Shuye Zhang

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.

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.

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.

Effects of Cooling Processes and Silica Filler Contents of Solder ACFs (Anisotropic Conductive Films) on the Joints Reliability

The effects of cooling processes of the hot bar lift-up time and the silica filler content of solder ACFs polymer resin on the crack formation at SnBi58 solder ACFs joint were investigated to obtain higher solder ACFs joint reliability. As a summary, complete removal of the solder crack at acrylic based ACF joints could be obtained by delaying hot bar lift-up time, until the temperature was cooled below the glass transition temperature of acrylic resin during cooling process, or adding more than 10 wt% of silica filler content into acrylic resins. Moreover, very reliable solder joint morphology and stable joint contact resistance were obtained after 1000 cycles T/C reliability due to removing solder cracks completely.

The comprehensive effects of visible light irradiation on silver nanowire transparent electrode

The silver nanowire (AgNW) transparent electrode is one of the promising components for flexible electronics due to its high electrical and thermal conductivity, optical transparency and flexibility. However, the application of the AgNW electrode with an improved performance is generally limited by its poor long-term stability. As the name suggests, the transparent electrode is usually exposed to visible light in various applications. Unlike other electrode materials, AgNWs show unique and complicated behavior under long-term visible light illumination. In this study, the comprehensive effect of visible light irradiation on the AgNW transparent electrode is initially investigated in detail. Light irradiation induces the migration of silver to enhance the nanowire contacts while also leading to the generation and growth of particles and diameter loss in the nanowire. Light irradiation accelerates the sulfidation and oxidation of the AgNWs as well, resulting in the emergence of degradation products on the nanowire surface. All these effects influence the sheet resistance of the AgNW electrode during light illumination. The light-induced change of sheet resistance also relates to the nanowire concentration due to the sensitivity of the wire-wire contact resistance near the percolation threshold. It is believed that this work will be a valuable reference for the design, processing and application of transparent electrodes used in numerous optoelectronic devices.

Facile and highly efficient fabrication of robust Ag nanowire–elastomer composite electrodes with tailored electrical properties

The fabrication of Ag nanowire (AgNW) patterns on a working substrate is a critical step to prepare flexible/stretchable electronics. The conventional method of making AgNW patterns is based on a 2-D mask, which suffers some serious drawbacks, such as low material usage, difficulty in preparing complex patterns, and susceptibility to damaged pattern edges. To overcome these drawbacks, this study proposes the use of a 3-D mask, which when combined with a vacuum filtration system, allowed efficient fabrication of complex AgNW patterns with clear edges. In addition, we controlled the microstructure of the patterned AgNW/polydimethylsiloxane (PDMS) electrodes transferred from the membrane filter, obtaining stretchable electrodes with anisotropic electrical properties. Moreover, we greatly improved the stability and reliability of the patterned electrodes in long-term stretch/release tests by developing a PDMS/AgNW/PDMS sandwich structure. Based on these improvements, robust stretchable electrodes with a stretchability of over 80% uniaxial strain and tunable gauge factors ranging from 0.07 to 520 (a much wider range than previously reported) were fabricated by optimizing the AgNW deposition density and PDMS peel-off direction. Finally, we demonstrated the applicability of the proposed method by fabricating stretchable circuits which were able to either maintain a stable resistance or sensitively monitor various human motions, revealing its tremendous potential for applications in flexible/stretchable electronics.

An Ultrastable Ionic Chemiresistor Skin with an Intrinsically Stretchable Polymer Electrolyte

Ultrastable sensing characteristics of the ionic chemiresistor skin (ICS) that is designed by using an intrinsically stretchable thermoplastic polyurethane electrolyte as a volatile organic compound (VOC) sensing channel are described. The hierarchically assembled polymer electrolyte film is observed to be very uniform, transparent, and intrinsically stretchable. Systematic experimental and theoretical studies also reveal that artificial ions are evenly distributed in polyurethane matrix without microscale phase separation, which is essential for implementing high reliability of the ICS devices. The ICS displays highly sensitive and stable sensing of representative VOCs (including toluene, hexane, propanal, ethanol, and acetone) that are found in the exhaled breath of lung cancer patients. In particular, the sensor is found to be fully operational even after being subjected to long-term storage or harsh environmental conditions (relative humidity of 85% or temperature of 100 °C) or severe mechanical deformation (bending to a radius of curvature of 1 mm, or stretching strain of 100%), which can be an effective method to realize a human-adaptive and skin-attachable biosensor platform for daily use and early diagnosis.

The Effect of Polymer Rebound on SnBi58 Solder ACFs Joints Cracks during a Thermo-Compression Bonding

In this study, a novel thermomechanical analysis (TMA) method was introduced to measure the exact polymer rebound amounts due to pressures removal after a thermo compression (TC) bonding process. Polymer resin was laminated between two silicon chips (7*7mm2), and then the compressive mode TMA measurement was done on the prepared samples. Constant compressive pressures were applied until the temperature was gradually increased to target temperature, and the forces were removed at the target temperatures. The polymer rebound was measured by monitoring the z-axis dimension change after the compressive forces was removed. The effects of bonding temperatures (from 150 to 250 oC) and the bonding pressures (1, 2 and 3MPa) on the SnBi58 (139 oC melting point) solder joints morphologies and joint resistances were evaluated to optimize bonding conditions and remove solder joint cracks for low Tg acrylic-based solder ACFs applications.

Effects of Polymer Ball Size and Polyvinylidene Fluoride Nanofiber on the Ball Capture Rate for 100-

There are still many problems in the conductive adhesive, such as particle size and particle dispersion in the matrix, which can affect the stability of the electrical conductivity. In order to resolve the particle dispersion issue in the anisotropic conductive films (ACFs) during the bonding process, we need to increase capture rate of conductive balls between the top and the bottom metal electrodes. In this paper, the methods of increasing capture rates were studied. The capture rates of the conventional 20- and 10-μm polymer ball ACFs were compared. In addition, the capture rates of the conventional polymer ball ACF and polyvinylidene fluoride (PVDF) nanofiber incorporated a polymer ball ACF were compared. As a result, the capture rates of 20-μm polymer ball ACFs were ~30% higher than 10-μm polymer ball ACFs. And the capture rate of PVDF nanofiber incorporated polymer ball ACF was 21% higher than the conventional polymer ball ACF. By using nanofiber and larger size of conductive balls, the capture rates can be increased.