Wolfgang H. Müller

Characterization of dual-stage moisture diffusion, residual moisture content and hygroscopic swelling of epoxy molding compounds

Experimental and FEM studies have been undertaken in order to characterize the non-Fickian behavior of moisture absorption, temperature-dependent residual moisture content and hygroscopic swelling of epoxy molding compounds exposed to moist environments. Moisture absorption and desorption tests of two molding compounds and two IC packages using these materials have been carried out by gravimetric methods. Finite element analysis has been performed to simulate the anomalous dual-stage moisture absorption. To consider the residual moisture remaining in the package after the desorption process, a simple method has been developed, which allows for consideration of bake-out conditions and provides much more flexibility, enabling calculation of the non-Fickian moisture desorption with a specific residual moisture content. The coefficient of moisture expansion (CME) has been also measured by coupling the results of thermal mechanical analyzer (TMA) and Thermal Gravitational Analyzer (TGA) at different temperatures. It has been shown that the moisture desorption model of this study can be used as an alternative for TGA.

A strategy for damage assessment of thermally stressed copper vias in microelectronic printed circuit boards

Abstract The thermal fatigue of plated-through vias remains a subject of concern, particularly when exposed to the high operating temperatures associated with automotive applications. In this paper the performance of different types of copper vias in different positions of a printed circuit board is analyzed. To this end a two-scale finite element analysis under the loading conditions of thermal cycling is employed. A new material model for the electrolytically deposited copper accounts for large elastic and plastic deformations and, additionally, for the growth of pores within the material. It is common practice to extrapolate the plastic straining computed within few steps of thermal cycling by means of a Coffin–Manson-Equation. We critical examine this strategy and point out, that a certain number of about 20 cycling steps is necessary to obtain meaningful extrapolations. Furthermore, an extrapolation of the computed porosity up to critical values allows similar conclusions. The presented strategy can serve as a predictive tool for plated through holes and vias and can reduce the need of repetitive experimental failure tests.

Morphology changes in solder joints––experimental evidence and physical understanding

The microstructures of solders in microelectronic components, lead containing as well as lead free, change over time. This is first of all due to comparatively high homologous temperatures which occur during reflow as well as during operation of the component. Moreover, because of the intrinsic thermal mismatch between the various materials that constitute the package substantial mechanical stresses and strains will arise and assist the process of microstructural change. In this paper we will, first, briefly provide experimental evidence for such microstructural change and how it relates to the solder bulk as well as to the various interfaces and passivation materials that are used when a solder joint is formed. Second, we will review state-of-the-art modeling techniques that allow to simulate such changes of microstructure provided certain material parameters are known. For this purpose we will set up all equations and then provide information on all the material parameters required for a numerical solution. We will present computer simulations based on this theoretical framework and study the influence of the material parameters on coarsening and aging and, in particular, examine the impact of mechanical stresses and strains. Finally we will address difficulties and challenges involved, experimental as well as modeling ones.

A solder joint fatigue life model for combined vibration and temperature environments

Electronic assemblies in field use are exposed to a wide range of environmental loads. The interaction of combined loads has to be considered in lifetime estimates of electronic packages. In this paper, we discuss lifetime prediction for lead-free soldered flip chips under vibration load in different temperature environments in terms of solder joint fatigue. Parameters for lifetime modeling are obtained from non-linear and temperature-dependent finite element analysis and lifetime experiments. We introduce temperature dependent coefficients and exponents for the Coffin-Manson-Basquin relationship considering elastic and plastic fatigue behavior. The results indicate that temperature is an important parameter affecting the solder joint vibration fatigue life.

Weibull statistics for multiple flaw distributions and its application in silicon fracture prediction

Focus of this paper is the prediction of silicon fracture in microchips of electronic devices. The strength of silicon chips strongly depends on flaw distributions introduced by wafer manufacturing processes. Consequently strength data scatter and a probabilistic approach to failure is required. For this purpose Weibull theory will be used and combined with test procedures and numerical tools for prediction of fracture probability. Multiple active flaw distributions on the surface of the chip and its sawing edges will be considered. Test procedures for strength measurements and Weibull parameter extraction will be presented. Failure prediction will be demonstrated for a microchip showing multiple active flaw distributions which is assembled on a leadframe.

