Solder Anisotropic Conductive Films Research Articles

A Study on Polyvinylidene Difluoride (PVDF) Anchoring Polymer Layer (APL) Solder Anisotropic Conductive Films (ACFs) for Fine-Pitch Flex-on-Flex (FOF) Interconnection

Fine-pitch flex-on-flex (FOF) assembly has been introduced for various electronic packaging applications because electronic products require higher functionality, flexibility, and I/O density. In this article, novel polyvinylidene difluoride (PVDF) anchoring polymer layer (APL) solder anisotropic conductive films (ACFs) were investigated for fine-pitch FOF applications. The capture rates of conductive particles and the insulation rates between neighboring electrodes have been significantly enhanced using PVDF APL solder ACFs as a result of suppressing the movement of conductive particles by the high tensile strength of the PVDF APL layer. For good solder wetting on electrodes, flux activators were added into the PVDF APL layer to remove the solder oxide layer. However, the tensile strength of the PVDF layer decreased as the flux activator content in the APL increased. The capture rates of conductive particles decreased as more flux activators were added because of the lower tensile strength of PVDF. Furthermore, optimized flux activator content in ACFs with 0.5 wt% in the APL and 2-wt% flux activators were also added in non- conductive films (NCFs) to obtain good solder wetting. Using the optimized APL solder ACFs, excellent contact resistances and insulation resistances were obtained before and after a pressure cooker test (PCT) and 85 °C/85% relative humidity (RH) reliability tests. As a conclusion, the PVDF APL solder ACF assembly can provide an excellent interconnect solution for fine-pitch FOF assemblies used for portable electronic product applications.

Low-Temperature Bonding of PZT (PbZrTiO3) and Flexible Printed Circuits Using Sn52In Solder Anisotropic Conductive Films for Flexible Ultrasonic Transducers

Solder anisotropic conductive films (ACFs) consist of conductive solder particles, polymer resin, and flux activator. The flux activator removes solder oxide of solder particles by generated acid for the solder wetting during a thermocompression bonding process. In this article, eutectic Sn52In solder (Tm = 118°) particles were used as the conductive solder particle in the solder ACFs to reduce <130° ACFs’ bonding temperature. Novel Sn52In solder ACFs were fabricated using a comma-roll film coater and used to interconnect Au-coated PZT (PbZrTiO3) and flexible printed circuits (FPCs). PZT and FPCs interconnects using Sn52In solder ACFs were successfully demonstrated with the formation of good solder joints and reliable electrical resistances. According to the energy-dispersive spectrometer (EDS) line scan analysis, excellent solder wetting was confirmed by the presence of Au–In and Ni–Sn intermetallic compounds (IMCs) at the PZT and FPCs interface. Sn52In solder ACFs’ joints showed no open-circuit failure due to the excellent metallurgical bonding during a pressure cooker test (100% relative humidity and 2 atm at 121°) and a dynamic bending reliability test (the minimum bending radius of 7 mm). Therefore, Sn52In solder ACFs’ bonding can provide excellent metallurgical interconnection for flexible ultrasonic transducer packaging.

Piezoelectric Ceramics and Flexible Printed Circuits’ Interconnection Using Sn58Bi Solder Anisotropic Conductive Films for Flexible Ultrasound Transducer Assembly

Interconnection technology using Sn58Bi solder anisotropic conductive films (ACFs) has been chosen to assemble piezoelectric ceramics on flexible printed circuits for ultrasound imaging applications. The bonding and dicing processes were performed, and the bending of test samples was evaluated. The results showed excellent integrity of solder ACFs’ interconnects compared with other Ni-/Au-coated polymer ball and Ni ball-based ACFs’ interconnects. The metallurgical interconnects of the solder ACFs produce lower electrical resistances after pressure cooker and bending reliability tests. Moreover, the solder ACFs with a high-speed bonding process resulted in more electrically conductive, more reliable, and finer-pitch bonding for producing higher process yield and higher density ultrasound transducer arrays.

Effects of Polyacrylonitrile Anchoring Polymer Layer Solder Anisotropic Conductive Films on the Solder Ball Movement for Fine-Pitch Flex-on-Flex (FOF) Assembly

In this paper, anchoring polymer layer (APL) solder anisotropic conductive films (ACFs) combined with an ultrasonic (US) bonding method are newly introduced. The effects of APL ACFs properties on conductive particle movement and interconnection stability for ultrafine-pitch chip-on-glass applications were reported earlier. It was found that the APL structure not only suppresses the conductive particles movement during the ACFs bonding process for ultrafine-pitch applications, but also prevents electrical short-circuit formation between neighboring bumps. However, to achieve a stable electrical interconnection, APL ACFs require additional process steps such as an oxygen plasma etch process to remove the APL polymer skins surrounding the conductive particles using a thermocompression bonding method. However, during the US bonding process, it was found that the polyacrylonitrile (PAN) polymer skin coated on conductive particles was successfully removed by vertical directional US vibration. Therefore, stable and metallurgical solder joint formation can be achieved without an additional oxygen plasma etching process.

