Printed circuit boards (PCB) are complex geometrical and functional systems that may be exposed to a combination of external and internal loads. In order to evaluate the dynamic behaviour of PCBs in early stages of the development process, modal finite element (FE) simulations are used. Realistic results for a wide frequency range can only be achieved if all the geometrical features, such as PCB assembly, copper layer thicknesses, prepreg structures, etc. with the appropriate material properties are taken into account. To model a printed circuit board including all details such as glass fiber-epoxy compounds and copper traces is possible, but is found to be very time-consuming. A method to model PCBs was developed taking into account the corresponding functional board layout and assembly. In order to ensure an appropriate representation of the layout-dependent local material properties for FE applications without considering the geometry in full detail, a simplified approach based on general composite theory, domain-specific mixture rules and generalized laminate theory was developed. The analytically calculated material property distributions of the PCB such as local stiffness values and densities can be transferred to the meshed geometry. To verify the developed method by comparison with experimentally achieved results, operational modal analysis (OMA) for a frequency up to 25 kHz was carried out by piezo patch transducer. It can be shown that both simulated mode shapes and natural frequencies of the non-assembled board show a very good agreement with the experimental results.

This paper presents a strain transfer investigation for Surface Acoustic Wave (SAW) strain sensors. For evaluation, a SAW strain sensor is assembled with a pretested bond material for potentially high strain transfer on a test holder. The setup is stressed with an axially homogeneous strain up to 500 ppm. The strain transfer ratio is computed from the applied load, the reference measurements with foil strain gauge, and the measured SAW strain sensor signal. The strain transfer performance of the bond material is also investigated with respect to the temperature dependency in the range between 22 °C and 85 °C. At these elevated temperatures an average strain transfer ratio of 0.606 ± 0.7% was measured. Mechanical load cycling tests up to 1000 cycles are used for the evaluation of the elastic fatigue of the bond material. The effects of mechanical load cycling and aging of the bond layer are analyzed with the SAW strain sensor response. After 1000 mechanical load cycles, the transferred strain into the SAW strain sensor is 0.582 ± 0.153%. Finally, the experimental results are compared with the results of a 3D FEM simulation which are deviating by less than 10%.

This investigation is aimed at the modeling of both the fabrication process and the reliability of press-fit interconnections on moulded interconnect devices (MID). These are multifunctional three-dimensional substrates, produced by thermoplastic injection moulding for large-series applications. The assembly process and subsequently the durability of press-fit interconnections has been modeled and proved with a finite element software. Especially, a simulation tool for process optimizations was created and applied. In order to obtain realistic results, a creep model for the investigated base material, a liquid-crystal polymer (LCP), was generated and verified by experiments. Required friction coefficients between metal pin and base material were determined by adapting simulations and experiments. Retention forces of pins pressed into substrate holes during as well after the assembly process, and after temperature loads were predicted by simulations. Additionally, the decreasing extraction forces over time due to creep in the thermoplastic base material have been predicted for different storage temperatures as well with finite element analyses. Following, the numerical results of the process and reliability modeling were verified by experiments. It is concluded that the behavior of the mechanical contact of the pin-substrate system, can be suitably described time- and temperature-dependent.

In this paper a finite element (FE) model has been developed for the simulation of thermo-mechanical stress in the sensor substrate. The bond topology has been optimized with respect to bond material properties. Based on the simulations, experiments with two bond technologies and three bond materials were carried out. The effect of thermo-mechanical stress on an α-quartz sample, attached to a heat-treatable steel carrier has been measured and evaluated with white-light interferometry. The bond quality has been evaluated with shear tests, thermal load cycle tests and micro-sections. The corresponding offset due to the remaining thermo-mechanical stress in the sensor substrate was measured and analyzed with a network analyzer. The investigated bond materials and technologies are providing low offset of the sensor signal and a high coupling of the measuring quantity into the SAW sensor.

This paper demonstrates our approach for the design optimization of wireless torque sensor with FE simulation tools. Such a system comprises the SAW transducer, a torque transfer clip and the machine's shaft. The design process was based on construction, modelling and simulation. In order to optimise the performance, the SAW device, the bonding material and the clip were regarded.

This investigation is aimed at the stability of press-fit interconnections for MID. Moulded Interconnect Devices (MID) are 3-dimensional substrates produced by thermoplastic injection moulding for large-series applications. The assembly process of press-fit interconnections has been correctly modelled with a FE-Software. With this model the behaviour of the mechanical contact pressure, which decreases due to creep in the thermoplastic base material, could be described. Especially, the modelling is a tool to optimize pin and hole geometries. In order to obtain realistic results, the creep law parameters for the applied base material, a liquid-crystal polymer (LCP), was fitted to experimental. The required coefficients of friction were determined by adapting the simulations to the experiments. The time-dependent retention force between a pin and the substrate directly after assembly or after temperature loads could be predicted with finite element analyses, which was used as reliability criterion.

