Golta Khatibi

The 'size effect' on the stress-strain, fatigue and fracture properties of thin metallic foils

In this investigation a microtensile machine in combination with a non-contacting laser-optical speckle correlation sensor to determine strain with high resolution was used to study the stress-strain behavior of thin metallic foils of Cu and Al with varying thickness ranging between 10 and up to 250 μm. The grain sizes varied between 2 and up to 250 μm. A size effect was detected resulting in an influence mainly on the fracture strain. This effect will be explained on the basis of texture differences, the number of activated gliding systems as a dependence on the ratio of grain size to foil thickness. To support these experimental findings the fracture topography has also been investigated. In addition, the fatigue crack propagation properties of the above mentioned Cu foils were studied as function of thickness. Using a specially designed fatigue testing set-up it was feasible to determine crack growth curves from free-standing foils. Depending on thickness, an unexpected crack growth behavior was detected. Using the ECCI technique it was feasible to study the interaction of the global dislocation arrangement with the crack tip.

Thermo-mechanical analysis of bonding wires in IGBT modules under operating conditions

Abstract The lifetime of IGBT (Insulated Gate Bipolar Transistor) modules is limited by thermo-mechanical fatigue. Thereby bonding wires represent the critical links where damage initiation is observed. For the first time Laser Doppler Vibrometer measurements and thermal imaging were employed to determine the temperature-dependent deformations of bond wires at different frequencies under operation conditions. This should be considered as an important step to facilitate more precise life-time predictions of power modules in long term usage.

A fast mechanical test technique for life time estimation of micro-joints

Abstract A novel accelerated mechanical testing method for reliability assessment of micro-joints in the electronic devices is presented as an alternative to time consuming thermal and power cycling test procedures. A special experimental set-up in combination with an ultrasonic resonance fatigue testing system and a laser Doppler vibrometer is used to obtain fatigue life curves of micro-joints under shear loading. Using this method fatigue life curves of Al wire bonded micro-joints were obtained up to 10 9 number of loading cycles and discussed with regard to micro-mechanisms of the bond failure. Failure analysis of the fatigued micro-joints showed that the predominant failure mechanism of power cycling tests, bond wire lift-off, was reproduced by the mechanical testing procedure. Life time of the micro-joints was modelled using a Coffin–Manson type relationship and showed a good correlation to life time curves obtained by power cycling tests. The major advantage of the proposed fast mechanical testing method is the significant reduction of the testing time in comparison with conventional thermal and power cycling tests. Furthermore subsequent examination of the failure surface provides a reliable tool for improvement of the bonding process. The proposed high frequency fatigue testing system can be applied as a rapid qualification and screening tool for various kinds of interconnects in electronic packaging.

Gigacycle fatigue of ultra high density sintered alloy steels

Abstract Sintered steel specimens with density levels of up to 7·6 g cm−3 have been prepared from Cr–Mo and Mo prealloyed powders. The fatigue response has been studied using an ultrasonic resonance testing device that enabled testing up to 109 cycles. It showed that the fatigue endurance strength can be drastically increased by raising the density and that the sintering conditions are effective, though less than the density. The existence of a true fatigue limit was disproved up to 109 cycles for all materials tested, with sintered steels thus being similar to wrought ones. Cr–Mo steels was shown to be superior to Mo alloyed grades due to the markedly finer as sintered microstructure and higher sintering activity. Fatigue crack initiation was found to originate from pores at first at multiple sites, with microstructural orientation being dominant compared to the direction of stress; with progressive loading, some cracks join to form a propagating macrocrack from which the final failure then starts.

A novel accelerated test technique for assessment of mechanical reliability of solder interconnects.

Abstract New generations of lead-free solder interconnects are widely used in consumer electronics. Reliability of the devices which are subjected to rough handling, depends on the fracture resistance of the solder interconnects to shock and mechanical loading. The conventional reliability testing procedures are reported to be expensive and time consuming. Thus alternative tests and evaluation methods for reliability assessment of solder joints are required. In this study a new method for quality assessment of solder interconnects under high strain vibrational shear loading is presented using an ultrasonic fatigue testing system in combination with a special experimental set-up. Using this technique lifetime curves for solder ball bonds of two different Sn–Ag–Cu lead-free alloys were obtained. Failure mechanisms of the solder ball bonds were studied using SEM methods and the reliability curves were discussed with regard to the failure modes and the composition of the lead-free alloys. The applicability of the proposed method is discussed with regard to the literature data.

Experimental and analytical study of geometry effects on the fatigue life of Al bond wire interconnects

The reliability of power electronic devices is mainly limited due to thermo-mechanical fatigue of the internal bond wire interconnections. The bond wire shape is already defined at the design stage of the device. Thus preliminary lifetime assessments become more and more important in order to satisfy the high quality demands and the short time to market of the devices. In this study a fast experimental test setup is used in order to determine the lifetime of a large number of wire bond shapes. Furthermore an analytical model is applied to calculate optimized wire bond shapes for a given set of parameters. The results of this investigation should help to optimize the shape parameters at an early stage of development using the presented analytical model in combination with the fatigue tests.

