Search

Plane two-dimensional finite element analysis was applied to study the effect of underfill thermomechanical properties on the potential of thermal fatigue failure for flip-chip ball grid array. Two-stage as well as constant thermomechanical properties of underfills were manipulated to represent extremes of practical underfills. The steady-state creep model was incorporated for the eutectic solder bump to represent its real behavior. It was found from the parametric studies that the underfll with high Young's modulus, low coefficient of thermal expansion, and high glass transition temperature leads to the longest service life.

The flexible interface model is adopted in this paper to analyze the three-dimensional (3D) elastic constitutive relationships, together with the local stress and strain distributions within constituent layers of an imperfectly bonded multilayered medium. Each constituent layer is assumed to be generally anisotropic with 21 independent elastic moduli, and three damage parameters are used to model the imperfect bonding state of each interface. Due to the generality of this model, elastic constitutive relationships for layered media composed of layers of other kinds of elastic symmetry can be readily achieved by algebraic reductions. From the comparison between theoretical predictions and experimental results on artificial bilaminated rock masses composed of isotropic layers, this model is shown to be a reasonable substitute of the conventional perfect bonding constitutive model.

The electromigration behavior of SnAg solder bumps with and without Cu column under-bump-metallizations (UBMs) has been investigated under a current density of 2.16 × 104 A/cm2 at 150 °C. Different failure modes were observed for the two types of samples. In those without Cu column UBMs, when SnAg solder bumps that had implemented 2 μm Ni UBMs were current stressed at 2.16 × 104 A/cm2, open failure occurred in the bump that had an electron flow direction from the chip side to the substrate side. However, in those with Cu column UBMs, cracks formed along the interface of Cu6Sn5 intermetallic compounds and the solder on the substrate side in the Sn-3.0Ag-0.5Cu solder bump that had an electron flow direction from the substrate side to the chip side. A three-dimensional simulation of the current density distribution was performed in order to obtain a better understanding of the current crowding behavior in solder bumps. The current crowding effect was found to account for the void formation on both the chip and the substrate side for the two kinds of solder bumps. One more important finding, as confirmed by infrared microscopy, is that the alleviation of current crowding by Cu column UBMs also helped decrease the Joule heating effect in solder bumps during current stressing. Therefore, the measured failure time for the solder joints with Cu column UBMs appears to be much longer than that of the ones with the 2 μm Ni UBMs. [ABSTRACT FROM AUTHOR]

An in situ SEM/energy dispersive spectroscopy (EDS) analysis revealed that the dispersed Sn-rich phase of the 95Pb-5Sn solder in a 95Pb-5Sn/63Sn-37Pb composite solder bump dissolved under current stressing. The dissolution was not prominent until the current density reached a threshold value of between 3.3 × 104 and 4.2 × 104 A/cm2. Supersaturation over thermal solubility of Sn-in-Pb was detected with current stressing. A polarity dissolution model was proposed for the dissolution behavior of the Sn-rich phase under current stressing. The dissolution mechanism under current stressing was discussed in relation to electromigration and thermomigration behavior of SnPb solders. [ABSTRACT FROM AUTHOR]

The bending and residual stress of flip chips caused by the mismatch of thermal expansion between the chip and the substrate have been measured by polychromatic microfocused synchrotron x-ray beam. Precise orientation information as a function of position on the chip was obtained from Laue diffraction patterns, so that the bending angle with respect to a reference position at the center of the chip can be calculated at each position. This in turn allows deducing the local curvature of the entire flip chip. Local stress distribution was then mapped by applying a modified Stoney’s stress-strain equation to the measured curvature. Our study shows that thermal stress on the circuits and the solder joints in a flip chip strongly depend on temperature and the distance from the center of the chip, indicating that interconnects at the corner and edge of a flip chip are of reliability concerns. [ABSTRACT FROM AUTHOR]

Electromigration-induced hydrostatic elastic stress in Pb-free SnCu solder joints was studied by in situ synchrotron x-ray white beam microdiffraction. The elastic stresses in two different grains with similar crystallographic orientation, one located at the anode end and the other at the cathode end, were analyzed based on the elastic anisotropy of the β-Sn crystal structure. The stress in the grain at the cathode end remained constant except for temperature fluctuations, while the compressive stress in the grain at the anode end was builtup as a function of time during electromigration until a steady state was reached. The measured compressive stress gradient between the cathode and the anode is much larger than what is needed to initiate Sn whisker growth. The effective charge number of β-Sn derived from the electromigration data is in good agreement with the calculated value. [ABSTRACT FROM AUTHOR]

This article reports a nanomechanical response study of the contact-induced deformation behavior in Al0.16Ga0.84N thin film by means of a combination of nanoindentation and the cross-sectional transmission electron microscopy (XTEM) techniques. Al0.16Ga0.84N thin film is deposited by using the metal-organic chemical vapor deposition method. Hardness and Young’s modulus of the Al0.16Ga0.84N films were measured by a Berkovich nanoindenter operated with the continuous contact stiffness measurements mode. The obtained values of the hardness and Young’s modulus are 19.76±0.15 and 310.63±9.41 GPa, respectively. The XTEM images taken in the vicinity just underneath the indenter tip revealed that the multiple “pop-ins” observed in the load-displacement curve during loading are due primarily to the activities of dislocation nucleation and propagation. The absence of discontinuities in the unloading segments of the load-displacement curve suggests that no pressure-induced phase transition was involved. [ABSTRACT FROM AUTHOR]

ABSTRACT Bio-based porous carbon/silica particles (denoted as RH-carbon/silica) were successfully prepared from agricultural waste rice husk by using acid-hydrothermal treatment and pyrolysis under nitrogen condition. As green filler, the cure behavior, thermal-mechanical properties, and thermal conductivity of the epoxy-carbon/silica biocomposites at different filler contents (5, 9, 17, 29 wt %) were characterized. Because of superior surface properties (surface area, porosity, and silica segment) and high content of carbon component in the RH-carbon/silica, the characteristics of the biocomposites were significantly improved with the increase of the filler content. At 29 wt % of filler content, the epoxy biocomposites exhibit lower curing temperature (148 °C), lower CTE (42 ppm/°C), higher Tg (123 °C), higher storage modulus (4059 MPa), and higher effective thermal conductivity (0.29 W/mK). In brief, the RH-carbon/silica particles that can serve not only as reinforcing agent but also as thermal transport medium used in epoxy composite, is a green and high-performance filler for this purpose. © 2016 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2017, 134, 44699. [ABSTRACT FROM AUTHOR]

A new representation for viscoelastic functions, the tensor relaxation-creep duality representation, is introduced. The derivation of a tensor time-differential constitutive equation for anisotropic viscoelastic materials using this new presentation is presented. The relaxation-creep duality characteristic ingrained in the new representation enables the interconversion of viscoelastic functions, which is not possible with the conventional Prony series representation of viscoelastic functions. The new representation therefore offers a better representation of the physics of viscoelasticity leading to a reduced number of viscoelastic parameters required to describe a viscoelastic function. The new representation has been demonstrated on two anisotropic viscoelastic crystallographic systems: (i) the symmetric systems with material- and time-independent eigenvectors and (ii) the symmetric systems with material-dependent but time-independent eigenvectors. [ABSTRACT FROM AUTHOR]