The estimation of product reliability during the design is one a key question in microelectronics. Lifetime is a function of such parameters as geometry, material properties and loads. These parameters have the tendency to vary from the designed ones. A probabilistic FE-simulation combined with statistical analysis is used in this work to create predictive models for the evaluation of inelastic strain amplitudes. Soldered joints are subjected to thermal-cycling conditions as a function of the different design parameters of muBGA and QFN. The models include local and global models and can be used for the prediction of the reliability of muBGAs and QFNs. In order to obtain statistical information on the effect of input parameters the analysis was carried out with parametric FE-models by multiple repetitions according to a three-factor Box-Behnken-design. Material properties (coefficient of thermal expansion), geometry (component size, substrate and chip pad diameters) and also load parameters (temperature range in thermal cycling) were used as random input variables for the deterministic model. The regression coefficients for evaluation of the influence of the independent variables and the cross-terms for BGAs and QFN were calculated. The developed quadratic regression models allow the reliability prediction with a precision almost comparable with complex finite element models. Thus results can be made available efficiently and this will allow user to understand the influences on the thermal-mechanical reliability. The model can be also used for the evaluation of the relative improvement of component design rather than the prediction of the absolute value of the lifetime.

A numerical optimisation strategy for interconnections in electronic packaging is demonstrated. The method is based on a toolbox for the parametric generation of finite- element models of package types such as Chip Scale Package (CSP), Micro Lead Package (MLP) or Ball Grid Array (BGA). The novelty of this work is the combination of this modeling toolbox with an optimisation software for automatic parameter variation. Resulting in a convenient tool to investigate the influence of geometry on the relevant quality characteristics of the device. Users can set the parameters to be varied, the ranges of parameter variation and the number of iterations. The optimisation software automatically generates the parameter sets depending on the number of iterations. The generation of a finite-element model for each parameter set, the meshing and the implementation of the required material properties are also automated by the toolbox. Thereafter, the simulation of the desired load conditions results in quality characteristics such as the maximum mechanical stress for each set. After completion of all iterations, the optimisation software provides a user interface for statistical analysis and graphic visualisation of the results. The wirebond geometry is also included in the toolbox. Influence on maximum mechanical stress and fatigue properties under thermal loads is examined during this study. As an example, the effect of the bonding tool geometry on the locations and the value of the maximum mechanical stress in the wirebond material during thermal shocking is determined. This combination of parametric finite-element model generation and automatic parameter variation represents a powerful tool for design automation in packaging technology and product development. The effects of several geometrical parameters on the thermal and mechanical behaviour of packaging interconnects can be predicted. In a virtual product-development process, time- and cost-intensive prototyping and testin- - g can thus be reduced.

The estimation of product reliability during the design is one of the key questions in microelectronics. The assembly lifetime is a function of such parameters as geometry, material properties and loads. All these influences exhibit systematic and stochastic variations. The effect of variability can be analysed by a probabilistic FE-simulation and statistical methods. This paper presents an approach for the prediction of thermal fatigue life of two CSP types, the μBGA and the QFN. Besides geometry parameters, also material properties and cycle temperatures are used as variable inputs. Based on a preliminary study the input parameters were defined as normal distributed. Sensitivities of the lifetime to the design parameters were computed and ranked after FE-simulations for both μBGA and QFN packages parameters had been performed. The fatigue life prediction of solder joints used in this work is based on a Coffin-Manson model and it was performed using the stress-strain data extracted from FE-simulations. As there exists a dependency on the solder deformation behaviour, the correct choice of the deformation model of lead-free solder alloys is an important aspect of this work. Summarising, in this work a probabilistic simulation method was developed to compute realistic failure distributions of two CSP types.

