Conductive Adhesive Films (ACFs and NCFs) Materials For Electronics Packaging Applications

Anisotropic Conductive Films (ACFs) have been widely used as excellent interconnection materials in semiconductor and display applications for chip-on-glass (COG), chip-on-flex (COF), chip-on-board (COB), flex-on-glass (FOG), flex-on-board (FOB), flex on flex (FOF) interconnections due to their fine-pitch capability, simple process, and cost effectiveness. However, less than 20 um COG and COF and less than 100 um FOF assemblies become challenging because of the electrical short between neighboring electrodes by agglomerated conductive particles in ACFs. Therefore, it is necessary to restrict the conductive particles movement during ACF bonding process, and solve the short problem at ultra-fine pitch. For this purpose, novel nanofiber ACFs have been invented by Paik et al, and successfully solved the electrical short problems at the fine pitch COG, COF, and FOF applications. For nanofiber ACFs, about 200 󼁾 500 nm diameter polymer nanofibers with coupled conductive particles are fabricated using an electro-spinning of polymer solutions mixed with conductive particles, and then nanofiber ACFs are successfully made by laminating NCFs (nonconductive films) on top and bottom side of the nanofibers with coupled conductive particles. This novel nanofiber ACFs show excellent electrical bump contact resistance, and electrical short free at ultra-fine pitch applications. Nanofiber ACFs can completely solve the electrical shortage problems at 7 μm bump-to-bump gap and 20 micron ultra-fine bump pitch of COG and COF and also less than 100 micron pitch FOF electronic packaging applications. It is because the movement of conductive particles during ACF assembly can be completely limited by the nanofiber anchoring effect resulting in about 3 times higher conductive particles capture rates compared with conventional ACFs. Detailed information of nanofiber ACFs formation, COG and COF assembly processes, COG and FOF characterization using nanofiber and conventional ACFs will be presented.

Flexible Chip-on-Flex (COF) and embedded Chip-in-Flex (CIF) packages by applying wafer level package (WLP) technology using anisotropic conductive films (ACFs)

Feasibility study of non-conductive film (NCF) for plasma display panel (PDP) application

In this study, self-fluxing additives were added in solder anisotropic conductive films (ACFs) in order to eliminate solder oxide resulting in an excellent solder wetting on metal pads during ACF bonding. Since the solder oxide causes poor wettability of solder particles, solder oxide was chemically removed by self-fluxing additives in solder ACFs. The test boards were 25 μm-thick polyimide based FPCs and 1 mm-thick FR-4 organic rigid PCBs which have 400 μm pitch Cu patterns with electroless nickel and immersion gold (ENIG) surface finish. Newly formulated solder ACFs were acrylic based adhesives film which can be fully cured above 150°C containing self-fluxing additives. The film contained 25 μm diameter Sn58Bi particles which has 138°C melting point and 8 μm diameter Ni particles as a spacer to maintain the gap between metal electrodes. According to the experimental results, the addition of self-fluxing additive caused significant improvement of solder wettability and reliability of solder ACF joints. Therefore, using a flux function added solder ACFs can be used for various applications such as FOB and FOF assemblies, and can provide an alternative interconnection method for high power and fine pitch assemblies for many other applications.

In this paper, a thermal acid generator was added as a flux activator in solder anisotropic conductive films (ACFs) to eliminate solder oxide resulting in stable solder wetting on metal pads. Since the native solder oxide causes poor wettability of the solder particles, solder oxide was chemically removed by the flux activator in solder ACFs. The test vehicles consisted of polyimide-based flexible printed circuits and FR-4 organic rigid printed circuit boards, which have 300- $\mu $ m pitch Cu pads with electroless nickel immersion gold metal surface finish. The thickness of the solder incorporated ACFs were $50~\mu $ m and it consists of epoxy-based adhesive resin, 20 wt% of 5–15- $\mu $ m diameter SAC305 (96.5Sn-3.0Ag-0.5Cu) solder particle, flux activator, and 5 wt% of 5- $\mu $ m diameter Au-coated Ni particles, which act as a spacers to maintain a stable gap between metal pads. According to the experimental results, solder wettability improved significantly, as the amount of the flux activator increased in the solder ACFs due to solder oxide removal. As a result, the flux activator caused significant improvement of electrical properties of solder ACF joints, such as contact resistance, power handling capability, and reliability. Therefore, the flux activator added solder ACFs can provide an excellent interconnection method for high-power flex-on-board and flex-on-flex assemblies.

