Y.J. Chen

Critical Concerns in Soldering Reactions Arising from Space Confinement in 3-D IC Packages

Six critical issues relating to interfacial reactions arising from space confinement in 3-D integrated-circuit (3-D IC) packaging are presented in this paper. The first issue arises from the concern that intermetallics (IMCs) may occupy a large portion of the solder joint volume. It will be demonstrated that this concern is real even for Ni under bump metallurgy (UBM) or surface finish, which reacts very slowly with solders. The second issue relates to impingement and ripening of IMC grains. When IMCs occupy a large portion of a joint, the IMC grains growing from the opposite sides of a joint will eventually touch each other. The morphology evolution from this point on determines the final microstructure of a joint. Structural defects might form as a result of the impingement and ripening of IMC grains. The third issue is the rise of impurity concentration due to solder consumption. Many of the impurities in solders, such as those from electroplating, are not soluble in IMCs. As Sn reacts to form IMCs, these impurities are rejected from IMCs into the remaining solder. Consequently, the concentrations of these impurities increase with the progress of reaction. Eventually, these impurities are trapped between IMCs growing from the opposite directions. The impact of these trapped impurities on the properties of solder joints is an important issue. The fourth issue is similar to the third except that the role of impurities is replaced by inert alloy elements of solders. The most obvious element is Ag. The fifth issue arises from the fact that, as the size of a joint becomes smaller, the surface-area-to-volume ratio increases. This makes the impact of thin-film layers on UBM and surface finish become ever higher. One well-known element is Au. The so-called Au embrittlement issue may become relevant again. The last issue is the volume shrinkage from chemical reactions. Internal stress or structure defects may form because of this shrinkage. Experimental evidence will be used in this paper to illustrate these six issues, and the implications of these issues will be discussed.

Reactions of Sn-4.0Ag-0.5Cu on Cu and Electroless Ni Substrate in Premelting Soldering Process

Early formation of Cu6Sn5 and (Ni,Cu)6Sn5 during soldering was investigated. Sn4.0Ag0.5Cu solder on Cu and electroless Ni-Au substrate was quenched in ice water after the initial stage of melting, and the solder–substrate interface was analyzed by transmission electron microscopy and energy-dispersive x-ray spectroscopy. At the contacting point, Cu3Sn and Cu6Sn5 were found at the solder–Cu substrate interface, while (Ni,Cu)6Sn5 and Ni2SnP were identified at the solder–electroless Ni interface. The contacting points were confirmed as the sites of heterogeneous nucleation. TEM analysis suggested that the nucleation of both intermetallic compounds can be attributed to diffusion-induced crystallization. Details of the early interfacial reactions are discussed.

Soldering reactions under space confinement for 3D IC applications

The microstructure features and the IMC kinetics of Cu-Sn and Ni-Sn soldering reaction under space confinement are studied in the present study. Experimentally, Cu/Sn(10 μm)/Cu and Ni/Sn(10 μm)/Ni sandwich structures were prepared through a hot-press bonding. Sn with a thickness of 10 μm was chosen to simulate the joints in 3D IC packages employing a severe space confinement. Microstructure characterisations showed that space confinement does not change the reaction products which are Cu6Sn5 and Cu3Sn for Cu-Sn reactions and Ni3Sn4 for Ni-Sn reactions. However, it was found that the interfacial morphology is very different to that of the conventional joints. IMCs in the space confined reactions occupy a very large or even entirety volume in the joint. In addition, voiding occurs in both Cu/Sn/Cu and Ni/Sn/Ni reactions, which having great impacts on the joint strength. According to the kinetics analysis, the IMCs growth rates measured in the present work are in the range of literature results, suggesting that the space confinement behaviour has very little effects on the IMC growth kinetics.

Flip-chip bonded MEMS capacitor Applied on tuning superconductive resonator

This paper presents the design methodology, fabrication and characterization of flip-chip bonded MEMS capacitor for tuning high temperature superconductive resonator. Because the main issue of MEMS capacitor integrated with high temperature superconductor (HTS) is high driving voltage due to accumulated thermal stress, a methodology to decrease the thermal stress is discussed. In this work, the thermal stress can be reduced to several MPa, and driving voltage is about 40 V. To avoid degradation of HTS thin film during MEMS process, MEMS capacitor is fabricated separately and then flip-chip bonded on the HTS resonator. The tuning effects of this MEMS capacitor applied on HTS resonator of 3 GHz resonant frequency are presented.

