Pixel detector hybridisation with anisotropic conductive films

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.

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.

Anisotropic conductive films (ACFs) are widely used for circuit interconnection due to their easy use, low temperature bonding, higher precision than soldering and eco-friendliness. However, current ACFs are generally prepared by randomly distributing conductive particles into suitable resins. The ACFs prepared by this approach have risks to result in shortcut when applied for high precision bonding (<100 μm). In order to alleviate this problem, we designed and prepared a new kind of ACFs with conducting particles well aligned in adhesive film, which is named as array-patterned ACFs (A-ACFs). A template with 12 μm periodic microcavities was prepared and used to load 5.4 μm silver-coated polystyrene particles. Through a series of process optimizations including particles-filling cycles and particles-transferring-pressure/temperature into the used polyurethane (PU) adhesive, well-aligned particles with a spacing of 6.6 μm in the PU film was obtained. Such prepared A-ACFs were used to bond two flexible printed circuits (FPC) not only with a spacing of 200 μm (FPC-200) but also with 40 μm (FPC-40). The bonding conditions including temperature and pressure for the FPC-200 connections were investigated in detail. The connecting resistance, insulation resistance, peeling force, and the particles’ morphologies between the bonded FPCs were investigated. The reliability of the two bonded FPCs were tested under 85 °C and 85% relative humidity. Results showed that the new kinds of A-ACFs are suitable for achieving high precision circuits bonding and show better accuracy than those of traditional ACFs (T-ACFs). Thus, this study might have new insight for designing A-ACFs and great potential for applications in high-precision devices.

Future collider experiments place demands on hybrid pixel detectors, particularly in terms of interconnect reliability across a range of die sizes and fine-pitch geometries. In development workflows based on Multi-Project Wafer (MPW) submissions, support for single-die assembly is crucial. To address these challenges, new hybridization methods have been introduced through collaborative efforts at CERN and within the AIDAinnova framework. One such approach utilizes Anisotropic Conductive Film (ACF) combined with Electroless Nickel Immersion Gold (ENIG) surface treatment to form interconnections without conventional solder bumps. Instead, conductive microparticles embedded in an adhesive layer are compressed under heat and pressure to establish electrical contact. This technique has demonstrated excellent yield and mechanical stability, especially for pitches below 100 µm, making it a strong candidate for advanced detector integration in high-energy physics applications.

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.

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.

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.

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.

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.

As demands for stretchable electronics have increased in the field of wearable devices, liquid metal, such as eutectic Gallium-Indium (eGaIn), has gained much attention due to its metallic conductivity with liquid reconfigurability. Although various applications have been suggested using eGaIn, an electrical connection has remained as a technique challenge. Wires have been directly inserted into the microfluidic channel filled with eGaIn, resulting in electromechanically unstable connection, bulky size, and time consuming fabrication steps for the electrode part. In this study, a novel solution for the electrode is proposed, connecting eGaIn wires directly to the metal electrode based on direct writing of eGaIn. The two electrode materials were considered as candidates, including electroless nickel immersion gold (ENIG) and immersion tin (Im-Sn) plated surfaces. Among them, only the ENIG-finished surface had stable electrical connection with eGaIn, allowing sufficiently low contact resistance 0.1Ω The suggested electrode part was mechanically durable under strain up to 100 %. As an application, a sensing skin embedding 10 sensing units was fabricated based on direct ink writing, using a flexible flat cable finished by ENIG plating.

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.

A test setup is developed, measured, and modeled to examine the insertion loss effects of Electroless-Nickel Immersion-Gold (ENIG) plating on various PCB topologies from 1 to 50 GHz. ENIG plating is observed to increase insertion loss compared with unplated samples nonlinearly in a manner that generally increases with frequency. Three main topolgies are analyzed; a 0.005 (0.127 mm) thick microstrip circuit using RT/duroid® 6002 laminate, a 0.008 (0.203 mm) thick microstrip using RO4003C™ laminate, and a 0.008 (0.203 mm) thick Grounded Co-Planar Waveguide (GCPW) using RO4003C™ laminate.

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

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.