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.     

Dynamic strength of anisotropic conductive joints in flip chip on glass and flip chip on flex packages

The work presented in this paper focuses on the behavior of anisotropically conductive film (ACF) joint under the dynamic loading of flip chip on glass (COG) and flip chip on flexible (COF) substrate packages. Impact tests were performed to investigate the key factors that affect the adhesion strength. Scanning electron microscopy (SEM) was used to evaluate the fractography characteristics of the fracture. Impact strength increased with the bonding temperature, but after a certain temperature, it decreased. Good absorption and higher degree of curing at higher bonding temperature accounts for the increase of the adhesion strength, while too high temperature causes overcuring of ACF and degradation at ACF/substrate interface––thus decreases the adhesion strength. Higher extent of air bubbles was found at the ACF/substrate interface of the sample bonded at the higher temperature. These air bubbles reduce the actual contact area and hence reduce the impact strength. Although bonding pressure was not found to influence the impact strength significantly, it is still important for a reliable electrical interconnect. The behaviors of the conductive particles during impact loading were also studied. From the fracture mode study, it was found that impact load caused fracture to propagate in the ACF/substrate interface (for COG packages), and in the ACF matrix (for COF packages). Because of weak interaction of the ACF with the glass, COG showed poor impact adhesion.     

Effects of different bonding parameters on the electrical performance and peeling strengths of ACF interconnection

The effects of different bonding parameters, such as temperature, pressure, curing time, bonding temperature ramp and post-processing, on the electrical performance and the adhesive strengths of anisotropic conductive film (ACF) interconnection are investigated. The test results show that the contact resistances change slightly, but the adhesive strengths increase with the bonding temperature increased. The curing time has great influence on the adhesive strength of ACF joints. The contact resistance and adhesive strength both are improved with the bonding pressure increased, but the adhesive strengths decrease if the bonding pressure is over 0.25 MPa. The optimum temperature, pressure, and curing time ranges for ACF bonding are concluded to be at 180–200 °C, 0.15–0.2 MPa, and 18–25 s, respectively. The effects of different Teflon thickness and post-processing on the contact resistance and adhesive strength of anisotropic conductive film (ACF) joints are studied. It is shown that the contact resistance and the adhesive strength both become deteriorated with the Teflon thickness increased. The tests of different post-processing conditions show that the specimens kept in 120 °C chamber for 30 min present the best performance of the ACF joints. The thermal cycling (−40 to 125 °C) and the high temperature/humidity (85 °C, 85% RH) aging test are conducted to evaluate the reliability of the specimens with different bonding parameters. It is shown that the high temperature/humidity is the worst condition to the ACF interconnection.     

Thermal stability performance of anisotropic conductive film at different bonding temperatures

In this work the effect of different bonding temperatures on the thermal stability of anisotropic conductive films (ACFs) was investigated. A thermogravimetric analyzer (TGA) was utilized to determine the thermal decomposition temperature of ACF. The experimental results showed that the temperature for maximum decomposition rate of ACF, T_m decreased with increasing bonding temperature. The results obtained from Fourier transform infrared spectroscopy (FTIR), which was used to examine the curing degree and also chemical changes in the ACF, showed some networks scissoring happened on C–N bond during the bonding process. This was the main reason why the ACF bonded at high temperature 225.0 °C, gave relatively low thermal stability. Four point probe was used to measure the contact resistance performance before and after thermal aging at 260 and 300 °C. The contact resistance results suggested that ACF bonded with 10 s at 205.0 °C, yielding a curing degree of 85.0% was the best bonding parameter to obtain a low contact resistance after thermal aging. FTIR results showed there was a significant increase in the absorbance peak of carbonyl group after thermal aging. Thermal oxidation reactions which were taken place at high bonding and aging temperature had broken the polymer networks in the ACF and caused electrical failure.     