Effects of Flux Activator Addition in Nanofiber/Solder Anisotropic Conductive Films (ACFs) on the Solder Wettability of Flex-on-Flex (FOF) Assembly

The advanced electronic packaging trends are changing to be more light weighted and miniaturized. Demands for flexible packaging are rapidly growing due to the rapid growth of wearable electronic industries, and advanced flexible interconnection methods are required. In order to satisfy these needs, fine pitch flex-on-flex (FOF) assembly has been introduced. FOF assembly can provide smaller package size, lower contact resistance, higher power handling capability, and flexibility. Anisotropic conductive films (ACFs) are interconnection materials, which have been widely used for the FOF assembly due to their simple and low-temperature processes. Previously, nanofiber ACFs were introduced by our research group, which showed remarkable capability of enhancing the conductive ball capture rate and preventing short circuit in the fine pitch assembly by suppressing the movement of conductive particles during the bonding process. Conventionally, an ultrasonic bonding method was used for a solder ACF bonding, because native solder oxide layer around solder balls can be physically broken by applying vertical ultrasonic vibrations, resulting in excellent solder metallurgical joints between electrodes. However, there are potential misalignment and damage issues generated by vertical ultrasonic vibrations. Therefore, the alternative bonding method is needed that can remove solder oxide layer to obtain excellent solder wetting without misalignment and damage issues by the ultrasonic bonding method. In this paper, thermocompression (TC) bonding and nanofiber/solder ACFs with addition of flux activators were used to chemically eliminate solder oxide. For the formation of stable intermetallic compounds, the contents of flux activators in nanofiber/solder ACFs were optimized. To investigate the flux effect during the TC bonding process, cross-sectional transmission electron microscopy was performed to analyze whether solder oxide layer is certainly removed by the flux. With the TC bonding and nanofiber/solder ACFs with addition of flux activators, stable solder wetting and excellent alignment were demonstrated for the fine pitch FOF assembly.

A Study on the Bonding Conditions and Nonconductive Filler Contents on Cationic Epoxy-Based Sn–58Bi Solder ACFs Joints for Reliable Flex-on-Board Applications

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.

Joint Morphologies and Failure Mechanisms of Anisotropic Conductive Films (ACFs) During a Power Handling Capability Test for Flex-On-Board Applications

In this paper, joint morphologies and failure mechanisms of anisotropic conductive films (ACFs) during a power handling capability test for flex-on-board applications were investigated in terms of four conductive particles: gold-coated polymer ball, Ni metal ball, SnBi58, and Sn–3Ag–0.5Cu particles. It was found that all the polymer ball and Ni ball joints could not handle the ACFs’ joint temperature of 200 °C presumably due to polymer resin expansion and physical contacts loss, and their corresponding contact resistance also became open circuit after the power test. In contrast, the failure mechanism of SnBi58 solder joint was that when temperature was over 138 °C and Sn element was quickly consumed by Sn-Ni intermetallic compounds resulting in brittle Bi-rich phase remaining, more severe crack occurred along the Bi-rich phase of SnBi58 joints. Sn–3Ag–0.5Cu solder ACF joints due to a higher solder melting point were the best among the four conductive particles in terms of the little crack formation in its joint morphology and the morphology and the remaining stable electrical resistance even after the highest power test to 300 °C, when FR4 printed circuit board burned. Beyond the FR-4 decomposition at 300 °C, ACF joints were burned due to epoxy resin decomposition from 330 °C.

Effect of Nanofiber Orientation on Nanofiber Solder Anisotropic Conductive Films Joint Properties and Bending Reliability of Flex-on-Flex Assembly

Anisotropic conductive films (ACFs) have been widely used as an interconnection adhesive material due to its light weight and simple and low-temperature assembly processes. However, because of the higher demands for further miniaturization, short-circuit problem of ACFs interconnection has been a major issue, when it comes to very fine pitch assembly. Also, due to the fast development of wearable devices, demands for flexible packaging as well as flexible interconnection methods such as flex-on-flex (FOF) are growing more, where the bending characteristics are important. Nanofiber incorporated ACFs were previously introduced by our research group. The nanofiber incorporated ACFs showed excellent conductive particle movement suppression capability that not only prevents short circuit but also improves the conductive ball capture rate, which eventually improves the joint reliability of fine pitch FOF assembly. In order to maximize the conductive particle movement suppression capability of the nanofiber, nanofiber was oriented using a drum-type receiver. In addition, the bending reliability of FOF assembly using nanofiber/solder ACFs with different nanofiber orientations has been also investigated. As a result, parallel nanofiber/solder ACFs showed excellent joint properties as well as bending reliability due to the stable metallurgical solder joint formation and high conductive particle movement suppression capability, and this new type of ACF technology will provide a promising solution for future flexible electronic packaging.

