Chao-Pin Yeh

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.

Correlation of analytical and experimental approaches to determine thermally induced PWB warpage

Thermomechanical design effects in the printed wiring board (PWB) design process are considered, In particular, a research project for developing advanced finite-element method (FEM)-oriented capabilities to simulate thermally induced PWB warpage is reported. The FE analysis results are validated by correlating them with measurements obtained from a separate experimental approach using the shadow Moire method. >

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.

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.

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.

Reliability analysis for fine pitch BGA package

This paper presents a linear parametric finite element analysis for the solder joints of a fine pitch BGA package for a selection of key design parameters including the effects of substrate, die attach material, and die size. Results show that, when the package assembly is subjected to the temperature excursion, the solder joint stress decreases as the thickness of the substrate increases. A softer die attach material results in higher solder stresses. This observation is different from some existing chip scale packaging due to structural differences. Results also show that the die size plays an important role on the stress/strain in the solder joints. In addition to the linear parametric analysis, a more complex, nonlinear study is also carried out. With this approach, solder joint configurations are predicted with Surface Evolver. Nonlinear solder properties are used to calculate the stress/strain evolution during thermal cycling. The energy approach is used to estimate solder joint fatigue life.

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.

A multidisciplinary approach for PWB design process optimization

Advanced packaging technologies have significantly increased the complexity o f the printed wirincl board design (PWB) process. Ever-stringent product specifications and reduced design cycle t ime have imposed even bigger challf?np??S on PWB designers The current PWB design process has many drawbacks that pose barriers t o meets these challenges Design information is the most important asset in PWB design To better use and manage multidisciplinary PWB design information, an effective information management approach has been proposed and implemented The proposed approach involves developing an advanced modeling method, termed Integrated Design Process Modeling Methodology (IDPMM). IDPMM consists of three representations that are capable o f characterrzing the data (entities, data relationships, etc ). the process (process flow. hierarchical decomposiliori, etc.), and the process optinlization aspects o f design process IDPMM has also been w e d with DeMAlD (Design Managers Aide for Intelligent Decomposition). a expert system dwe loped by NASA Langley, for ach~eving true optinial PWB desigri As a case sttrtly. IDPMM was applied to a simple exemplar PWB design pt-oblom. The case study results showed that IDPMM could improve the erriciency of the co!iveniional PWB design process by considerably I-educing the number of fettdbacks and design cycle t ime For a lal-ge-scale PWB dnsigrr project which involves numerous complex rnultirliscipllnary interactions and feedhacks, the efficienry can he fur thr~r imp~ovpr l

Solder joint design optimization for fine pitch component applications

Fine pitch leaded components, such as TSOP/QFP, have been widely used in portable electronics in recent years. One of the most critical issues in the electronic packaging industry is to find effective ways to reduce manufacturing related solder joint defects in these electronic components. During the fine pitch component reflow process, major failure mechanisms include solder bridging, solder opens, insufficient soldering, etc. This research aims to develop a physics-based validated modeling methodology, allowing for effective simulation of the solder joint formation process and prediction of the above solder defects. The goal of the methodology is to determine the optimal solder joint configuration (i.e. bond pad size, stencil aperture design, solder volume, etc.) in a cost-effective manner. The solder joint formation process during the solder solidification stage has been simulated using the Surface Evolver software tool. This paper consists of three integral parts: (1) simulation of the 3D solder joint formation process for the final solder joint geometry configurations; (2) determination of the optimal pad/stencil aperture; and (3) determination of the optimal solder paste volume and material. The methodology can also enable one to analyze the safety margin for a given pad/stencil aperture design. Finally, the simulation models were used to accurately pinpoint the deficiencies in certain pad/stencil aperture designs, which can cause solder bridging, opens or insufficient soldering.