Effect of microwave preheating on the bonding performance of flip chip on flex joint

A microwave (MW) preheating mechanism of anisotropic conductive adhesive film (ACF) has been introduced in order to reduce the bonding temperature for flip chip technology. Thermal curing of epoxy shows a very sluggish and non-uniform curing kinetics at the beginning of the curing reaction, but the rate increases with time and hence requires higher temperature. On the other hand MW radiation has the advantage of uniform heating rate during the cycle. In view of this, MW preheating (for 2/3 s) of ACF prior to final bonding has been applied to examine the electrical and mechanical performance of the bond. Low MW power has been used (80 and 240 W) to study the effect of the MW preheating. It has been found that 170 °C can be used for flip chip bonding instead of 180 °C (standard temperature for flip chip bonding) for MW preheating time and power used in this study. The contact resistance (0.015–0.025 Ω) is low in these samples where the standard resistance is 0.017 Ω (bonded at 180 °C without prior MW preheating). The shear forces at breakage were satisfactory (152–176 N) for the samples bonded at 170 °C with MW preheating, which is very close and even higher than the standard sample (173.3 N). For MW preheating time of 2 s, final bonding at 160 °C can also be used because of its low contact resistance (0.022–0.032 Ω), but the bond strength (137.3–145 N) is somewhat inferior to the standard one.     

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.     

平板显示器中应用ACF的驱动IC封装技术

文章叙述了ACF（Anisotropic Conductive Film,各向异性导电膜）与驱动IC（Integrated Circuit,集成电路）芯片封装的历史,并强调了驱动IC封装在实现显示器微型化、高分辨率、低成本及高显示质量等方面的重要性。文章还对微细间距COF（Chip on Flex）连接用ACF的材料设计进行了介绍。文章指出低温固化ACF可以改善LCD（Liquid Crystal Display,液晶显示屏）模块的生产效率,降低大型LCD模块表面的热应力;同时指出COG（Chip on Glass）连接后LCD面板的翘曲变形引起LCD模块漏光事故。ACF焊接温度的降低可以有效减少翘曲变形,避免在应用COG封装大型LCD模块的驱动IC时所产生漏光。     

A new COG technique using low temperature solder bumps for LCD driver IC packaging applications

A new chip on glass (COG) technique using flip chip solder joining technology has been developed for excellent resolution and high quality liquid crystal display (LCD) panels. The flip chip solder joining technology has several advantages over the anisotropic conductive film (ACF) bonding technology: finer pitch capability, better electrical performance, and easier reworkability. Conventional solders such as eutectic Pb-Sn and Pb-5Sn require high temperature processing which can lead to degradation of the liquid crystal or the color filter in LCD modules. Thus it is desirable to develop a low temperature process below 160/spl deg/C using solders with low melting temperatures for this application. In our case, we used eutectic 58 wt%Bi-42 wt%Sn solder for this purpose. Using the eutectic Bi-Sn solder bumps of 50-80/spl mu/m pitch sizes, an ultrafine interconnection between the IC and glass substrate was successfully made at or below 160/spl deg/C. The average contact resistance of the Bi-Sn solder joints was 19m/spl Omega/ per bump, which is much lower than the contact resistance of conventional ACF bonding technologies. The contact resistance of the underfilled Bi-Sn solder joints did not change during a hot humidity test. We demonstrate that the COG technique using low temperature solder joints can be applied to advanced LCDs that lead to require excellent quality, high resolution, and low power consumption.     

