Da-Jeng Yao

Infrared microscopy of hot spots induced by Joule heating in flip-chip SnAg solder joints under accelerated electromigration

Joule heating effect in solder joints was investigated using thermal infrared microscopy and modeling in this study. With the increase of applied current, the temperature increased rapidly due to Joule heating. Furthermore, modeling results indicated that a hot spot existed in the solder near the entrance point of the Al trace, and it became more pronounced as the applied current increased. The temperature difference between the hot spot and the solder was as large as 9.4°C when the solder joint was powered by 0.8A. This hot spot may play an important role in the initial void formation during electromigration.

Mesoscale actuator device: micro interlocking mechanism to transfer macro load

Abstract A novel proof-of-concept prototype Mesoscale Actuator Device (MAD) containing microscale components has been developed. The MAD is similar to piezoelectrically driven inchworm motors with the exception that mechanically interlocking microridges replace the traditional frictional clamping mechanisms. The interlocked microridges, fabricated from single crystal silicon, are designed to increase the load carrying capability of the device substantially. Tests conducted on the current design demonstrate that interlocked microridges fabricated with 30% KOH solution support a 9.6 MPa shear stress or that a pair of 5×5 mm locked chips supports a 500 N load. For high frequency operation, an open loop control signal is implemented to synchronize the locking and unlocking of the microridges with the elongating and contracting of the actuator. The system was successfully operated from 0.2 Hz to 500 Hz (or speeds from 2 μm/s to 5 mm/s). The upper limit (500 Hz) is imposed by software and hardware limitations and not related to physical limitations of the prototype device.

Mesoscale actuator device with micro interlocking mechanism

A novel proof-of-concept prototype Mesoscale Actuator Device (MAD) has been developed. The MAD is similar to piezoelectric driven inchworm motors with the exception that mechanically interlocking microridges replace the traditional frictional clamping mechanisms. The interlocked microridges, fabricated from single crystal silicon, increase the load carrying capability of the device substantially. Tests conducted on the current design demonstrate that the interlocked microridges support 16 MPa in shear or that a 3/spl times/5 mm locked chip supports a 25 kgf load. To operate the MAD device at high frequencies an open loop control signal is implemented. Synchronizing the locking and unlocking of the microridges with the elongating and contracting actuator requires minor perturbations in the voltage signal supplied. The system was successfully operated from 0.2 Hz to 500 Hz (or speeds from 2 /spl mu/m/s to 5 mm/s). The upper limit (500 Hz) is imposed by software limitations and not related to physical limitations of the current device.

DNA ligation of ultramicro volume using an EWOD microfluidic system with coplanar electrodes

An automatic ultramicro-volume DNA ligation process using a coplanar electrode type of electrowetting-on-dielectric (EWOD) microfluidic system was designed for economy of reagent. The droplets, containing DNA, a ligation enzyme and a multi-salt reaction buffer, served to complete the ligation using a developed EWOD system. Droplets of ultramicro volume (0.3 µL) were successfully generated from reservoirs between one plate with coplanar electrodes and another plate with a hydrophobic surface free of electrodes. In one successful cloning, the total usage of reagents in an ultramicro-volume EWOD chip was 2.1 µL; no volume was wasted, in comparison with 85% waste with the standard protocol and 80% waste with a free-cover coplanar EWOD chip. The results also show that the entire process was accomplished without damage to the chip surface and without biomaterial annulment. With a design consisting of electrode pathways in a flexible pattern and multiplex reservoirs, an EWOD digital microfluidic system with coplanar electrodes would be improved as an efficient parallel DNA-cloning system in the construction of an artificial library or expression library.

A flexible hydrophilic-modified graphene microprobe for neural and cardiac recording.

UNLABELLED A graphene-based flexible microprobe developed by microelectromechanical system technology shows high resolution for the detection of electrophysiological signals from various bio-objects. The hydrophilization post-treatment using steam plasma was performed on the graphene surface to decrease the interfacial impedance between graphene and electrolyte, and thus improve the signal-to-noise ratio during neural and cardiac recording. The signal-to-noise ratio of the action potentials from axons of a crayfish measured by hydrophilic-modified graphene microprobe (27.8±4.0dB) is higher than that of untreated device (20.3±3.3dB). Also, the form of the QRS complex and T wave in the electrocardiogram of the zebrafish heart can be clearly distinguished using the modified device. The total measured noise levels of the overall stability of the system were 4.2μVrms (hydrophilic graphene) and 7.64μVrms (hydrophobic graphene). The graphene-based implant can be further used for in vivo, long-term recording and retina prosthesis. FROM THE CLINICAL EDITOR In this study a graphene-based flexible microprobe developed using microelectromechanical system technology was demonstrated to enable high resolution detection of electrophysiological signals, including EKG in zebrafish models. Both hydrophilic and hydrophobic graphene were studied, paving the way to potential future clinical applications of this new technology.