A Study on the Failure Mechanism and Enhanced Reliability of Sn58Bi Solder Anisotropic Conductive Film Joints in a Pressure Cooker Test Due to Polymer Viscoelastic Properties and Hydroswelling

A study of the failure mechanism and the enhanced reliability performance of SnBi58 solder ACF joints were investigated by considering the total influence of polymer viscoelastic and hygroscopic properties on the cross-sectional morphologies and the electrical properties of Sn58Bi solder joints in a pressure cooker test (PCT) for 120 h. The Sn58Bi solder failure mechanism is demonstrated that, when polymer resin absorbed moisture, the Tg values of polymer resins decreased, and the coefficient of thermal expansion (CTE) values of polymer resins increased, due to a hydroswelling effect. Considering the total influence of polymer viscoelastic and hygroscopic properties on the cross-sectional morphologies and the reliability of Sn58Bi solder joints in a PCT for 120 h, four typical polymer resins were evaluated. As a result, cationic epoxy (128 ppm/°C) with the smallest CTE mismatch (112 ppm/°C) with SnBi58 solder joints (16 ppm/°C) in a PCT aging test (121°C, 100% humidity, and 2 atm) showed a stable electrical property in terms of joint contact resistance and remained a complete cross-sectional Sn58Bi solder joint morphology after PCT reliability for 120 h.

Effects of Bonding Pressures and Bonding Temperatures on Solder Joint Morphology and Reliability of Solder ACF Bonding

In this paper, in order to improve the reliability of anisotropic conductive film (ACF) interconnections, solder ACF joints were investigated in terms of solder joint morphology. ACFs are film-type interconnection adhesive materials that consist of polymer adhesive resins and randomly dispersed conductive particles. Recently, in order to obtain high reliability of ACF joints, solder ACFs that use solder particles as conductive particles of ACFs have been introduced combined with an ultrasonic bonding method. However, further researches are needed in the area of crack initiation and the propagation of solder ACF joints at high temperature and high humidity conditions due to the stress generated by hygroscopic expansion of ACF resin. Therefore, in order to solve the crack initiation of solder ACF joints, solder ACF joint morphology should be controlled and optimized to minimize the solder crack-related reliability problems. In this paper, solder ACF joint reliability was investigated depending on the solder morphologies of ACF joints bonded with various bonding pressures and temperatures. According to the results, as bonding pressure increased from 2 to 6 MPa, aspect ratio (joint diameter/joint gap) increased due to the increased joint area and decreased joint gap. In the pressure cooker test (PCT) reliability tests, as solder aspect ratio increased, electrical resistances were more stable after 60 h of the tests due to higher joint strength. In the target bonding temperature profile of 250 °C, solder joints showed a concave shape. However, joints bonded at in the target bonding temperature profile of 200 °C showed a convex shape. It is mainly due to the lower degree of cure of resin when ACF temperature reached to solder Melting Point (MP) in the target bonding temperature profile of 250 °C compared with that in the target bonding temperature profile of 200°, because ACF temperature reached to solder MP in shorter time in the target bonding temperature profile of 250 °C. Concave-shaped solder joints showed higher PCT reliability than the convex-shaped solder joints due to the lower stress concentration. These results indicate that solder ACF joint morphology was a significantly important factor for highly reliable solder ACF joints in high temperature and high humidity reliability conditions.

Effect of Flux Activators on the Solder Wettability of Solder Anisotropic Conductive Films

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.

A Study on the Solder Ball Size and Content Effects of Solder ACFs for Flex-on-Board Assembly Applications Using Ultrasonic Bonding

In this paper, the electrical and reliability properties of various solder anisotropic conductive film (ACF) joints for flex-on-board (FOB) assembly using ultrasonic bonding were assessed in terms of short circuits, contact resistance, current handling capability, wetted solder areas, and reliability evaluation. It was found that the wetted solder areas increased and reliability was enhanced as the size and content of the solder ball increased. Regardless of solder ball size and content, the contact resistances of solder ACF joints were almost the same, because the current went through the electrode/electrode contact, which is like a short circuit to the solder joint. In addition, the exact value of current handling capability of all the ACF joints could not be evaluated, because metal lines of printed circuit board were burnt before ACF joint failure. After 120 h of pressure cooker test (PCT), the open-circuit failures were mainly observed at Ni ACFs, and 5-15-μm solder ball size from 10 to 30 wt% solder content and 25-32-μm size and 10 wt% solder content of solder ACFs, presumably due to lower wetted solder areas on metal electrodes. In addition, the crack was noted after 120 h of PCT in the solder ACF joints who has lower wetted solder areas. As a result of this paper, the solder ball size and content of solder ACFs joints were optimized at 25-32-μm sizes and above 30 wt% content for 300-μm pitch FOB assembly.

