Array-Patterned Anisotropic Conductive Films for High Precision Circuit Interconnection

For several decades, non-conductive pastes (NCPs) have been widely used for the mass production of ultrasound transducers assembly using piezoelectric ceramics and FPCs. The NCPs interconnection is established through direct metal to metal contact. In more details, the surface of piezoelectric ceramics is roughly grinded to make point contacts before metallization. And the NCPs are filled in non-contacted area between metallized piezoelectric ceramics and metal electrodes of FPCs. However, the point contacts result in higher electrical resistance. Also, the electrical conduction and the reliability of NCP interconnection can be deteriorated especially in moisture and at high temperature environment due to the polymer expansion. In addition, the piezoelectric ceramics such as PZT cannot maintain the polarization above its Curie temperature. Therefore, the curing temperature should be below 150°C. Therefore, longer curing times at 150°C are required for NCPs to avoid the depolarization of piezoelectric ceramics. Furthermore, sometimes surface grinding of piezoelectric ceramics may produce cracks which lead to the reduction of production yield. Therefore, it is desirable for lowering the electrical resistances, increasing the reliability and shorter curing time without grinding process. As a result, low temperature anisotropic conductive films (ACFs), which consist of adhesive polymer resin and conductive particles, were introduced in order to increase the electrical conduction and reliability without grinding process. In this study, various ACFs, including 3 types of conductive particles (Sn58Bi solder, Au/Ni coated polymer and Ni balls) and 3 types of thermosetting polymer resins (cationic epoxy, imidazole epoxy and acrylic resins), were investigated with respect to mechanical/electrical properties and reliability. In order to lower the ACFs bonding temperature below 150°C, low melting temperature eutectic Sn58Bi solder particles with the melting point of 138°C was used. For the thermo-compression bonding, the piezoelectric ceramics were placed at the bottom and flexible printed circuits boards (FPCBs) were placed on the top of piezoelectric ceramics to guarantee the real temperature of the piezoelectric ceramics below 150°C. As a result, piezoelectric ceramics and metal electrodes interconnection was successfully performed below 150°C with stable contact resistance and solder joint formation even after reliability test and the dicing process. After dicing into FPCBs, no short circuit was found between neighboring electrodes. And ACFs with Sn58Bi solder particles and cationic epoxy resin showed the lowest electrical resistance after bonding, excellent mechanical and electrical performance among various ACFs.

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

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.

Flexible Printed Circuit (FPC) boards are being widely used for a number of applications to enable products in a three dimensional format thereby utilizing the “dead” space within the product envelop. During the past few years, the usage and complexity of FPC made of polyimide, polyester or Teflon have grown substantially and are expected to continue to grow even more in the next coming years. In the past these FPC's have been connected to other FPC's or to a PCB with help of mainly connectors but the interest of replacing these connectors with a lower cost and smaller footprint connections with help of Anisotropic Conductive Film (ACF) is being widely explored. This paper will report the development of fine pitch ACF bonding down to 0,2mm and the mechanical and thermo-mechanical reliability of the ACF interconnects. The influence of difference process parameters and ACF materials on the ACF process yield will be discussed as well.

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.

Effects of bonding pressure on the thermo-mechanical reliability of ACF interconnection

Due to the increasing demand for higher performance, greater flexibility, smaller size, and lighter weight in mobile and wearable electronic products, there have been growing needs of various electronic packaging products and fine-pitch interconnection technologies. To realize various state-of-art mobile and wearable electronic products, fine pitch and flexible packaging & interconnection technologies are needed. As one of the promising fine pitch and flexible packaging and interconnection technologies, conductive adhesive films materials such as ACFs(Anisotropic Conductive Films) and NCFs(Non Conductive Films) are widely used now. In general, ACFs have two technical limitations such as (1) ultra-fine pitch shortage free interconnection and (2) higher current handling capability. For (1) ultra-fine pitch applications without electrical shortage, new concept of Nano-fiber ACFs and APL(Anchoring Polymer Layer) ACFs have been successfully invented by KAIST both for less than 20 micron pitch COG(Chip On Glass), COP(Chip On Polymer). and COF(Chip On Flex). And they can be also used for less than 50 micron pitch FOF(Flex On Flex) and FOB(Flex On Board) applications. In addition, for (2) higher current handling capability applications, new solder ACFs have been also introduced by KAIST to replace the conventional metal particles based ACFs interconnection. By solder ACFs, 30% low contact resistance, 4 X higher current handling capability, and excellent reliability were successfully achieved compared with conventional ACFs. Furthermore, ACFs materials-based packaging and interconnection method can provide the flexible interconnect solution for OLED(Organic LED) COP and COF/CIF(Chip In Flex) packages to realize wearable electronic products.Recently the 3-D die chip stacking using the Through silicon via (TSV) technology has been widely used for stacking memory and ASIC chips. In the 3D-TSV vertical interconnection, Cu pillar/Sn-Ag eutectic solder bump is one of the promising bonding method. After the 3D-TSV chips are interconnected with the bumps, the gap between stacked chips should be filled with underfill materials. However, capillary flow underfill between 3D-TSV stacked chips become difficult because of increased chip/wafer area, decreased gap height, and multiple chip/wafer stacking. Therefore, new bonding method using NCFs materials, which performs both Cu-pillar/Sn-Ag flip chip bump to metal pads interconnection and underfils at the same time has been extensively used.In this presentation, the ACFs materials for ultra-fine pitch and higher current carrying interconnection and the NCFs for 3D-TSV chip stacking applications will be introduced.