Magnetic bead-based DNA detection with multi-layers quantum dots labeling for rapid detection of Escherichia coli O157:H7

This work demonstrated the feasibility of detecting 250zM Escherichia coli O157:H7 eaeA target DNA by using a magnetic bead-based DNA detection assay with designed labeling strategy within 40-60min. The magnetic beads were used as the solid support for the binding probe and isolated the target DNA from the sample. The detection signals could be amplified from the multi-layers biotin-streptavidin conjugated quantum dots based on binding with specific designed biotinlyted linker. This assay method would provide a simple, rapid, and ultra-sensitive detection method for DNA or other biomolecular analysis.

A three-dimensional flexible microprobe array for neural recording assembled through electrostatic actuation

We designed, fabricated and tested a novel three-dimensional flexible microprobe to record neural signals of a lateral giant nerve fiber of the escape circuit of an American crayfish. An electrostatic actuation folded planar probes into three-dimensional neural probes with arbitrary orientations for neuroscientific applications. A batch assembly based on electrostatic forces simplified the fabrication and was non-toxic. A novel fabrication for these three-dimensional flexible probes used SU-8 and Parylene technology. The mechanical strength of the neural probe was great enough to penetrate into a bio-gel. A flexible probe both decreased the micromotion and alleviated tissue encapsulation of the implant caused by chronic inflammation of tissue when an animal breathes or moves. The cortex consisted of six horizontal layers, and the neurons of the cortex were arranged in vertical structures; the three-dimensional microelectrode arrays were suitable to investigate the cooperative activity for neurons in horizontal separate layers and in vertical cortical columns. With this flexible probe we recorded neural signals of a lateral giant cell from an American crayfish. The response amplitude of action potentials was about 343 µV during 1 ms period; the average recorded data had a ratio of signal to noise as great as 30.22 ± 3.58 dB. The improved performance of this electrode made feasible the separation of neural signals according to their distinct shapes. The cytotoxicity indicated a satisfactory biocompatibility and non-toxicity of the flexible device fabricated in this work.

Electromigration in Pb-free SnAg3.8Cu0.7 solder stripes

Electromigration behavior in the eutectic SnAg3.8Cu0.7 solder stripes was investigated in the vicinity of the device operation temperature of 100°C by using the edge displacement technique. Measurements were made for relevant parameters for electromigration of the solder, such as drift velocity, threshold current density, activation energy, as well as the product of diffusivity and effective charge number (DZ*). The threshold current densities were estimated to be 4.3×104A∕cm2 at 80°C, 3.2×104A∕cm2 at 100°C, and 1.4×104A∕cm2 at 120°C. These values represent the maximum current densities that the SnAg3.8Cu0.7 solder can carry without electromigration damage at the three stressing temperatures. The electromigration activation energy was determined to be 0.45eV in the temperature range of 80–120°C. The measured products of diffusivity and the effective charge number, DZ*, were −1.8×10−10cm2∕s at 80°C, −5.0×10−10cm2∕s at 100°C, and −7.2×10−10cm2∕s at 120°C.

A cone-shaped 3D carbon nanotube probe for neural recording

A novel cone-shaped 3D carbon nanotube (CNT) probe is proposed as an electrode for applications in neural recording. The electrode consists of CNTs synthesized on the cone-shaped Si (cs-Si) tip by catalytic thermal chemical vapor deposition (CVD). This probe exhibits a larger CNT surface area with the same footprint area and higher spatial resolution of neural recording compared to planar-type CNT electrodes. An approach to improve CNT characteristics by O(2) plasma treatment to modify the CNT surface will be also presented. Electrochemical characterization of O(2) plasma-treated 3D CNT (OT-CNT) probes revealed low impedance per unit area (∼64.5 Ω mm(-2)) at 1 kHz and high specific capacitance per unit area (∼2.5 mF cm(-2)). Furthermore, the OT-CNT probes were employed to record the neural signals of a crayfish nerve cord. Our findings suggest that OT-CNT probes have potential advantages as high spatial resolution and superb electrochemical properties which are suitable for neural recording applications.

Thermal Gradient in Solder Joints Under Electrical Current Stressing

The thermal gradient and temperature increase in SnAg3.5 solder joints under electrical-current stressing have been investigated by thermal infrared microscopy. Both positive and negative thermal gradients were observed under different stressing conditions. The magnitude of the thermal gradient increases with the applied current. The measured thermal gradients reached 365°C/cm as powered by 0.59 A, yet no obvious thermal gradient was observed when the joints were powered less than 0.25 A. The temperature increase caused by joule heating was as high as 54.5°C when powered by 0.59 A, yet only 3.7°C when stressed by 0.19 A. The location of heat generation and path of heat dissipation are believed to play crucial roles in the thermal gradient. When the major heat source is the Al trace, the thermal gradient in the solder bumps is positive; but it may become negative because the heat generated in the solder itself is more prominent.