Fatigue life prediction of micro wire bonds in electronic packages

Abstract

About 90% of all wire bonded interconnects in semiconductor packages consist of thermosonic wire bonds with diameters in the range of 15 mto 75 m. Recently Cu and Pd coated Cu wires on Al, Ni-Au and bare Cu bond pads are used in fine-pitch packages allowing higher lead counts and smaller pad sizes. The wire bonds related dominant failure modes in the packages are liftoff and fracture of the wire neck. Particularly, the heat affected zone above the nail head has been recognized as one of the vulnerable sites of the package. The standard methods for evaluation of the quality of the wire bonds are pull or shear tests which provide information about their robustness under static loads. However, in the practice the packages are exposed to high thermal and mechanical alternating stresses. The component level reliability tests for evaluation of the plastic packages subjected to thermo-mechanical loads include, among others, thermal cycling (TC) with temperature swings (T) in the ranges of about -40C to 150C depending on the application type. In longterm, thermo-mechanically induced cyclic shear and tensile stresses may lead to failure of the wire bonds. Fatigue is considered as the root cause of failure of metallic wire bonds subjected to thermal (TC) and multi-axial mechanical cyclic (MMC) loading conditions. In the recent years, accelerated mechanical fatigue testing has been proposed as a novel concept and an attractive cost-effective and time-saving method for rapid evaluation of a variety of electronic components. The principle idea of this approach is replacement of thermally induced loading with equivalent and adequate mechanical loading. As a result, vulnerable sites of the devices can be detected in a very short time and physically meaningful lifetime curves can be obtained. However, establishment of realistic lifetime prediction curves based on the fatigue data obtained by accelerated mechanical testing requires not only a qualitative but also a quantitative correlation between the mechanical and thermal cycling data. This can only be achieved by a deep understanding and careful analysis of the thermo-mechanical response of the devices under accelerated mechanical testing and operational conditions in each special application case. One of the main motivations of this dissertation was to establish a relationship between the lifetime of wire bonds subjected to multi-axial mechanical and thermal cyclic loading conditions in order to develop a cost effective methodology for fast evaluation of the packages with focus on the wire bond fatigue. The present dissertation deals also with investigation of low and high cycle fatigue behaviour of thin metallic wires before (freestanding, fresh wires) and after bonding process in plastic packages. Finite Element Methods (FEM) and experimental techniques were applied to study the static and cyclic thermal and mechanical response of wires under the following boundary conditions: i) wire bonds embedded in an encapsulated package, ii) bond wires in a non-encapsulated package and iii) freestanding micro wires in the initial state and heat treated conditions. Several material models were developed for FEM considering the different microstructure and temperature dependent mechanical properties of the micro wires. From Nf (number of cycle to failure) values obtained from various simulations and experiments, the correlation coefficients have been extracted. These correlation coefficients can also be used as acceleration factors between accelerated multi-axial mechanical test and TC testing for fast evaluation of a certain package type, with focus on wire bond fatigue. Furthermore, crystal plasticity finite element method (CPFEM), based on crystal plasticity theory, was applied on freestanding and bonded wires for a better understanding of the fatigue behaviour and prediction of grain orientation and direction for metallic wires at the micro and sub-micro scale. The results of CPFEM simulations have been explained by the changes of values for some material parameters of thin metallic wires in single and polycrystalline grains compared to bulk metal.