Karl W. Wyatt

Drop/impact simulation and test validation of telecommunication products

Portable communication devices suffer impact-induced failure in usage. The products must pass drop/impact tests before shipment. The drop/impact performance is an important concern in product design. Due to the small size of this kind of electronic products, it is very expensive, time-consuming, and difficult to conduct drop tests to detect the failure mechanism and identify the drop behaviour. Finite element analysis provides a vital, powerful vehicle to solve this problem. The methodology of computer modeling, finite element method simulation and test validation techniques developed in ASMR at Motorola over the last two years are introduced in this paper. Two drop simulation and test validation cases in ASMR are reported in detail. The models are created with HYPERMESH, and the analysis is carried out with LS-DYNA3D. The analysis focuses on housing breakage, LCD cracking and structural disconnection under drop/impact shock. Apart from the computer simulation, a drop laboratory has been built in ASMR. With a customized drop tester, the drop orientation of the specimen can be controlled. The impact force relation to barrier, acceleration and strain inside the specimen during drop can be recorded in terms of time history curves. The test device, drop test and correlation of analysis and test data are illustrated in the paper. The simulation and test technology are applied to reliability identification and design support to Motorolas products.

Electro-thermo-mechanical responses of conductive adhesive materials

Micromechanics models which aim to provide an understanding of conductive adhesive materials from the level of micro-particles (less than 30 mm) are presented in this paper. The pressure-induced conducting mechanisms are investigated. A deformation analysis reveals a logarithmic pressure-resistance relationship and is capable of addressing the conducting phenomena for both rigid and deformable particle systems within a contact mechanics framework. This logarithmic relationship also provides analytical support for findings reported in the literature of conductive adhesive research. It is observed that electrical contacts are made by squashing conducting particles for a deformable particle system while the particle penetration creates a crater in metallization to make contacts for a rigid particle system. The current analysis provides simple closed-form solutions for the elastic deformation of single-particle contacts and based on the assumption that the contact forces are evenly distributed in a conductive film, the pressure-resistance responses are correlated to the particle volume fraction. The high volume fraction, while ensuring that there are a sufficient number of particles to make contacts, may limit the particle deformation due to overall increased stiffness, resulting in the increased resistance on a per particle basis. The current analysis also offers insight into design considerations whereby limited amount of deformation (low processing temperature) and sufficiently low electrical resistance are to be simultaneously satisfied. For the mechanical performance, the uniaxial nonlinear stress-strain relationship is obtained for conductive adhesive systems in terms of polymer and particle material properties. The Mori-Tanakas method is utilized to account for particle-particle and particle-matrix interactions. The behaviour in thermal expansion within the elasto-plastic deformation range is also obtained in a similar fashion. In all these calculations, only a very simplified finite element analysis for the problem of a particle embedded into an infinitely extended matrix material needs to be carried out.

An Interfacial Delamination Analysis for Multichip Module Thin Film Interconnects

Analysis of interfacial delamination for multichip module thin-film interconnects (MCM/TFI) is the primary objective of this paper. An interface crack model is integrated with finite-element analysis to allow for accurate numerical evaluation of the magnitude and phase angle of the complex stress intensity factor. Under the assumption of quasi-static delamination growth, the fate of an interfacial delamination after inception of propagation is determined. It is established that whether an interfacial delamination will continue to grow or become arrested depends on the functional behavior of the energy release rate and loading phase angle over the history of delamination growth. This functional behavior is numerically obtained for a typical MCM/TFI structure with delamination along die and via base, subjected to thermal loading condition. The effect of delamination interactions on the structural reliability is also investigated. It is observed that the delamination along via wall and polymer thin film can provide a benevolent mechanism to relieve thermal constraints, leading to via stress relaxation.

Cure kinetics and mechanical properties of conductive adhesive

The curing reaction of a conductive adhesive was studied with a differential scanning calorimeter (DSC) under isothermal conditions in the range of 100-160/spl deg/C. An autocatalyzed kinetic model was used to describe the curing reaction. The rate constant and the reaction orders were determined and used in the model to predict the progress of the curing reactions. A good agreement is found between the proposed kinetic model and the experimental reaction rate data. The reaction rate constants were correlated with the isothermal temperature by the Arrhenius equation. The activation energy for the curing reaction is determined to be 94.9 kJ/mol. The reaction order which represents the effects of the unreacted materials is found to be a parabolic function of temperature. But the effects of the reacted materials on the reaction rate change sharply at around 120/spl deg/C. Unlike some previous results on epoxy curing kinetics, the sum of the two reaction orders is not a constant for this conductive adhesive. Thermogravimetric Analyzer (TGA) was used to study the weight loss during thermal processes. The degradation temperature of the conductive adhesive was found to be 250/spl deg/C. The properties of the corresponding unfilled epoxy were also studied with the DSC and TGA. Results were compared with those obtained from the conductive adhesive. Tests were conducted to investigate the mechanical and electrical property changes during cure.