Failure behaviors of anisotropic conductive film (ACF) and non-conductive film (NCF) interconnects were investigated by measuring the connection resistance. 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. The interconnection reliability was evaluated by multiple reflow process. The connection resistance of the ACF joints was markedly higher than that of NCF joints, mainly due to the constriction of the current flow and the intrinsic resistance of the conductive particles in ACF joints. The connection resistances of both interconnections decreased with increasing bonding force, and subsequently converged to about 10 and 1 mOmega at a bonding force of 70 and 80 N, for the ACF and NCF joints, respectively. During the reflow process, 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.

As pitch and space get finer in COG package, interconnection problems such as short circuit or open open circuit have been raised. In order to solve these problems, a novel concept of ACFs called nanofiber ACFs, a combination of nanofiber technology and electronic packaging, was introduced as one promising method in fine pitch assembly. In this study, nanofiber ACF technology was first applied to COG assembly, and nanofiber ACFs were investigated in terms of particle movement and electrical properties. For the nanofiber ACFs, polyvinylidene fluoride (PVDF) and polybutylene succinate (PBS) were used as nanofiber polymer materials. For the conductive particles, insulated conductive particles (ICPs) were used and incorporated into each nanofibers by electrospinning technology and adhesive films were laminated to fabricate nanofiber ACFs. The free movement of conductive particles was successfully suppressed in COG assembly by using PVDF and PBS nanofiber ACFs, resulting in excellent particle capture rate and electrical properties.

Due to the increasing demand for higher performance, greater flexibility, smaller size, and lighter weight in mobile electronic products, there have been growing needs of wearable electronic products in near future. Wearable watches, glasses, wrist bands, shoes, and clothes have been recently introduced in the market. In order to realize flexible wearable electronics, flexible components have to be packaged and interconnected with flexibility using flexible packaging technologies. As a one of the promising flexible packaging and interconnection technologies using flexible substrates such as polymer films and fabrics, fabrics can provide great flexibility and durability. Therefore, fabric-based fine pitch interconnection using ACFs materials will provide the solution for realizing various flexible wearable electronic products. In the electronic packaging technology for fabric-based wearable electronics, FOF(Flex On Fabric) technologies using FPCs substrates and various fabrics with fine pitch circuits using ACFs interconnection materials become very important, because ACF interconnection provides excellent electrical flexibility compared with conventional solder-or socket-based interconnection methods which have a limitation of bending and flexing.

Regarding the application of the chip on glass (COG) with anisotropic conductive film (ACF) to the liquid crystal displays (LCDs), the problems with the warpage of COG packages, interfacial delamination, and increasing contact resistance of bumps, during or after thermal and moisture loading, are major reliability issues encountered in the industry. The goal in this paper is to investigate the effect of the parameters, such as bonding pressure and temperature during manufacturing, thermal and moisture expansion of the ACF, and its elastic modulus and fillets, associated with thermal cycling (from room temperature to 85 ), on the warpages of the ACF-bonded COG packages. The full-field Twyman-Green interferometry is used for measuring the warpages of the COG packages due to the fabrication with the various bonding pressure and temperature and during thermal cycling. Three-dimensional finite-element models (FEM) are used for calculating the warpage in terms of such parameters, and those results are compared with experimental observations in order to understand the mechanics. The results show that the tremendously saddle-warped shapes are found at the COG packages after manufacturing, and their maximum warpages increase with the bonding pressure but are insensitive to the bonding temperature. However, these large warpages have been obviously relaxed as the temperature goes beyond 50 during the first thermal cycling. After the first cycling, the warpages of the COG packages become much less severe and remain stably, then slightly increase with the temperature. The deformed behaviors of the COG package under the thermal loads have been resolved by comparing the FEM results with the experimental observations. The moisture-induced expansion strain of the saturated ACF in the condition of 29 /85%RH was determined to be 0.147% by means of the combination of experimental and theoretical analyses. It is also found that, for relatively thin fillets, the coefficient of thermal expansion (CTE) and moisture absorption of the ACF do not have any obvious influence on the warpages of the COG packages, but for relatively thick fillets they do somewhat. Moreover, the warpages of the COG package affected by ACF fillet thickness, elastic modulus, and CTE have been thoroughly studied by the finite-element analysis and the results have been interpreted in detail. Overall, for reducing the warpage of the COG package, two approaches are suggested: giving the packages at 85 preheated after manufacturing and precisely controlling low mismatch of CTE between chip and glass substrate, since both play significant roles.