Understanding of Delamination Mechanism of Anisotropic Conductive Film (ACF) Bonding in Thin Liquid Crystal Display (LCD) Module

The chip-on-glass (COG) technique using anisotropic conductive film (ACF) has been developed for liquid crystal display (LCD) panels with excellent resolution and high quality for several years. However, many serious manufacturability and reliability issues were observed from previous studies. In those, delamination occurring at the ACF interface is one of the common concerns. Few works presented analysis of delamination mechanism through the whole COG bonding process with the combination of LCD module scale and ACF interconnect scale. In this paper, the delamination mechanism of COG/ACF interconnection was studied by using finite element analysis. Equivalent block and global-local modeling methods were implemented with nonlinear elastic-plastic and sequential coupled thermal-mechanical analysis. The critical parameters of the COG bonding process and geometry of integrated circuit (IC) and glass were investigated to understand the mechanism of ACF delamination. It was found that the delamination could be reduced by decreasing the temperature difference between bonding head and glass substrate or using thin and short IC. The local model analysis revealed that the interface of glass/ACF epoxy encountered the higher stress than that in the interface of IC/ACF epoxy and had the higher possibility to delaminate. Therefore, increasing the bonding-strength between glass and ACF epoxy is the direction to reduce the probability of ACF delamination.     

Effect of Conductive Particle Properties on the Reliability of Anisotropic Conductive Film for Chip-on-Glass Applications

This paper describes how the material properties of conductive particles in anisotropic conductive films (ACFs) affect the electrical conductivity and the reliability of ACF interconnections for chip-on-glass (COG) applications. For the conductive particles, Au/Ni-coated polymer particles with a 5-diameter were used. Two different types of conductive particles were characterized with respect to their mechanical and electrical properties, such as ball hardness, recovery behavior, and electrical resistance. In addition, two ACFs were fabricated in the form of a double-layered structure, in which the thickness of the ACF and a nonconductive film (NCF) layer were optimized to have as many conductive particles as possible on the bump after COG bonding. The electrical contact resistance of an ACF interconnection in a COG structure depends mainly on the electrical properties of conductive particles in the ACF. The electrical reliability of an ACF interconnection in a COG structure also depends more on the electrical properties than the mechanical properties of conductive particles under a high-temperature and humid condition. Conductive particles with a lower electrical resistance, higher mechanical hardness, and lower recovery rate show better reliability than conductive particles with a higher electrical resistance, lower mechanical hardness, and higher recovery rate. Cross-sectional scanning electron microscopic (SEM) pictures of a COG interconnection show the deformation of two different conductive particles after the reliability tests. The ACF interconnections in the edge or corner of a driver IC show less reliable joints due to high absorption of moisture.     

Theoretical Prediction and Experimental Measurement of the Degree of Cure of Anisotropic Conductive Films (ACFs) for Chip-On-Flex (COF) Applications

The degree of cure of anisotropic conductive films (ACFs) was theoretically predicted and experimentally measured to investigate the effect of the degree of cure of ACFs on the electrical and mechanical stability of ACF joints and the␣reliability of chip-on-flex (COF) assemblies. The cure reaction of ACFs, observed by an isothermal differential scanning calorimetry (DSC) analysis, followed an autocatalytic cure mechanism, and the degree of cure of ACFs as a function of time and temperature was mathematically derived from an autocatalytic cure kinetics model. To simulate the ACF temperature field accurately during the COF bonding process, the thermal properties of the ACF such as the thermal diffusivity (α), specific heat capacity (C _p), and thermal conductivity (λ) were characterized experimentally. The degrees of cure of ACFs as functions of the bonding time during the COF bonding process were theoretically predicted by the incorporation of autocatalytic kinetics modeling and ACF temperature simulation. The predicted degrees of cure of ACFs were well matched with the experimental data measured by attenuated total reflectance/Fourier-transform infrared (ATR/FT-IR) analysis. The contact resistances of the ACF joints and the peel adhesion strengths of the COF assemblies were evaluated for electrical and mechanical interconnection stability. According to these results, the ACF contact resistances decreased and the ACF peel adhesion strengths increased as the degree of cure of ACFs increased. In addition, to investigate the effect of the degree of cure of ACFs on the reliability of COF assemblies, an 85°C/85% relative humidity (85°C/85% RH) test was performed. These results showed that the reliability of COF assemblies also strongly depends on the degree of cure of the ACFs.     