The reliability of flip-chip-interconnects is primarily affected by the thermo-mechanical deformation of the solder ball or bump, which is a consequence of the well-known CTE-mismatch problem. The common approach to determine the stresses and strains for reliability predictions in the critical interconnections is the use of finite elements simulations of the assembly. Simulation results should be verified by experiments in order to prevent systematic errors. A combination of tests and FE-simulations can help to reduce the simulation errors and to verify the computed results. The objective of this work is to apply modern test methods for thermo-mechanical investigations on CSP- and flip-chip-assemblies and to evaluate these quantitatively and qualitatively. Applied test methods were electronic-speckle-pattern-interferometry (ESPI) and a test chip which exhibits specifically designed thin film structures on the chip surface of the flip-chip assembly

This paper describes two important issues associated with CSP package reliability. Failure due to thermomechanical stress is one of the dominant failure causes of CSPs. FE-simulations are frequently used to analyse local stresses and strains in soldered joints under thermal loads. Consequently stress and strain data are a basis for the lifetime estimation of the assemblies. In order to make a design optimisation it is necessary to identify those design and manufacturing parameters that will affect reliability. Among the factors which have influence on life-time the principal ones are materials, geometry, and assembly process parameters. In assembly technology, geometrical features can be reproduced only within certain tolerances. Also the material properties of all parts are subjected to a certain scatter. The same holds for boundary conditions, such as thermal loads. Only some of the parameters that can affect reliability can be modelled directly in a finite elements analysis. The principal variables were analysed by physical investigation of a large number of assemblies. This analysis yielded variation ranges to be used in the FEA. From FE simulation results sensitivities of lifetimes on these variables were computed and ranked. A second aspect is the statistical nature of reliability data. Thermal cycling experiments typically lead to a wide distribution of lifetimes. Therefore a designer should also be capable of the computation of generic distribution functions. This is a technique which has not yet been established. Therefore we worked out a method which combines the scatter ranges of the input variables with probabilistic methods like Monte Carlo simulation and also with FEA in order to allow for a statistical lifetime prediction. In this way, it was possible to compute also the uncertainty ranges of output parameters. In our case the influence of input parameters on thermal-cycling lifetime was estimated. Parametric studies were conducted to study qualitativ- - e and quantitative effects of several parameters on the lifetime distribution. The Weibull function is well suited to describe the simulated life distributions. Our results revealed that the slope of the Weibull curves from simulations is similar to experimental values. Therefore it was concluded that the dominant sensitivities have been modelled. Summarising, in this work a probabilistic simulation method was developed in order to calculate realistic failure distributions and to compare these directly with experimental failure data. In this way we contribute to the concept of "simulation rather than testing"

The first part of the work reported here is dedicated to the 3D finite-elements modelling of CSP. It will take into account both thermal-mechanical characteristics of the assembly materials and geometric factors. Here an approach of modular and parametric modelling with standardized modules linked via contact elements is proposed. Also the aspects of CAD systems for parameterized modelling are treated. The modular concept of modelling also will allow for efficient calculations with a variable number of bumps. The second part of the paper is dedicated to the materials descriptions, which is necessary for the simulation of CSP assemblies. Measured materials data as well as suitable models for numerical representations are discussed. The relevant thermo-mechanical properties of package and assembly materials are presented. Besides actual solder alloys also polymers like moulding compounds are treated. In order to calibrate and check the simulation results, an ESPI sensor was used for measuring the thermal deformations of specimens under thermal loads in comparison to finite elements simulations. FE-models validated by experimental results present the deformations and stress state of the package under thermal cycling.

Model reduction is a new numerical technique [1, 2] that allows us to obtain an accurate low-dimensional representation of high-dimensional finite elements models. Additionally, in the case of linear models the time to perform model reduction is comparable to the solution time of a stationary problem. Hence model reduction can be employed as a fast solver for a transient or harmonic problem during the optimization process [3]. The goal of the paper is to explore new possibilities and methodology to apply model reduction during design of new packages. The described approach has been limited by us to thermo-mechanical finite element models developed in ANSYS.

To predict the performance and the reliability of microsystems in the development process, simulation is a powerful tool which is still gaining greater importance. The accuracy of the simulation results depends substantially on the quality of the available materials data. Realistic results can only be achieved in thermo-mechanical computations when the temperature dependency as well as the nonlinearities and the time-dependence of the materials properties are taken into account. The first part of the work reported here is dedicated to the data and materials models for the simulation of MEMS assemblies. As part of this work measured materials data as well as suitable models for their numerical representation are demonstrated. In the presentation, the results of the measurements of most of the relevant properties of package and assembly materials are presented. These comprise bonding wires, moulding compounds and conductive adhesives under typical service conditions. The corresponding materials modelling for finite element simulations will be shown. Furthermore, the procedure to predict the performance of MEMS assemblies and interconnections is regarded. On the basis of such an analysis the necessary models and data for prediction of the thermo-mechanical behaviour are presented. It will be demonstrated how these can be utilized in designing MEMS. The finite-elements-simulations will present coupled physical problems as induced by interactions of thermal and mechanical effects. In that way, it will be possible to take into account complex load profiles under realistic service conditions. In a representative example of an industrial pressure sensor the influence of the attachment material and its viscoelasticity-plasticity on the accuracy of the device will be calculated.