In this paper, a chip-in-flex (CIF) assembly that has an excellent bending performance, including a minimum bending radius without a chip fracture and the capacity to withstand dynamic bending, is developed. Chip-on-flex (COF) and CIF assemblies are fabricated using anisotropic conductive films (ACFs) as interconnection materials. The COF package is composed of 40-μm-thin silicon chips, ACF, and flexible substrates. The CIF package is fabricated by attaching a cover adhesive film and a polyimide film on the COF package to encapsulate the silicon chip. Through static bending tests, the optimal thickness of the cover adhesive film is established. The optimized CIF assembly allows a minimum bending radius of 4 mm without a chip fracture, while the chip in the COF assembly fractures at a bending radius of 10 mm. A finite-element analysis of the static bending test is performed to understand the internal stress state of the assemblies. A bending reliability test of the CIF package is also conducted at a bending radius of 7.5 mm for 160k cycles, by measuring the daisy-chain resistance during the test. The effect of the elastic modulus of the ACF resin on the fatigue endurance is investigated through the bending fatigue test. The higher modulus of the ACF resin resulted in excellent fatigue reliability with stable ACF joints showing neither delamination nor resin crazing after 160k cycles of bending.

The nonconductive film (NCF) adhesives have potentials to replace the anisotropic conductive film (ACF) adhesive in the chip-on-glass (COG) or other packages in liquid crystal displays (LCDs) due to their feasible features of finer bump pitch and cost reduction. Prior to applying the NCF to the COG packages, reliability issues, such as interfacial delamination and increase of bump contact resistance under thermal and moisture loads, have to be overcome. The objective of this paper is to investigate the effect of moisture ingression on stress field of the COG packages with NCF bonding. The coefficients of moisture diffusion and the hygro-strains of the NCF adhesive have been determined by an innovative approach of measuring the out-of-plane deformation of the bimaterial specimens during moisture absorption at the condition of 30degC/85%RH using Twyman-Green interferometry, associated with a finite element analysis and bimaterial theory. In this study, the results show that the average of the moisture diffusion coefficients and saturated hygro-strains of the NCF under 30degC/85%RH are 1.09 times 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-6</sup> mm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> /s and 1.51 times 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-3</sup> , respectively. After being validated by the diffusion theory, the moisture- transient finite element analysis reveals that the moisture absorption of the COG package under 30degC/85%RH is nearly saturated after 1000 h. It is also found that there are large tensile stresses at pad corner and mediate ones across bump joint, and these tensile stresses gradually increase with moisture absorption and up to the maximum at the state of moisture saturation. Through the parametric study by the validated finite element model, the results show that the tensile bump-joint and interfacial stresses increase nonlinearly with the NCF elastic modulus and linearly with the saturated moisture-induced strain (SMIS) of the NCFs. Furthermore, the COG package with polyimide bumps is found to have lower moisture-induced tensile stresses than that with gold bumps. It is further shown that the tensile stresses from the longitudinal-section and transverse-section models are similar only with a few percent difference in values. It can be concluded that the stress states of the NCF-bonded COG packages induced by transient moisture ingression have been fully addressed through experimental, theoretical and numerical approaches in this study.

