Fabric-based fine pitch interconnect technology using anisotropic conductive films (ACFs)

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.

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.

FOG(Flexible Printed Circuit On Glass) ACF(Anisotropic Conductive Film) bonding achieves electric conductivity interconnection by ACF material. The conductive microspheres from ACF material are trapped between ITO(Indium Tin Oxides) bumps and substrate pads, which are the fundamental cause of achieving electric conductivity interconnection. Prior work has studied the mechanism of achieving electric conductivity interconnection and the interconnection resistance calculation model, presenting the quantity and distribution of trapped conductive microspheres determine the contact resistance of FOG ACF bonding. But those studies haven't presented the mechanism of trapped conductive microspheres in LCD FOG ACF bonding. In this paper, the problem has been researched by Ripley's K function, K-S(Kolmogorov-Smirnov) test and PPP(Poisson Point Process) theory. It has been proved that the distribution of trapped microspheres in each bump belongs to the random distribution and the distribution of trapped microspheres' quantity can be calculated with PPP module in LCD FOG bonding. Finally, we have analyzed the probability of trapped certain number of conductive microspheres in different conductive microsphere density in ACF material and bumps' size, which is useful to optimize ACF bonding and improve the packaging reliability in LCD module industry.

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.

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

Transient thermal simulation was performed to analyze thermal response of the assembly process for a package using Anisotropic Conductive Film (ACF). Two assembly processes were modeled: a simplified process where the package was fixed at two different temperatures during assembly, and a detailed process where the package experienced a ramping heating process, followed by a constant temperature curing process. A 3D conjugate Computational Fluid Dynamics (CFD) study was first conducted, followed by a 3D conduction-only analysis due to the minimal effect of convection and radiation. Results from the detailed process modeling indicated that during the initial ramping, within 0.02 second, the die and nozzle head experienced a small temperature drop due to the cooling effect of the ACF material and substrate. The ACF material also displayed a steep increase in temperature after contacting the die, followed by a short decay, then ramped up again. At the end of the 10-second ramping process, the ACF reached a temperature of almost 203°C, while the die was at 206°C. During the 5 seconds of curing, all parts reached steady state in less than 2 seconds.

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

ABSTRACTNew metal powders containing organic thin film layer with multi-functional properties are prepared and used in the manufacturing of anisotropic conductive film (ACF) material for electronic package application. The organic thin film layer possesses both insulating property and thermo-compressed flowable property. The former insulating property ensures metal powder to have good insulation property at room temperature, and the latter thermo-compressed flowable property maintains the electric conductance of ACF material after high temperature compression process. Besides, the organic thin film layer also exhibited good steric hindrance to enhance the dispersion and anti-settling of metal powers in resin matrix. The functional organic groups on the surface thin film will also improve the interface adhesion between metal powder and resin matrix. Such metal powder with surface chemical modification is suitable for electronic material application, for example, anisotropic conductive film (ACF).

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.

Current mobile phones are more or less information and entertainment devices with the ability to communicate with other devices through wireless networks. In addition, it is more and more a mobile office for business user offering access to needed files and services in company's intranet to conduct daily business. To be able to perform these tasks, mobile phone needs very high performance and high functionality combined with small size and low energy consumption. It is also very important to optimize the total system cost. Utilizing different functional blocks as modules gives the designer and product marketing higher freedom to increase product variety and tailoring by using common building blocks to create a product. One of the key challenges is the physical interconnection between the modules, when number of connections increases and high performance with low interference is required. An interesting option for this interconnection dilemma is to use anisotropic conductive film (ACF) to connect the modules. ACF offers minimal form factor, high-density and low cost, as well as high reliability. Successful execution of board-to-board (flexible or rigid) interconnection using ACF needs careful consideration of design issues, materials selection, and process development. Sufficient thermal transfer to the ACF during the process is needed for proper curing. An interposer is used between the bonding head and the materials to be bonded to eliminate any issues with planarity. As the interposer has definite effect on the thermal transfer to the ACF, we have studied effects of different interposer materials and thickness to the quality of the ACF interconnection.

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.

Flip chip interconnection technology has been considered the best solution for high-density packaging of a variety of electronic devices. However, there still remain several technical obstacles. The infrastructures are not perfectly ready for die handling/assembling, die-quality assurance, and so on. Especially, an effective bare die testing/screening method is necessary for the electronics industry. The novel Anisotropic Conductive Film (ACF), described in this paper, is composed of high density pillars (linear metal material) which form sequential conductive paths in the direction of the thickness of the thermoplastic film. Developments are ongoing for creating the ultimate IC packaging system for connecting bump-less dice on printed circuit boards, under appropriate thermal pressure conditions. The material and structure of this ACF are optimized for bare die testing/screening applications. The fine pitch terminals, with Au bumps on a bare die, can be probed with reasonable pressure by using this ACF. The electrical contact performance of the ACF was improved in the structure with lower modulus resin, less thickness and optimized pillar pitch. This ACF will hopefully be utilized as an effective interconnecting material for semiconductor device testing/screening systems.

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.

This paper discusses flip chip on FR‐4 and ceramics using non‐conductive adhesive (NCA), anisotropic conductive film (ACF), or anisotropic conductive paste (ACP). Several ACF and ACP materials with different types of adhesive resin and conductive particles and one NCA material were evaluated. Flip chips were assembled on test vehicles for temperature cycling and high‐temperature high‐humidity tests. The reliability performance of the processes was compared. Flip chip processes using NCA, ACF, or ACP could give satisfactory reliability and high assembly yield for some applications, when the bonding parameters were optimised.

The flip-chip interconnection technology based on anisotropic conductive films (ACFs) has recently become an attractive solution for the assembly of micro-electromechanical systems (MEMS) and application specific integrated circuits (ASIC) in MEMS packages. In the present work, we have studied the fine pitch capability of ACF interconnects for MEMS applications such as fingerprint sensors and capacitive micromachined ultrasonic transducers, in which interconnects spread around MEMS and ASIC surface. The silicon test chips and substrates with different interconnect pitch were assembled using a single layer ACF. The electrical performance of ACF interconnects with varying pitch from 110 to 200 μm was compared. Furthermore, the distribution of conductive particles and the electrical resistance of ACF interconnects at both peripheral and central parts of the chips were evaluated. Effect of thermal shock cycling test (−40 to +125 °C) on samples was investigated. The results showed insignificant difference in the electrical performance between ACF interconnects with pitch varying from 110 to 200 μm. The particle distribution and the electrical resistance of ACF interconnects at different chip regions were similar. No significant effect of the thermal shock cycling test was observed. No failures (open/short circuit) occurred, both before and after the thermal shock cycling test.