Die cracking in flip-chip-on-board assembly

The die cracking in a flip-chip-on-board (FCOB) assembly occurs very often during the solder reflow process and the fracture path follows a straight downward crack extension and then kinks at a certain depth. Such a fracture path selection in FCOB structures is determined here based on a mode-I domination criterion. This is done by solving the corresponding problem of a kinked crack in a FCOB structure for a series of kinking angles. The numerical results indicate that the energy release rate maximum assumes initially at a zero-degree angle, which points to the straight downward extension in the thickness direction, and then shifts to a large angle, which corresponds to kinking. The kinking angle and the depth at which the kink starts have been predicted and compared agreeably with cross sectioning of failed samples from a manufacturing floor. The maximum allowable manufacturing defect size, below which the die cracking can be prevented, is then determined by a beam theory. The analysis suggests that in a typical FCOB structure a defect size of more than 35 micron will result in die cracking failure after solder reflow. The numerical results also reveal that a good tolerance of defects can be achieved by limiting the board thickness to be less than twice of the die.

Process induced residual stresses in isotropically conductive adhesive joints

Mechanical and thermomechanical tests were conducted to characterize conductive adhesives in terms of dimensional stability and viscoelastic properties. Dynamic testing results were converted from the frequency domain into the time domain. Those results were then incorporated into the ABAQUS finite element (FE) code, in which a finite element analysis (FEA) was conducted to investigate the stress development and stress relaxation process in the conductive adhesive joints by taking into account the viscoelastic properties of the conductive adhesive. Experimental results showed that the stresses in the conductive adhesive can relax significantly at elevated temperatures. Based on the experimental data and FEA result, a new cure schedule is proposed for the conductive adhesive studied.

Experimental evaluation of solder joint thermal strain in a CSP using digital speckle correlation

Thermomechanical strain was directly measured using a computer image processing technique in an area encompassing the solder-copper interface near the corner of an outermost solder ball in a chip scale package (CSP). Due to the ultra-high spatial resolution of the technique, the shear strain concentration was revealed and quantified over the area with linear dimensions in terms of microns. Discussions are given on the significance of the measurement for identifying the root cause for solder joint failure, and on the application prospects for the technique as an experimental tool in emerging areas of research where microscopic strain measurement is needed.

Solder joint formation simulation and finite element analysis

The solder joint formation during reflow and related subsequent solder joint reliability of surface mounted electronic devices are critical issues in the field of electronic packaging. Solder joint reflow and reliability are highly dependent on joint configuration which are governed by bond pad size, alloy material, solder paste volume, leadframe geometry, and leadframe/pad alignment, etc. The objective of this work is to develop numerical models and methods to: (1) simulate the solder joint formation during the reflow process; (2) determine the stress/strain distribution within the joint; and (3) further predict the reliability of the solder joints. The solder joint formation process during the solidification stage can be simulated using the Surface Evolver program developed by University of Minnesota. The thermomechanical force/displacement (or stress/strain) analysis can be carried out using ANSYS, a general purpose Finite Element Analysis program. The effort also includes the linking of the two programs. Examples of applications of this work include study of components falling off during second (facing down) reflow and thermal stress analysis (due to CTE mismatch) of PQFP mounted on FR4 PWB.

Reliability Study of Holographic Optical Elements Made with DuPont Photopolymer.

We report reliability-test results of transmission-type holographic optical elements (HOEs) made with the DuPont photopolymer HRF-600. The reliability tests performed include 6000 cycles of liquid-to-liquid thermal-shock cycling (-55 degrees C-125 degrees C), 2200 cycles of air-to-air thermal cycling (-55 degrees C-125 degrees C), 1500 h of humidity testing (85 degrees C and a relative humidity of 85%), and 675 h of burn-in testing at 125 degrees C. A total of 210 holograms was tested, with 532 data points collected for diffraction-efficiency measurements. The results show that the average efficiency change after these tests is in the range of -4% to 0% and the standard deviation is only ~10%.

Flush-porting method and device for reduction of wind-induced noise in a microphone

The present invention provides a method (600) and device (400) for minimizing wind-induced noise in a microphone. The device includes: a housing (412) having a recessed area shaped to accommodate a microphone transducer (410); the microphone transducer (410), situated within the recessed area such that a thin film situated over the microphone transducer is flush with/overlaying a top of the recessed area and affixed at least to the sides of the recessed area, for receiving sound; and the thin film (402) has at least one aperture (404, . . . 406) for allowing sound to impinge on the microphone transducer (410), and has a minimal thickness that maintains structural integrity.