Although cationic epoxy is reported as the best adhesive candidate for Sn–58Bi solder anisotropic conductive films (ACFs) reliability, ACFs bonding conditions have not been optimized yet. In this paper, ACFs bonding parameters, such as pressure and time at 200 °C, have been optimized to produce flex-on-board (FOB) applications with a higher reliability, using the cationic epoxy-based Sn–58Bi solder ACFs joints with 10- $\mu \text{m}$ average sizes Al2O3 spacers. It was difficult to achieve above 90% cure degree by less than 20 s at 200 °C, and Sn elements started to diffuse from micrometer-sized Sn–58Bi joints after 10 s. Both 80% and 90% cure degree can provide 520-kgf/cm peel adhesion strength. To obtain a larger amount of Sn phase remaining at solder joints for lower contact resistance and higher reliability and adhesion strength, 10 s at 200 °C bonding condition was selected. As large amounts of Al2O3 spacers were added into ACFs resin, the reliability of joints became lower. Direct contacts of two electrodes were obtained at 1.5 wt% Al2O3 spacers, however, no direct electrode contact was observed at 3 wt% Al2O3 spacers. As a summary, optimized bonding conditions at 200 °C for 10 s at 1-MPa pressure and solder ACFs with 3 wt% 10- $\mu \text{m}$ diameter Al2O3 spacers were suggested for nondirect contacts of electrodes interconnection and higher reliability of cationic epoxy-based Sn–58Bi solder ACFs joints for FOB applications.

The radio frequency (RF) and high frequency performance of the flip chip interconnects with anisotropic conductive film (ACF) and non-conductive film (NCF) was investigated by measuring the scattering parameters (S-parameters) of the flip chip modules. The effects of two chip materials, Si and gallium arsenide (GaAs), and of the metal pattern gap between the signal line and ground plane in the coplanar waveguide (CPW) on the RF performance of the flip chip module were also investigated. The transmission properties of the GaAs were markedly improved on those of the Si chip, which was not suitable for the measurement of the S-parameters of the flip chip interconnect. Extracted impedance parameters showed that the RF performance of the flip chip interconnect with NCF was slightly better than that of the interconnect with ACF, mainly due to the self inductance of the conductive particle surface and the mutual inductance between the conductive particles in the ACF interconnect.

Transmission property of flip chip package with adhesive interconnection for RF applications

The purpose of this paper is to investigate the parameters such as thermal loads, adhesive material properties and fillets, bump materials, and moisture, affecting the warpage and bump-joint stresses of the chip-on-glass (COG) packages with anisotropic conductive film (ACF). Two- and three-dimensional finite-element models (FEMs) are employed for calculating the warpage and bump- joint stresses in terms of such parameters. In addition, the real-time full-field Twyman-Green interferometry is used for measuring the out-of-plane deformations of the specimens and providing validation of the warpage calculated from finite-element models. Also, Suhir's close-form solution of a die-substrate assembly is applied for looking into insight of thermomechanics, especially for adhesive stresses and correlated with finite-element results. The results show that the fillets strongly affect the warpage of the COG specimens by changing the convex shape of warpage for those without fillets to the saddle shape for those with fillets. However, unlike those with fillets, the warpage of the COG is insensitive to those with or without (gold or polymer) bumps. It is also found that the two-dimensional (2-D) FEM in consistence with Suhir's solution can be used for approximately addressing both warpage and bump-joint stress issues. The COG warpage results for thermal loads, from 2-D FEM, indicate that the maximum warpage is very sensitive to the modulus of the ACFs (especially for low modulus) rather than the thermal expansion coefficients of the ACFs, which are consistent with Suhir's solution. The stress results show that the COG with gold bumps provides higher bump-joint stresses (especially for compressive transverse normal stress, which is important for maintaining good electrical contact between bumps and tracks through the conductive particles in the ACF) than that with polymer bumps for the isothermal loaded case. Note that negative isothermal load here applied represents the cooling temperature change from an uniform elevated temperature of the ACF curing in the COG packaging. Furthermore, the transverse normal bump-joint stress increases three fold, when the fillets are present for the COG either with polymer bumps or with gold bumps. The similar mechanism is found in the case of the COGs under thermal-gradient loads. It is also seen that the higher the elastic modulus (E) and thermal expansion coefficient (CTE) of the ACF, the larger the compressive transverse normal bump-joint stress. It is also interestingly found that the volume swelling of the ACF due to moisture absorption provides very relatively large tensile transverse normal bump-joint stresses for the COGs with bumps. These tensile stresses could be one order of magnitude larger than the compressive values resulted from other parameters discussed here. As a result, the approaches to well controlling ACF moisture absorption and selecting low-moisture-expansion ACF materials are very critical to improving the reliability of the COG packages