models. The results reveal that the model combining the Poisson function for fewer than six particles per pad with the modified box model provides the most accurate predictions of the failure probability of ACF in IC/substrate assemblies.     

Effects of epoxy functionality on the properties and reliability of the anisotropic conductive films for flip chips on organic substrates

The effects of the functionality of an epoxy monomer on the composite properties and reliability of anisotropic conductive films (ACFs) in a flip-chip package were investigated. Three epoxy monomers with different functionalities (f=2–4) were considered. The ACFs prepared using epoxy monomers with higher functionality resulted in lower molecular weight between crosslinks (M_c). As the M_c decreased, the elastic modulus (E′) and coefficient of thermal expansion (CTE) were improved. These results were highly consistent with the rubber elasticity theory. The reliability performance of the flip chip on organic substrate assemblies using ACFs were also investigated as a function of epoxy functionality. The ACFs prepared by using higher functional epoxy monomers showed improved reliability performance.     

Formulation and characterization of anisotropic conductive adhesive paste for microelectronics packaging applications

There has been a steadily increasing interest in using electrically conductive adhesives as interconnecting materials in electronics manufacturing. In this paper, several anisotropic conductive adhesive (ACA) pastes were formulated, which consist of diglycidyl ether of bisphenol F or diglycidyl ether of bisphenol A as polymer matrix, imidazoles as curing agents, and different sizes of silver (Ag) powders or gold (Au)-coated polymer spheres as conductive particles. The effects of ACA resin and different curing agents, as well as different conductive particles, on flexible substrate of the flip-chip joint were studied. The results show that the size and type of different conductive particles have very limited influence on an ACA flip-chip joint. The ACA resin as well as the curing agent can affect the reliability of the joint. The same results can be applied for the failure analysis of ACA flip-chip technology.     

ACF互联可靠性影响因素的研究进展

总结了材料、芯片结构和环境因素等对异向导电胶膜(Anisotropic Conductive Film;ACF)互联可靠性的影响。文献表明,ACF基体树脂的吸湿膨胀系数、粘结强度及玻璃转化温度严重影响组装产品的互联可靠性,而导电粒子的导电性、芯片的结构和焊盘的表面处理方式等对可靠性也有较大影响。文献中的可靠性试验表明,在上面提到的因素中,湿气是影响器件中ACF可靠性的主要因素。     

Research on the contact resistance,reliability, and degradation mechanisms of anisotropically conductive film interconnection for flip-chip-on-flex applications

Although there have been many years of development, the degradation of the electrical performance of anisotropically conductive adhesive or film (ACA or ACF) interconnection for flip-chip assembly is still a critical drawback despite wide application. In-depth study about the reliability and degradation mechanism of ACF interconnection is necessary. In this paper, the initial contact resistance, electrical performance after reliability tests, and degradation mechanisms of ACF interconnection for flip-chip-on-flex (FCOF) assembly were studied using very-low-height Ni and Au-coated Ni-bumped chips. The combination of ACF and very-low-height bumped chips was considered because it has potential for very low cost and ultrafine pitch interconnection. Contact resistance changes were monitored during reliability tests, such as high humidity and temperature and thermal cycling. The high, initial contact resistance resulted from a thin oxide layer on the surface of the bumps. The reliability results showed that the degradation of electrical performance was mainly related to the oxide formation on the surface of deformed particles with non-noble metal coating, the severe metal oxidation on the conductive surface of bumps, and coefficient of thermal expansion (CTE) mismatch between the ACF adhesive and the contact conductive-surface metallization. Some methods for reducing initial contact resistance and improving ACF interconnection reliability were suggested. The suggestions include the removal of the oxide layer and an increase of the Au-coating film to improve conductive-surface quality, appropriate choice of conductive particle, and further development of better polymeric adhesives with low CTE and high electrical performance.     