Damage accumulation and fracture in aged lead-free Sn-3.5Ag solder joints

Owing to concerns regarding the environment and health, lead-containing solders have now been eliminated and substituted by their lead-free counterparts. Hence, the present article is devoted to the clarification of their mechanical strength and reliability. Lead-free Sn-3.5Ag solder joints of various thicknesses were exposed to different thermal treatments in order to study the effect of material changes due to ageing. Thereafter, tensile tests were performed showing a pronounced decrease of strength after excessive heat treatment. The theoretical analysis is facilitated by simulations according to the finite element method. Thereby, the influence of material changes in the solder could be separated from the effect of thermal recovery in the copper base material. Crack initiation in the solder is described by an approach of damage mechanics derived from a thermodynamic framework. Excessive heat treatment leads to Kirkendall voids reducing the ultimate tensile strength of solder joints. Therefrom, one can estimate the reduction of tensile strength as a function of time and temperature.Graphical abstract

Microstructural Investigation of Interfacial Features in Al Wire Bonds

In the present study the microstructure of ultrasonically bonded Al wires on AlSiCu and AlSi metallization was investigated by means of scanning electron microscopy, electron back-scattered diffraction, and high-resolution transmission electron microscopy techniques. Detailed microstructural investigations were conducted on samples in the as-bonded condition, subsequent to power cycling tests, and after long-time thermal exposure to reveal the temperature-dependent evolution of the interfaces and the metallization layer. Typical interfacial features were found to be ultrafine and nanoscaled grains of Al and Al2O3, amorphous Al oxide particles, voids, and pores, with regions of high density of dislocations and dislocation loops within the larger grains of the wire and metallization. The observed interface features confirm the suggested mechanism of formation of bonding interface by emergence of submicron grains at the thin interfacial boundary between the metallic pair as a result of dynamic recrystallization and interdiffusion. While isothermal and/or thermomechanical cycling lead to strong grain growth in the metallization layer and the Al wire, the nanostructured interfacial regions mainly remain, indicating a high thermal stability and strength of the interface. Furthermore, evaluation of a large number of wire bonds prepared using standard bonding conditions showed the presence of a certain percentage of nonbonded areas and microstructural variations between the interconnects processed under nominally identical conditions. However, it was found that, if a sufficient effective bonding interface is provided, the long-time reliability of Al wire bonds is maintained due to the stability and strength of the nanostructured interface.

Sn-Ag-Cu Nanosolders: Solder Joints Integrity and Strength

Although considerable research has been dedicated to the synthesis and characterization of lead-free nanoparticle solder alloys, only very little has been reported on the reliability of the respective joints. In fact, the merit of nanoparticle solders with depressed melting temperatures close to the Sn-Pb eutectic temperature has always been challenged when compared with conventional solder joints, especially in terms of inferior solderability due to the oxide shell commonly present on the nanoparticles, as well as due to compatibility problems with common fluxing agents. Correspondingly, in the current study, Sn-Ag-Cu (SAC) nanoparticle alloys were combined with a proper fluxing vehicle to produce prototype nanosolder pastes. The reliability of the solder joints was successively investigated by means of electron microscopy and mechanical tests. As a result, the optimized condition for employing nanoparticles as a competent nanopaste and a novel procedure for surface treatment of the SAC nanoparticles to diminish the oxide shell prior to soldering are being proposed.

Growth behavior and physical response of Al-Cu intermetallic compounds

This review covers recent investigations and concludes our findings for the growth of Cu/Al intermetallic compounds (IMC). [1, 2] The corresponding copper-aluminum interfaces were either established by a physical vapor deposited (PVD) Cu layer on a PVD aluminum pad or a Cu thermosonic nailhead bond on a PVD aluminum-based pad metallization. The identification, growth kinetics and mechanical strength of the different Al-Cu intermetallic compounds have been investigated. The annealing matrix of these investigations covered the temperature range from 150-300 °C for 25-2000 h. The identification of the Al-Cu phases utilizes X-ray diffraction analysis (XRD), selected area diffraction pattern (SAD) and scanning electron microscopy (SEM) & energy dispersive X-ray spectroscopy (EDX). The main three IMC phases Al4Cu9, AlCu and Al2Cu were identified over the whole temperature range, whereas two additional phases (Al3Cu2, Al6Cu94) contribute to the total IMC growth at temperatures above 200 °C. Individual diffusion constants D0 and activation energies Ea of 1.0 eV for Al4Cu9 and AlCu, 1.2 eV for Al2Cu and 1.3 eV for the total IMC growth have been obtained. As the two slow growing phases Al3Cu2 and Al6Cu94 were not observed below 200 °C, lower activation energies for the total IMC stack were expected and have been measured to be in the range of 1.05-1.1 eV for thin film and bonded samples for temperatures below 200° C. Therefore it is recommended to use these lower activation energies for lifetime predictions in the typical regime of device application temperatures. The impact of IMC thickness and annealing conditions on bond strength was studied using ball shear test. The test results did not show any hints on interface strength degradation across the full experimental matrix even for the groups where Al was already fully consumed, in case of a tungsten barrier or adhesion layer between Al metallization and silicon-based dielectrics was used.