Hybrid pixel detectors require a reliable and cost-effective interconnect technology adapted to the pitch and die sizes of the respective applications. During the ASIC and sensor R&D phase, and in general for small-scale applications, such interconnect technologies need to be suitable for the assembly of single-dies, typically available from Multi-Project-Wafer submissions. Within the CERN EP R&D programme and the AIDAinnova collaboration, innovative hybridisation concepts targeting vertex-detector applications at future colliders are under development. This contribution presents recent results of a newly developed in-house single-die interconnection process based on Anisotropic Conductive Film (ACF). The ACF interconnect technology replaces the solder bumps with conductive particles embedded in an adhesive film. The electro-mechanical connection between the sensor and the read-out chip is achieved via thermo-compression of the ACF using a flip-chip device bonder. A specific pad topology is required to enable the connection via conductive particles and create cavities into which excess epoxy can flow. This pixel-pad topology is achieved with an in-house Electroless Nickel Immersion Gold (ENIG) plating process that is also under development within the project. The ENIG and ACF processes are qualified with the Timepix3 ASIC and sensors, with 55 µm pixel pitch and 14 µm pad diameter. The ACF technology can also be used for ASIC-PCB/FPC integration, replacing wire bonding or large-pitch solder bumping techniques. This contribution introduces the ENIG plating and ACF processes and presents recent results on Timepix3 hybrid 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.

This study conducts an experimental investigation into the bending behavior and electrical resistance properties of Flex (PI with Cu)-On-Film (ITO–coated PET) packages considering the flexible effects under static bending loads with sequential bending cycles. The FOF packages, consisting of flex (with fine-pitch Cu circuits) and film (ITO–coated PET plastic) are bonded by Anisotropic Conductive Film (ACF). During the ACF bonding process, the deformed conductive particles in the adhesive matrix between the Cu electrodes and the ITO film under the effects of the compression force, and these broken conductive particles will play an electrical conductive role. In the subsequent bending tests (loading &unloading with many cycles), the conductive particles experience a compression or tension stress, which causes a further propagation of the cracks formed in the original packaging process. The results show the measured electrical resistance in the specified conductive channel increases with the bending cycles increasing, and the microscopic observations show that the higher electrical resistance is the result of a smaller overall contact area between the conductive ACF particles and the Cu electrode/ITO film due to fatigue crumbling of the particles.

Reliability of chip on glass module fabricated with direct printing method

The performance of flexible substrates for lead-free applications was studied using Finite Element Method (FEM). Firstly, the thermal induced stress in the flex substrate during the lead free solder reflow process was predicted. The shear stress at the interface between the copper track and flex was plotted. This shear stress increases with the thickness of the copper track and the thickness of the flex. Secondly, an anisotropic conductive film (ACF) flip chip was taken as a typical lead-free application of the flex substrate and the moisture effect on the reliability of ACF joints were studied using a 3D macro-micro modeling technique. It is found that the time to be saturated of an ACF flip chip is much dependent on the moisture diffusion rate in the polyimide substrate. The majority moisture diffuses into the ACF layer from the substrate side rather than the periphery of the ACF. The moisture induced stress was predicted and the predominant tensile stress was found at the interface between the conductive particle and metallization which could reduce the contact area and even cause the electrical failure.

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

The degree-of-cure of an anisotropic conductive film (ACF) was theoretically predicted and experimentally measured to investigate the effect of the degree-of-cure of the ACF on the electrical and mechanical stability of ACF joints and the reliability of COF assemblies. The cure reaction of the ACF observed by isothermal DSC analysis followed an autocatalytic cure mechanism, and the degree-of-cure of the ACF as functions of time and temperature was mathematically derived from an autocatalytic cure kinetics model. To accurately simulate the ACF temperature field during COF bonding process, the thermal properties of the ACF such as thermal diffusivity (alpha), specific heat capacity (Cp), and thermal conductivity (lambda) were experimentally characterized. The degree-of-cures of the ACF as a function of bonding times during COF bonding process were theoretically predicted by the incorporation of the autocatalytic kinetics modeling and the ACF temperature simulation. The predicted degree-of-cures of the ACF were well matched with the experimental data measured by an ATR/FT-IR analysis. The contact resistances of ACF joints and the peel adhesion strengths of COF assemblies were measured to evaluate the electrical and mechanical interconnection stability. According to the results, the contact resistances decreased and the peel adhesion strength increased as the degree-of-cure increased. In addition, to investigate the effect of the degree-of-cure of the ACF on the reliability of COF assemblies, the 85degC and 85% relative humidity (85degC/85% RH) test was performed. The results showed that the reliability of COF assemblies also strongly depends on the degree-of-cure of the ACF.

Flip chip attachment on flexible LCP substrate using an ACF