Effect of bump characteristics and temperature variation on the online contact resistance of anisotropic conductive joints

Anisotropic conductive film (ACF) suffers a major drawback in regard to reliability even though it has merits, such as reduction in interconnection distance, high performance, and environmental friendliness. The factor of thermal warpage may lead to a highly unreliable electrical connection in the assembly. The work presented in this paper focuses on the online contact-resistance behavior of the ACF joint during thermal shock and compares the results of two different types of dies (Au/Ni bump and bumpless). For this work, we used a flip chip of 11 × 3 mm² in dimension. The flex substrate used was made of polyimide film with an Au/Ni/Cu electrode and daisy-chained circuit for a matching die-bump pattern. The ACF that was used is an epoxy resin in which nickel and gold-coated polymer balls are dispersed. Tests for three different thermal-cycling profiles (125°C to −55°C, 140°C to −40°C, and 150°C to −65°C) were carried out. The samples bonded at a temperature of 180°C, and a pressure of 80 N was used. The initial contact resistances of Au/Ni bump and bumpless samples were 0.25 ω and 0.4 ω respectively. A comparative study was carried out from the results obtained. The results showed that for the flip-chip-on-flex (FCOF) packages having an Au/Ni bump, the increase in online contact resistance is higher than that of the FCOF packages having bumpless chips. For example, in the thermal-cycling profile of 140°C to −40°C, the online contact resistance for the Au/Ni bump raised to 4.6 ω after 180 cycles, whereas it was only 1.3 ω for the bumpless sample. The bump height and bump materials were found to be the main factor for such variation. Results show that, above the glass-transition temperature (T_g), the ACF matrix becomes less viscous, which reduces its adhesive strength and lets the higher bump height of the chip result in a higher standoff of the package and thus sliding is easier to take place. The responses by the assemblies in hot and cold conditions are examined, and in-chamber behavior of the assembly is studied and explained.     

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.     

环境对各向异性导电胶膜性能参数的影响

各向异性导电胶膜(ACF)的玻璃转化温度T_g是它的一个重要性能参数,用差示扫描热示计(DSC)分别测定商用各向异性导电胶膜固化前和固化后的玻璃转化温度,并确定不同的固化时间对它的玻璃转化温度的影响,以及高温高湿(85℃,85%RH)环境对它的玻璃转化温度影响,得到各向异性导电胶玻璃转化温度下降是其粘接强度下降的原因之一。     

Adhesion and Reliability of Anisotropic Conductive Films (ACFs) Joints on Organic Solderability Preservatives (OSPs) Metal Surface Finish

The effect of final metal finishes of Cu electrodes on the adhesion and reliability of anisotropic conductive film (ACF) joints was investigated. Two different metal surface finishes, electroless Ni/immersion Au (ENIG) and organic solderability preservatives (OSPs) coated on Cu, were selected in this study for ACF bonding. The adhesion strength of ACF/OSP joints was higher than that of ACF/bare Cu and ACF/ENIG joints. The fracture sites of the ACF/bare Cu and ACF/ENIG joints were ACF/metal interfaces, while those of ACF/OSP joints were inside the ACF. Transmission electron microscope (TEM) and Fourier-transform infrared (FT-IR) analyses showed that the OSP coating layer on the Cu electrodes reacted with the epoxy resin of the ACFs but still remained at the bonding interface. According to the in-depth X-ray photoelectron spectroscopy (XPS) analysis, additional C-N bonds formed after the OSP-epoxy reaction and the outermost nitrogen of the OSP layer participated in curing of the epoxy resin of the ACF. Therefore, the OSP layer acted as an adhesion promoter to ACFs. Furthermore, this role of the OSP layer enhanced the reliability of the ACF/OSP joints under high temperature and humid environments, as compared to the ACF/ENIG joints.     