Tongue Pressure Sensing Array Integrated with a System-on-Chip Embedded in a Mandibular Advancement Splint

Obstructive sleep apnea (OSA), which is caused by obstructions of the upper airway, is a syndrome with rising prevalence. Mandibular advancement splints (MAS) are oral appliances for potential treatment of OSA. This work proposes a highly-sensitive pressure sensing array integrated with a system-on-chip (SoC) embedded in a MAS. The device aims to measure tongue pressure distribution in order to determine the efficacy of the MAS for treating OSA. The flexible sensing array consists of an interdigital electrode pair array assembled with conductive polymer films and an SoC capable of retrieving/storing data during sleep, and transmitting data for analysis after sleep monitoring. The surfaces of the conductive polymer films were patterned with microdomed structures, which effectively increased the sensitivity and reduced the pressure sensing response time. The measured results also show that the crosstalk effect between the sensing elements of the array was negligible. The sensitivity of the sensing array changed minimally after the device was submerged in water for up to 100 h.

Micromachined Planar Supercapacitor with Interdigital Buckypaper Electrodes

In this work, a flexible micro-supercapacitor with interdigital planar buckypaper electrodes is presented. A simple fabrication process involving vacuum filtration method and SU-8 molding techniques is proposed to fabricate in-plane interdigital buckypaper electrodes on a membrane filter substrate. The proposed process exhibits excellent flexibility for future integration of the micro-supercapacitors (micro-SC) with other electronic components. The device’s maximum specific capacitance measured using cyclic voltammetry was 107.27 mF/cm2 at a scan rate of 20 mV/s. The electrochemical stability was investigated by measuring the performance of charge-discharge at different discharge rates. Devices with different buckypaper electrode thicknesses were also fabricated and measured. The specific capacitance of the proposed device increased linearly with the buckypaper electrode thickness. The measured leakage current was approximately 9.95 µA after 3600 s. The device exhibited high cycle stability, with 96.59% specific capacitance retention after 1000 cycles. A Nyquist plot of the micro-SC was also obtained by measuring the impedances with frequencies from 1 Hz to 50 kHz; it indicated that the equivalent series resistance value was approximately 18 Ω.

A Wireless Monitoring System Using a Tunneling Sensor Array in a Smart Oral Appliance for Sleep Apnea Treatment

Sleep apnea is a serious sleep disorder, and the most common type is obstructive sleep apnea (OSA). Untreated OSA will cause lots of potential health problems. Oral appliance therapy is an effective and popular approach for OSA treatment, but making a perfect fit for each patient is time-consuming and decreases its efficiency considerably. This paper proposes a System-on-a-Chip (SoC) enabled sleep monitoring system in a smart oral appliance, which is capable of intelligently collecting the physiological data about tongue movement through the whole therapy. A tunneling sensor array with an ultra-high sensitivity is incorporated to accurately detect the subtle pressure from the tongue. When the device is placed on the wireless platform, the temporary stored data will be retrieved and wirelessly transmitted to personal computers and cloud storages. The battery will be recharged by harvesting external RF power from the platform. A compact prototype module, whose size is 4.5 × 2.5 × 0.9 cm3, is implemented and embedded inside the oral appliance to demonstrate the tongue movement detection in continuous time frames. The functions of this design are verified by the presented measurement results. This design aims to increase efficiency and make it a total solution for OSA treatment.

Consumption of Cu pad during multiple reflows of Ni-doped SnAgCu solder

Abstract The effects of solder volume and the number of reflows on the Cu consumption rate were studied. Nickel-doped Sn3Ag0.5Cu0.02Ni solder balls of different diameters (300, 400, 600 and 760 μm in diameter) were reflowed over Cu soldering pads with an opening of 600 μm in diameter. The reflow profile had a peak temperature of 235 ± 2°C and a 90 s duration during which the solder was molten. The nominal ramp rate and cooling rate were both 1.5 K s−1. The first reflow consumed the largest amount of Cu, and the subsequent reflows only consumed marginally more Cu. It was argued that the solder joints became saturated with Cu during the first reflow so that during the subsequent reflows the Cu substrates were consumed at markedly reduced rates. The Cu consumption increased with the diameter of the solder balls. This was because a larger amount of Cu was needed in order to saturate a solder joint with a larger solder volume. The doping of Ni did not change the Cu consumption rate despite the fact that the am...

Rapid dielectrophoresis assembly of single carbon nanocoil on an AFM probe tip

The assembly of a single carbon nanocoil on the tip of an atomic force microscope probe has been demonstrated by utilizing dielectrophoresis in an isopropyl alcohol solution. The nanocoil adheres to the probe tip where the maximum electrical field exists. Its orientation due to the relative position between the probe tip and the dielectrophoresis electrode has been investigated. The capillary effect which constrains the nanocoil assembly in the limited aqueous space has been discussed. The experiment results have been verified through finite element simulation. The total assembly process can be achieved within several seconds at room temperature environment by using a simple apparatus.