Studies on various chip-on-film (COF) packages using ultra fine pitch two-metal layer flexible printed circuits (two-metal layer FPCs)

Various fine pitch chip-on-film (COF) packages assembled by (1) anisotropic conductive film (ACF), (2) nonconductive film (NCF), and (3) AuSn metallurgical bonding methods using fine pitch flexible printed circuits (FPCs) with two-metal layers were investigated in terms of electrical characteristics, flip chip joint properties, peel adhesion strength, heat dissipation capability, and reliability. Two-metal layer FPCs and display driver IC (DDI) chips with 35 μm, 25 μm, and 20 μm pitch were prepared. All the COF packages using two-metal layer FPCs assembled by three bonding methods showed stable flip chip joint shapes, stable bump contact resistances below 5 mΩ, good adhesion strength of more than 600 gf/cm, and enhanced heat dissipation capability compared to a conventional COF package using one-metal layer FPCs. A high temperature/humidity test (85 °C/85% RH, 1000 h) and thermal cycling test (T/C test, ?40 °C to + 125 °C, 1000 cycles) were conducted to verify the reliability of the various COF packages using two-metal layer FPCs. All the COF packages showed excellent high temperature/humidity and T/C reliability, however, electrically shorted joints were observed during reliability tests only at the ACF joints with 20 μm pitch. Therefore, for less than 20 μm pitch COF packages, NCF adhesive bonding and AuSn metallurgical bonding methods are recommended, while all the ACF and NCF adhesives bonding and AuSn metallurgical bonding methods can be applied for over 25 μm pitch COF applications. Furthermore, we were also able to demonstrate double-side COF using two-metal layer FPCs.     

The effect of reflow process on the contact resistance and reliability of anisotropic conductive film interconnection for flip chip on flex applications

The work presented in this paper focuses on the effect of reflow process on the contact resistance and reliability of anisotropic conductive film (ACF) interconnection. The contact resistance of ACF interconnection increases after reflow process due to the decrease in contact area of the conducting particles between the mating I/O pads. However, the relationship between the contact resistance and bonding parameters of the ACF interconnection with reflow treatment follows the similar trend to that of the as-bonded (i.e. without reflow) ACF interconnection. The contact resistance increases as the peak temperature of reflow profile increases. Nearly 40% of the joints were found to be open after reflow with 260 °C peak temperature. During the reflow process, the entrapped (between the chip and substrate) adhesive matrix tries to expand much more than the tiny conductive particles because of the higher coefficient of thermal expansion, the induced thermal stress will try to lift the bump from the pad and decrease the contact area of the conductive path and eventually, leading to a complete loss of electrical contact. In addition, the environmental effect on contact resistance such as high temperature/humidity aging test was also investigated. Compared with the ACF interconnections with Ni/Au bump, higher thermal stress in the Z-direction is accumulated in the ACF interconnections with Au bump during the reflow process owing to the higher bump height, thus greater loss of contact area between the particles and I/O pads leads to an increase of contact resistance and poorer reliability after reflow.     

Application of Al/PI composite bumps to COG bonding process

This work demonstrates the probing, testability and applicability of Al/PI (aluminum/polyimide) composite bumps to the chip on-glass (COG) bonding process for liquid crystal display (LCD) driver chip packaging. The experimental results showed that the thickness of Al overlayer on PI core of the bump, the location of pin contact, and the bump configuration affect bump probing testability. The bump with type IV configuration prepared in this work exhibited excellent probing testability when its Al overlayer thickness exceeded 0.8 μm. We further employed Taguchi method to identify the optimum COG bonding parameters for the Al/PI composite bump. The four bonding parameters, bonding temperature, bonding time, bonding pressure and thickness of Al overlayer are identified as 180° C, 10 s, 800 kgf/cm² and 1.4 μm, respectively. The optimum bonding condition was applied to subsequent COG bonding experiments on glass substrates containing Al pads or indium tin oxide (ITO) pads. From the results of resistance measurement along with a series of reliability tests, Al pad is found to be a good substrate bonding pad for Al/PI bump to COG process. Excellent contact quality was observed when the bumps had Al overlayer thickness over 1.1 μm. As to the COG specimens with substrate containing ITO pads, high joint resistance suggested that further contact quality refinement is necessary to realize their application to COG process     