Computer simulation of the processing of engineering materials with lasers: theory and first applications

Current numerical simulations of laser materials processing are usually based on a greatly simplified process model in order to allow for short computation times. This significantly decreases their flexibility and ability for simulation of the great variation of todays processes and, consequently, accounting for their subtle but important differences. The theory utilized for building a simulation tool presented in this paper can be said to be truly three-dimensional as opposed to other reported work that uses symmetric boundary conditions. The interaction of the free surface of the liquid face is modelled which provides a significant increase in the amount of process detail captured. The investigators contend that numerical modelling of the above can only be achieved credibly using high performance computing methods, in particular parallelization techniques.

Probability of silicon fracture in molded packages [ICs]

The reliability of semiconductor devices in automotive environmental conditions requires optimized package design with respect to geometry, materials and manufacturing processes. Process steps at high temperatures can result in high package stresses at operating temperatures due to mismatch of thermal expansion coefficients of the package compound materials. Device failure may occur caused by failure modes, such as delamination of material interfaces or bulk material fracture. This paper addresses the predictability of silicon bulk fracture in microchips in electronic devices. The typical scatter of silicon strength data requires a probabilistic approach. Weibull theory is applied for the evaluation of the stress state and the computation of fracture probability in a microchip.

Investigation of the solder joint fatigue life in combined vibration and thermal cycling tests

In this paper, we discuss lifetime prediction for flip chips under temperature and vibration loading in terms of the failure mechanisms related to solder joint fatigue. Our approach does not need additional data from the experiment but can be used in the design stage. For lifetime prediction solder fatigue coefficients from the literature and results from Finite Element Analysis (FEA) are processed by a MATLAB-routine. The predictions are compared to range of in-house experiments on combined loading. In the experimental setup, a statistically relevant number of specimens with single bump in-situ resistance monitoring are used to address the statistical scatter of the lifetime. Therefore, statements on the statistical distribution of solder joint failure in combined loading tests can be formulated. A laser vibrometer is used to determine exact accelerations and deflections of the Printed Circuit Board (PCB). In the failure analysis, ion-etched cross sections of the failed solder bumps are prepared. The features of microstructural transformation and crack-paths are discussed for temperature cycling-only, vibration-only, and combined load experiments. Finally, the model prediction is compared to the experimentally determined solder joint lifetimes and the ranges of good agreement are discussed as well as the range with less agreement.

Effect of nonlinear hygro-thermal and residual stresses on the interfacial fracture in plastic IC packages

This study presents a multi-disciplinary approach in order to investigate the influence of hygroscopic stresses and stress relaxation in Epoxy Molding Compounds (EMC) on the apparent interfacial fracture toughness of Cu/EMC interfaces. Bi-material beams were designed and produced via transfer molding similarly to the process used in state-of-the-art semiconductor packaging. An empirical method was used to investigate the so-called ldquostress-freerdquo temperature of the package using warpage measurement of the beams at elevated temperatures. Cure shrinkage was measured by introducing a fitting parameter to the Finite Element (FE) analysis to match the simulation results to the experimental warpage. The bi-material beams were exposed to a humid environment and their warpage was observed to change during the moisture absorption. Warpage measurements together with a viscoelastic FE analysis allow for the calculation of two important material properties, namely, cure shrinkage and the Coefficient of Hygroscopic Swelling (CHS). Fracture toughness of the EMC/Cu interface as a function of exposure time to moisture was measured. End-Notched Flexure (ENF) tests at various intervals after moisture loading were performed. The Virtual Crack Closure Technique (VCCT) was applied to measure the adhesion strength in terms of interfacial fracture toughness by considering the sample manufacturing and sorption history.

SHEARING TESTS APPLIED TO PANTOGRAPHIC STRUCTURES

With the advancements in 3D printing technology, rapid manufacturing of fabric materials with complex geometries became possible. By exploiting this technique, different materials with different structures have been developed in the recent past with the objective of making generalized continuum theories useful for technological applications. So-called pantographic structures are introduced: Inextensible fibers are printed in two arrays orthogonal to each other in parallel planes. These superimposed planes are inter-connected by elastic cylinders. Five differently-sized samples were subjected to shear-like loading while their deformation response was analyzed. Results show that deformation behavior is strong non-linear for all samples. Furthermore, all samples were capable to resist considerable external shear loads without leading to complete failure of the whole structure. This extraordinary behavior makes these structures attractive to serve as an extremely tough metamaterial.