Reliability of nickel flip chip bumps with a tin-silver encapsulation on a copper/tin-silver substrate during the bonding process

This study focused on the feasibility of using Ni flip chip bumps with a Sn-2.5Ag (wt.%) solder encapsulation. The interfacial reaction and die shear property of the Ni flip chip bump with a Sn-Ag solder cap bonded on the electroplated Cu/Sn-Ag substrate were investigated with increasing bonding time. After bonding for 1 s (Cu_xNi_1−x)₆Sn₅ and Cu₆Sn₅ intermetallic compound (IMC) layers were formed at the upper and lower interfaces, respectively, with the former IMC being the predominant phase during bonding. The transformation of the solder cap into the (Cu_xNi_1−x)₆Sn₅ IMC depleted the solder after bonding for 30 s, and then the Ni concentration in the IMC gradually decreased with increasing bonding time. The shear property peaked after 30 s, and then decreased with increasing bonding time. The fractures occurred at the solder/Cu₆Sn₅ interface and inside the (Cu_x Ni_1−x)₆Sn₅ IMC after bonding for 1 s and 30 s, respectively, after which the fracture location shifted toward the (Ni_xCu_1−x)₃Sn₄/(Cu_x Ni_1−x)₆Sn₅ interface with increasing bonding time.     

Residual stress and interfacial reaction of the electroplated Ni-Cu alloy under bump metallurgy in the flip-chip solder joint

Pure Ni, the Ni-Cu alloy, and pure Cu layers as the under bump metallurgy (UBM) for a flip-chip solder joint were deposited by electrolytic plating. For the pure Ni layer, residual stress can be controlled by adding a wetting agent and decreasing current density, and it is always under tensile stress. The Ni-Cu alloys of different Cu compositions from ∼20wt.%Cu to 100wt.%Cu were deposited with varying current density in a single bath. The residual stress was a strong function of current density and Cu composition. Decreasing current density and increasing Cu content simultaneously causes the residual stress of the metal layers to sharply decrease. For the pure Cu layer, the stress is compressive. The Cu layer acts as a cushion layer for the UBM. The residual stress of the UBM strongly depends on the fraction of the Cu cushion layer. Interfacial reaction of the UBM with Sn-3.5 wt.% Ag was studied. As the Cu contents of Ni-Cu alloys increased, the dissolution rate increased. Several different intermetallic compounds (IMCs) were found. The lattice constants of alloys and the IMC increase with increasing Cu contents because the larger Cu atoms substitute for the smaller Ni atoms in the crystallites. The Cu content of the IMC are strongly dependent on the composition of the alloys. Ball shear tests were done with different metal-layer schemes. The failure occurs through the IMC and solder.     

A printing technology combining screen-printing with a wet-etching process for the gate electrodes of organic thin film transistors on a plastic substrate

We have developed a practical printing technology for the gate electrode of organic thin film transistors (OTFTs) by combining screen-printing with a wet-etching process using nano-silver (Ag) ink as a conducting material. An Ag film was deposited onto a PVP (polyvinylphenol)-coated PC (polycarbonate) plastic substrate by screen-printing with nano-Ag ink, where Ag content of 20 wt.% was mixed using a terpineol solvent. Subsequently, the film was cured at 200 °C for 60 min, and then finally wet-etched through patterned positive photo-resist masks. The screen-printed Ag electrode exhibited a minimum line width of ∼5 μm, a thickness of ∼65 nm, and a resistivity of ∼10⁻⁶ Ω cm, producing good geometrical and electrical characteristics for a gate electrode. Additionally, it also provided good step coverage with the PVP dielectric layer, and consequently leakage current between the gate and source/drain electrodes was eliminated. Moreover, the electrical characteristic of the screen-printed Ag electrode was not significantly changed even after a bending test in which the Ag electrodes were bent with a bending radius of 6 mm and 2500 iterations of cyclic bending. OTFTs with the screen-printed Ag electrode produced a saturation mobility of 0.13 cm²/Vs and a current on/off ratio of 1.79 × 10⁶, being comparable to those of an OTFT with a thermally evaporated Al gate electrode.