Numerical study on the bonding tool position, tip profile and planarity angle influences on TAB/ILB interconnection reliability

Tape automated bonding (TAB) is a widely used interconnection technology for high-pincount and fine-pitch IC packaging. In this study, a three-dimensional computational model was developed for analyzing TAB inner lead bonding (ILB) process. This experimental study on the thermomechanical properties of copper leads was achieved using high precision micro-force tensile tests. A stress–stain relation between the copper lead and different temperature ranges was successfully implemented into the finite element model to study large plastic deformation in ILB formation. The resulting ILB lead profile and bump sinking values obtained from the simulations agreed well with the experimental observations from actual manufacturing data with the same bonding parameters. The tool position and lead length effects are analyzed to study the residual stress distribution after ILB. A 10-lead model was developed to study how the tool tip profile and planarity ‘angle affect the co-planarity between the bonding tool and the stage. The numerical results show that the permissible tool profile variance should not exceed 1.25 μm and the acceptable planarity angle is 0.005° to achieve the minimum bump deformation requirement.     

Adhesion strength and contact resistance of flip chip on flex packages––effect of curing degree of anisotropic conductive film

The use of anisotropic conductive adhesives film (ACF) as an interconnect materials for flip-chip joining technique is increasingly becoming a vital part of the electronics industry. Therefore, the performances of the ACF joint turn into an important issue and depend mostly on the curing condition of the ACF matrix. ACF is a thermosetting epoxy matrix impregnated with 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. Therefore, the cure process is critical to develop the ultimate electrical and mechanical properties of ACF. The purpose of the present work is to investigate optimum curing conditions to achieve the best performance of ACF joints. Differential scanning calorimeter was used to measure the curing degree. Adhesion strength was evaluated by 90° peeling test. The contact resistance has also been studied as a function of bonding temperature and curing degree. Results show a strong dependence of curing condition on the electrical and mechanical performances. Adhesion strength increases exponentially with the curing degree. Whereas the contact resistance decreases with the curing degree and achieve the minimum value at 87% of curing. Co-relation of the curing degree of the ACF was also studied through the detailed investigation of the fracture surfaces under scanning electron microscopy.     

Reliability of adhesive interconnections for application in display module

The reliability of adhesive interconnections using anisotropic conductive film (ACF) and non-conductive film (NCF) was evaluated by measuring connection resistance during 500 cycles of thermal shock testing. The four-point probe method was used to measure the connection resistance of the adhesive joints constructed with Au bump on Si chip and Cu pad on flexible printed circuit (FPC). The connection resistance of the ACF joints was markedly higher than that of the NCF joints, mainly due to the constriction of the current flow and the intrinsic resistance of the conductive particles in the ACF joints. The connection resistances of both interconnections decreased with increasing bonding force, and subsequently converged to about 10 and 1 mΩ at a bonding force of 70 and 80 N, for the ACF and NCF joints, respectively. During the thermal shock testing, two different conduction behaviors were observed: increased connection resistance and the termination of Ohmic behavior. The former was due to the decreased contact area caused by z-directional swelling of the adhesives, whereas the latter was caused by either contact opening in the adhesive joints or interface cracking.     