Ultrasonic-Assisted Thermocompression Bonding Method of Solder Anisotropic Conductive Film Joints for Reliable Camera Module Packaging

In this paper, to improve the reliability of anisotropic conductive film (ACF) joints in the camera module packaging without any mechanical damages, ultrasonic-assisted thermocompression (TC) bonding was investigated using solder ACFs. This technology uses small lateral-direction ultrasonic vibration during the TC ACF bonding to break solder oxide layers for sufficient solder wetting on the electrodes at ACF joints. Bonded solder ACF joints were investigated in terms of solder ACF joint morphology, resistance, adhesion strengths, and reliabilities. In addition, the stability of camera module components after bonding process was also observed. Ultrasonic-assisted TC bonded solder ACF joints showed excellent solder wetting compared with conventional TC bonded solder ACF joints due to broken solder oxide layers by applied ultrasonic vibration. The Au contents diffused from electroless nickel immersion gold-finished Cu pads to solders was about 12 atomic% in the ultrasonic-assisted TC bonding. The Au contents were three times higher than that of the solders in the conventional TC bonding. In the ultrasonic-assisted TC bonding, ultrasonic amplitude was optimized. At the optimized amplitude, solders were well wetted and showed no mechanical damages of camera module components. Adhesion strength of ultrasonic-assisted TC bonded solder ACF joints was almost the same as that of TC bonded ACF joints due to fully cured resins in these samples. During the unbiased autoclave test (121 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">°</sup> , 2 atm, 100%RH), ultrasonic-assisted TC bonded ACF joints showed no open-circuit failure compared with conventional TC bonded Ni ACF joints due to metallurgical alloy joints between solder balls and electrodes by sufficient solder wetting. These results prove that ultrasonic-assisted TC bonding using solder ACFs can be applied for highly reliable ACF assembly in the camera module packaging without any mechanical damages.

Ultrasonic Bonding of Anisotropic Conductive Films Containing Ultrafine Solder Balls for High-Power and High-Reliability Flex-On-Board Assembly

New solder anisotropic conductive films (ACFs) consist of a thermosetting polymer resin and fine solder balls instead of the conventional metal particles or metal-coated polymer particles. These solder balls have lower melting temperature than conventional metal conductive particles, which enable them to be melted during ultrasonic (US) bonding, and form intermetallic alloy joints with metal pads of flex on board. In this paper, excellent solder ACF joints are demonstrated using a US-bonding method for high-power and high-reliability flex-on-board (FOB) assemblies. Ultrasonically bonded solder ACF joints were characterized in terms of their electrical properties and reliability. Solder alloy bonding was achieved using two kinds of solder balls, i.e., Sn-58Bi and SAC305 (96.5Sn-3.0Ag-0.5Cu). At the same time, the acrylic ACF resin was completely cured after 5 s of US bonding. Solder alloy ACF joints show about 20% decrease in daisy-chain electrical resistance and 100% increase in the current-carrying capability compared with conventional physical-contact-based Ni ACF joints. Solder alloy ACF joints also show significantly improved reliability with stable electrical resistances up to 24 h in an unbiased autoclave test, whereas conventional Ni ACF joints show severe electrical open failures within 4 h.

High Power and High Reliability Flex-On-Board Assembly Using Solder Anisotropic Conductive Films Combined With Ultrasonic Bonding Technique

To realize an ultrathin and fine-pitch flex-on-board module assembly with high power handling capability and excellent reliability, ultrasonic (US) bonding technology with solder anisotropic conductive films (ACFs) was successfully demonstrated. This technology provides fluxless soldering within a polymer adhesive matrix on metal electrodes using solder ACFs combined with the room temperature US bonding technique. According to experimental results, the temperature of the solder ACF joints showed rapid heating rates up to 400°C/s and reached at above 250°C under US vibration. The ACF temperature could be precisely controlled from 75°C to 260°C by adjusting US vibration amplitudes from 4 to 13 μm. At above the melting temperatures of solder particles, US-bonded solder ACF joints showed stable solder metallurgical joints alloyed with metal electrodes and more than 90% curing of ACFs within 5 s bonding time. At the same time, US-bonded solder ACF joints showed about 30% reduced electrical joint resistances, more than 90% improved current handling capability and significantly enhanced reliability in an unbiased autoclave test (121°C, 2 atm, 100%RH) compared with conventional Ni ACF joints. The significance of this paper is that the fluxless solder metallurgical joining and adhesive bonding can occur at the same time, and realizes not only enhanced ACF joint properties but also reduced ACF bonding time within 5 s at room temperature.