Pearl Agyakwa

Physics-of-Failure Lifetime Prediction Models for Wire Bond Interconnects in Power Electronic Modules

This paper presents a review of the commonly adopted physics-of-failure-based life prediction models for wire bond interconnects in power electronic modules. In the discussed models, lifetime is generally accounted for by loading temperature extremes alone. The influence of the time spent at temperature on bond wear-out behavior and damage removal phenomena resulting from thermally activated processes is not addressed. The phenomenological considerations based on some unusual observations highlight the need for new approaches to wire bond life prediction models and thus motivate the proposal of a new time-domain damage-based crack propagation model.

A time-domain physics-of-failure model for the lifetime prediction of wire bond interconnects

A new physics-of-failure lifetime prediction model for wire bonds is proposed. It discards the usual cycle-dependent modeling methodology and is instead based on a time domain representation. The bonding interface damage condition is estimated at regular time intervals through a damage model which includes the effect of temperature and time dependent material properties. Thus the impact of time at temperature and other rate sensitive processes on the bond degradation rate can be accurately represented. In addition, the model accounts not only for the damage accumulation processes but also the damage removal phenomena during thermal exposure.

Suitable Thicknesses of Base Metal and Interlayer, and Evolution of Phases for Ag/Sn/Ag Transient liquid-phase Joints Used for Power Die Attachment

Real Si insulated gate bipolar transistors with conventional Ni/Ag metallization and dummy Si chips with thickened Ni/Ag metallization have both been bonded, at 250°C for 0 min, 40 min, and 640 min, to Ag foil electroplated with 2.7 µm and 6.8 µm thick Sn as an interlayer. On the basis of characterization of the microstructure of the resulting joints, suitable thicknesses are suggested for the Ag base metal and the Sn interlayer for Ag/Sn/Ag transient liquid-phase (TLP) joints used for power die attachment. The diffusivities of Ag and Sn in the ξAg phase were also obtained. In combination with the kinetic constants of Ag3Sn growth and diffusivities of Ag and Sn in Ag reported in the literature, the diffusivities of Ag and Sn in the ξAg phase were also used to simulate and predict diffusion-controlled growth and evolution of the phases in Ag/Sn/Ag TLP joints during extended bonding and in service.

Unusual Observations in the Wear-Out of High-Purity Aluminum Wire Bonds Under Extended Range Passive Thermal Cycling

This paper reports on the reliability of ultrasonically wedge-bonded 99.99% (4N) and 99.999% (5N) pure aluminum wires under different passive thermal cycling ranges, namely, -40°C to 190°C, -60°C to 170°C, -35°C to 145 °C, and -55°C to 125°C. The rate of bond strength degradation during cycling was found to be more rapid in the wire bonds subjected to lower peak temperatures (Tjmax) and lower temperature ranges (ΔT) for both wire types. This observed effect of ΔT cannot be described by the commonly accepted empirical relationships based on damage accumulation, such as the Coffin-Manson law. In addition, the 4N wire bonds were found to degrade more rapidly than the 5N bonds under the cycling ranges investigated. Microstructural characterization and nanoindentation of the bond interfaces indicated differences in microstructural restoration in wires subjected to the different cycling ranges. These differences have been attributed to annealing phenomena occurring in the wires during the high-temperature phase of cycling, which are believed to remove some of the damage accumulated during the low-temperature phase. A model is proposed for the prediction of wire bond wear-out rate, which incorporates both damage accumulation and damage removal mechanisms. We conclude that the rate of annealing during cycling varies exponentially with temperature; the annealing effects which occur can reduce damage accumulation and therefore influence wire bond reliability.

Room-Temperature Nanoindentation Creep of Thermally Cycled Ultrasonically Bonded Heavy Aluminum Wires

Recent findings suggest that creep occurs during thermal cycling of ultrasonically bonded wires, the extent of which is influenced by the nature of the temperature cycle, particularly its peak temperature. In this work, this hypothesis is investigated through a study of the power-law creep behavior of bonded 375-μm aluminum wires that have been thermally cycled. Data from a study of two wire purity levels (99.999% and 99.99%) and two different cycling profiles (−55°C to 125°C and −60°C to 170°C) are presented. Room-temperature creep stress exponents are derived for the wire bonds from constant-load nanoindentation tests and compared with their respective microstructures.

High power density, low stray inductance, double sided cooled matrix-converter type switch

This paper presents an advanced integration approach for vertical power semiconductor devices. Based on recently demonstrated surface bump technology, it advances previous work by implementing a flip-chip stacking concept, which results in an improved solution for space exploitation, device performance optimization and assembly process simplification. As a case study, the design of a high-voltage bidirectional switch is considered, for which a prototypal assembly is developed and preliminary functional tests are carried out.

Thermal and mechanical design optimization of a pressure-mounted base-plate-less high temperature power module

We discuss the mechanical and thermal design of a high temperature pressure-mounted base-plate-less power module for application in a continuous high temperature (150°C) ambient.

Damage Evolution in Al Wire Bonds Subjected to a Junction Temperature Fluctuation of 30 K

Ultrasonically bonded heavy Al wires subjected to a small junction temperature fluctuation under power cycling from 40°C to 70°C were investigated using a non-destructive three-dimensional (3-D) x-ray tomography evaluation approach. The occurrence of irreversible deformation of the microstructure and wear-out under such conditions were demonstrated. The observed microstructures consist of interfacial and inter-granular cracks concentrated in zones of stress intensity, i.e., near heels and emanating from interface precracks. Interfacial voids were also observed within the bond interior. Degradation rates of ‘first’ and ‘stitch’ bonds are compared and contrasted. A correlative microscopy study combining perspectives from optical microscopy with the x-ray tomography results clarifies the damage observed. An estimation of lifetime is made from the results and discussed in the light of existing predictions.

Methodology for identifying wire bond process quality variation using ultrasonic current frequency spectrum

Life time prediction of power electronic modules is becoming increasingly important in order to reduce unscheduled maintenance and unexpected failures. Recent developments in life time estimation of standard power electronic modules determine the nominal life time degradation under operating conditions and/or under harsh environments. However, it is important to obtain life time degradation information considering underling process variation originating from the manufacturing line. In this work, a methodology for obtaining a non-destructive assessment of bonding quality is investigated. This is done with the view of capturing information about bonding quality prior to service exposure, and hence determining the effect of the observed variation in bond quality on reliability. Analysis of the frequency spectra of signals obtained from the ultrasonic generator of a wire bond machine reveals it is a process sensitive parameter. The analyzed results show a good correlation between the frequency and amplitude values of the generator output signals and bond quality. 3D x-ray scans of bonds provide further non-destructive evaluation and validate the observed link between the observed generator output signals and bond quality.

A highly integrated high-voltage bi-directional switch

In recent years, a bondwire-less integration approach has been demonstrated for vertical power devices [1]. It is based on the use of bumps (e.g. small blocks of copper) to connect the top of vertical power components. This enables a significant improvement in power density and performance compared with standard modules, the major benefits being a dramatic reduction of the stray inductance and the possibility for double-sided cooling. The approach of [1] was extended to stacked device integration in [2], which proposed a prototypal half-bridge switch design. In this paper, an approach is presented which advances the previous work by implementing a front-to-front device stacking concept, thus resulting in an improved utilization of space, optimized device performance and a more simplified assembly process. As a case study, the design and implementation of a high-voltage bi-directional switch is considered, and upon which preliminary functional tests are carried out.