Particle on Bump (POB) technique for ultra-fine pitch chip on glass (COG) applications by conductive particles and adhesives

Chip on glass (COG) technology is widely used in liquid crystal display (LCD) modules for connecting driver ICs to the displays especially for middle and small size panels. The most common COG technology currently used in display applications is based on anisotropic conductive films (ACF). As the increasing demand in higher resolution and cost reduction, the bump pitch of the driver ICs becomes finer and finer. With the reduction of bump pitch, the current ACF based COG technology is confronted with two issues: one is the increase of the chances of open circuit; the other is the increase of the chances of forming shorts. A new approach for ultra-fine pitch chip on glass (COG) bonding, named ”Particle on Bump (POB)”, is proposed in this paper. In this technique, conductive particles are planted on the top surface of bumps of a driver IC through Au–Sn intermetallic connection. The driver IC is then assembled on the glass substrate of a LCD panel with an insulated adhesive by thermal press. The new method ensures that electrical connections are established only between bumps and corresponding pads. The Au–Sn reflow process for particle planting and corresponding COG bonding process were investigated in detail. The results showed that reliable connections were formed between particles and bumps through an Au–Sn intermetallic layer and final COG interconnections thus formed performed well in reliability tests. It is concluded that the POB technique overcomes the shortcomings of current ACF technique and has good potential to provide a viable ultra-fine pitch flip chip on glass solution for display applications.     

Current-carrying capacity of anisotropic-conductive film joints for the flip chip on flex applications

The effect of the substrate-pad physical properties (surface roughness and hardness) on the current-carrying capacity of anisotropic-conductive film (ACF) joints is investigated in this work. Flip chips with Au bumps were bonded to the flexible substrates with Au/Cu and Au/Ni/Cu pads using different bonding pressure. It was found that the current-carrying capacity of ACF joints increased to a maximum value with the rise of the bonding pressure; then, it reduced if the bonding pressure continually increased. The maximum average value per unit area of Au/Ni/Cu pad and Au/Cu pad ACF joints is about 93 μA/μm² and 118 μA/μm², respectively, at 100-MPa bonding pressure. The variation trend of connection resistance is the opposite of current-carrying capacity. The variation of current-carrying capacity (or connection resistance) of Au/Cu pad joints is larger than that of Au/Ni/Cu pad joints. The current-carrying capacity is related to the variation of the resistance of ACF joints. The connection resistance of ACF joints depends primarily on the particle constriction resistance (R_coi), R_coi ∞ 1/a, where “a” is the radius of contact spot. A smaller contact area results in larger joule heat generation per unit volume (Q_g), Q_g ∞ 1/a⁴, which preferentially elevates the temperature of the constriction. The raised temperature increases the resistance because of the temperature-dependent coefficient of the metal resistivity. The theory of tribology is used to explain the difference between Au/Cu pad and Au/Ni/Cu pad ACF joints. For the Au/Cu pad ACF joints, the deformation of the particles’ upper and bottom sides is nearly symmetrical; the contact between conductive particles and pad has the character of “sliding contact,” especially under high pressure. For the Au/Ni/Cu pad ACF joint, the contact between particles and pad determined the conduction characteristics of ACF joints. It has the character of “static contact.” Thus, the current-carrying capacity (or connection resistance) of Au/Cu pad joints is more sensitive to the bonding pressure.     

The contact resistance and reliability of anisotropicallyconductive film (ACF)

The effect of bonding pressure on the electrical and mechanical properties of anisotropic conductive film (ACF) joint using nickel particles and metal-coated polymer ball-filled ACFs was investigated. The contact resistance decreases as the bonding pressure increases. Contact resistance of ACF is determined by the contact area change between particles and contact substrates. Electrical conduction through the pressure engaged contact area between conductive particles and conductor substrates is the main conduction mechanism in ACF interconnection. In addition, environmental effects on contact resistance and adhesion strength such as thermal aging, high temperature/humidity aging and temperature cycling were also investigated. Interestingly, the contact resistances of the excessively bonded samples deteriorated more than those of optimally bonded ones. Increasing contact resistance and decreasing adhesion strength after harsh environmental tests were mainly due to the loss of contact by thermal stress effect and moisture absorption, and also partially due to the formation of metal oxide on the conductive particles