Fengshun Wu

Flex Reliability of RFID Inlays Assembled by Anisotropic Conductive Adhesive

Accompanied by the cost reduction and the assembly line speed increase, the drive to enhance the reliability of RFID tags is ever increasing due to the incremental puzzles on the tag response function. Anisotropic conductive adhesive (ACA) bonding is one of the main stream in a mass production of RFID inlay, but due to its nature of adhesive bonding and contact conduction, the increasing response problems emerges, especially when the ACA joint encounters the occasion that it needs to be flexed. A simple method to evaluate the flex reliability of the RFID inlays is developed in this study. A gold film is sputtered on the RFID flip chip to form a conductive layer, making the inlay strap becomes a conductor, and thus the contact resistance variety of ACA bonding joint can be detected. The ACA assembled inlays using this kind of dummy die are delivered to a serial tests: wind, static tensile and pure bend, while the DC resistance of these inlays is recorded simultaneously. It is found that the pure bend instead of static tensile results in irrecoverable enhancement of contact resistance of ACA joint. Besides, pure bend test with die facing down exhibit a less damage on the resistance than that with die facing up, because the former has a press effect while the latter exert a tear effect on the joint.

Reliability Evaluation on Flexible RFID Tag Inlay Packaged by Anisotropic Conductive Adhesive

Interconnecting the RFID flip-chip on flexible antenna substrates by anisotropic conductive adhesive (ACA) is a preponderant technique to accomplish the low cost manufacturing of radio frequency identification (RFID) tag inlay. However, for the nature of hot compression bonding by adhesive, the frequency response function of RFID inlays often turns unstable and eventually goes to a response failure. In this work, ACA was prepared by mixing Ag particles into thermoset epoxy resin, and RFID flip chips were assembled on the aluminum antenna by ACA under 180 degC, 12 sec, 1.4 MPa. It was found that the ACA has a comparatively high Tg and the fractograph shows that Ag conductive particles were captured by the gold bumps other than the aluminum antenna. The static tensile test and wind test reveals that the contact resistance of ACA joint rises up with the time or period increasing. Further, the outwards bend wind destructs more than the inwards bend wind on the contact conductivity of ACA due to its heavier tearing fatigue effect. The inlays remained 10 cm response distance after HHT test of 85 degC, 85 %RH, 168 hrs. In a word, the flexible RFID tag inlay assembled by ACA prepared here has an excellent flexibility and high reliability.

Effect of Interfacial IMCs Proportion on the Reliability of Miniature Lead-Free Solder Joint

The effect of interfacial IMCs (intermetallic compounds) proportion on the reliability of miniature lead-free solder joint was studied. The miniature lead free solder joint was formed with Sn-3.5Ag-0.5Cu or Sn-3.5Ag solder alloys and two copper bars that the ends were plated with Ni and Au. Several sizes of micro-joints were gotten to measure the effect of IMCs proportion. Optical microscope (OM) was used to observe the changes of interfacial IMCs of different miniature joints. Maximal shear strength force and maximal impact force were tested to evaluate the effect of miniaturization on the reliability. The results showed that the interfacial IMCs proportion of solder joints increased when they were aged at 150degC for 0 hours, 120 hours, and 200 hours or the sizes of solder joints changed from 100 mum to 20 mum. The maximal shear strength and maximal impact force of the solder joints decreased when the interfacial IMCs proportion increased

Analysis of Dark Stain on Chip Surface of High-Power LED

In the process of aging or in service of some high power blue LEDs, dark stain may occur on the LED chip surface, which cast a shadow over the chip and thus led to the losing of luminous flux. In the degraded samples, it was found that firstly the dark stain was formed on the chip surface equably, and then the color of this stain gradually darken with time. In this paper, this kind of dark stain was detected and analyzed. The morphology of the dark stain and the elements contained in it were measured by scanning electron microscopy (SEM) and energy dispersive analysis by X-ray (EDAX), respectively. A large mount of oxygen and carbon were found in the black zones, which implied that some chemical reactions occurred on the interface. After we removed the black materials off the chip surface, the quantity of luminous flux evidently increased. The origin of oxygen and the possible reason of the dark stain were also discussed

Effects of Heating Factors on Brittle Fractures of Solder Joints by High Speed Shear Test

During the solder process, the components, distribution and thickness of intermetallic compound (IMC) between solder and pad can strongly affect the strength of solder joints, which eventually determine the brittleness of joints. In this paper, the influence of heating factor (Qeta), defined as the integral of the measured temperature over the dwell time above liquids in the reflow profile on impact toughness of solder joints was evaluated quantitatively by means of high speed shear test (HSST) and conventional static shear test (SST), in which the shear rate was 1.5m/s and 8times10-5m/s respectively. The results of high speed shear testing indicate that with heating factor increasing, the impact toughness of solder joints increases to a peak and then decreases. Therefore, it is obvious that Qeta= 600~1200 sdegC is an optimal heating factor range for Sn96Ag3.5Cu0.5, in which the assembly exhibits the greatest impact toughness under HSST. The results also show that the failure mode and position of solder joints in these two tests are quite different, and the results of HSST appears more susceptible to the reflow process than that of SST.

The Study on the Interaction of Interfacial Reactions between UBM/Solder and Solder/Pad in Miniature Joints

This stud} investigated the interaction of interfacial reactions between Cu/Sn3.5Ag and Sn3.5Ag/AuNi diffusion couples with different stand-off heights (SOH) in immature solder joints. After reflowing. many (Cux,Ni1-x)6Sn5 IMCs were found to surround the original IMCs at both interfaces of the solder joint, which were attributed to Cu and Ni atoms diffusion throughout the w hole solder. This phenomenon was defined as the interaction of two interfacial reactions in miniature solder joints. The concentrations of Ni and Cu in (Cux,Ni1-x)6Sn5 ternary IMC at each side depended on the diffusion rates to Ni and Sn. the height of the solder joint and the experimental conditions. At Sn3.5Ag/AuNi interface, the concentrations of Ni and Cu decreased and increased in (Cux,Ni1-x)6Sn5 ternary IMCs with SOH decreasing, respectively. At Cu/Sn3.5Ag interface, the concentrations of Ni and Cu changed in opposite rules. Cu had a faster diffusion rate in molten Sn to generate a higher concentration in (Cux,Ni1-x)6Sn5. which also took a sharp slope of the percentage of IMC thickness/SOH with SOH increasing at Sn3.5Ag/AuNi interface.

Preparation of Conductive Particles by Electroless Nickel Plating on Polystyrene Microsphere

Anisotropic conductive adhesive (ACA) is an advanced packaging material with lots of merits, but its manufacture is of high tech upon multiple branches of knowledge. In this letter, a step of preparing the conductive particles of ACA was investigated, that is, electroless nickel plating on polymer microspheres. The process of the coating formation consists of four steps: chemical etching, sensitization, activation, and electroless plating. 0.23 mum in thickness uniform and smooth Ni-P coating on 3.7 mum in diameter polystyrene microspheres was formed upon 25 min plating. The advantage of this plating process is to ensure an easy and accurate control on the thickness and the quality of the plating coat