Pearl Agyakwa

• Materials Science BSc, Materials Science PhD
• Senior Researcher at University of Nottingham

This study details the novel self-assembly of sodium titanate converted Ti-based microspheres into hierarchical porous 3D constructs, with macro, micro, and nanoporosity, for the first time. Ti6Al4V microspheres were suspended into 5 M NaOH (60 °C/24 h) solutions, with extensive variations in microsphere:solution ratios to modify microsphere intera...

This paper deals with the performance of sintered nano-silver bonds used as wide-bandgap power module die attachment technology. The paper specifically explores the fine-scale microstructures of highly porous sintered attachments under power cycling to provide a deeper understanding of the significance of porosity as a reliability-related microstru...

This paper comments on an ultrasonically welded copper busbar terminal to a copper substrate in a power module through thermal cycling. Welded samples were subjected to rapid cycling conditions to better understand the structure of the welds and their failure mechanism. At regular thermal cyclic intervals, the samples were evaluated with 3D X-ray t...

This study investigates the power cycling reliability of nanosilver sintered joints formed by a time-reduced sintering process, designed for use on a die bonder. A range of sintering parameters, reflecting different levels of manufacturability, were used to produce sintered joints in respect of shear strength and porosity, within a process cycle ti...

This study investigates a time-reduced sintering process for die attachment, prepared, within a processing time of several seconds using dry nanosilver film. The effects of three main sintering parameters, sintering temperature (220 to 300 °C), sintering time (1 to 9 s) and bonding pressure (6 to 25 MPa) on the resultant shear strength are investig...

A time-lapse study of thermomechanical fatigue damage has been undertaken using three-dimensional (3D) X-ray computer tomography. Morphologies were extracted from tomography data and integrated with data from microscopy modalities at different resolution levels. This enables contextualisation of some of the fine-scale properties which underpin the...

13.5 mm × 13.5 mm sintered nano-silver attachments for power devices onto AlN substrates were prepared at 250 °C and a pressure of 10 MPa for 5 minutes and compared with Pb5Sn solder joint die attachments under constant current power cycling with an initial temperature swing of 50-175 °C. Both the effective thermal resistance and microstructural ev...

In this paper, the microstructural evolution of pre-annealed heavy copper wires ultrasonically bonded onto Cu-AlN sub-strates is investigated. Bonds are subjected to thermal cycling between-55 and 150°C. The bond interfaces are non-destructively imaged prior to and after thermal cycling using 3D X-ray tomography. They are then examined using electr...

This paper characterizes thermal and reliability performance of a SiC MOSFET power module with embedded decoupling capacitors and without anti-parallel diodes. Active and passive temperature cycling, supported by transient thermal impedance characterisation and scanning acoustic microscopy, are used to evaluate key degradation mechanisms. The forwa...

Pressure-assisted sintering processes to attach power devices using wet nanosilver pastes with time scales of minutes to a few hours have been widely reported. This article presents our work on time-efficient sintering using nanosilver dry film and an automatic die pick and place machine, resulting in process times of just a few seconds. The combin...

Pressure-assisted sintering processes to attach power devices using wet nanosilver pastes with time scales of minutes to a few hours have been widely reported. This paper presents our work on time-efficient sintering, using nanosilver dry film and an automatic die pick and place machine, resulting in process times of just a few seconds. The combine...

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...

Routine monitoring of the wire bonding process requires real-time evaluation and control of wire bond quality. In this paper, we present a nondestructive technique for detecting bond quality by the application of a semisupervised classification algorithm to process the signals obtained from an ultrasonic generator. Experimental tests verified that...

In this work we report on a reliability investigation regarding heavy copper wires ultrasonically bonded onto active braze copper substrates. The results obtained from both a non-destructive approach using 3D X-ray tomography and shear tests showed no discernible degradation or wear out from initial conditions to 2900 passive thermal cycles from −...

Transient thermal impedance measurement is commonly used to characterize the dynamic behaviour of the heat flow path in power semiconductor packages. This can be used to derive a “structure function” which is a graphical representation of the internal structure of the thermal stack. Changes in the structure function can thus be used as a non-destru...

The samples of sintered Ag joints for power die attachments were prepared using paste of Ag nanoparticles at 240°C and 5 MPa for 3 to 17 minutes. Their microstructural features were quantitatively characterized with scanning electronic microscopy, transmission electron microscopy, X-ray diffraction and image analysis method. The resulting normalize...

In a previous paper, a new time-domain damage-based physics model was proposed for the lifetime prediction of wire bond interconnects in power electronic modules. Unlike cycle-dependent life prediction methodologies, this model innovatively incorporates temperature- and time-dependent properties so that rate-sensitive processes associated with the...

This communication introduces a modification to the Wagner method to reveal and correct considerable systematic errors existing in the previously established analytical methods used to calculate the interdiffusion coefficients of binary multiphase systems where all the phases have narrow homogeneity ranges and there are step changes of concentratio...

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 join...

This paper concerns the non-destructive visualisation of the evolution of damage within ultrasonically bonded alumini-um wires using three dimensional x-ray computed tomography. We demonstrate the potential to observe the progressive accumulation of damage within the same wires during passive thermal cycling between -55°C and 190°C. Tomography data...

This work is to investigate the relevant packaging / assembling technologies for developing a SiC-based planar power module which is aimed to meet the requirements such as operating temperature of -60 °C to 200 °C, SiC devices connected to 540 V DC bus and non-hermetic module. The results reported in this paper include: (i) design of a compact wire...

The specific thermal resistance values of several thermal interface materials (TIMs) intended to thermally enhance Cu contact pairs and their degradation under isothermal ageing at 170 °C have been investigated using Cu stack samples consisting of 10 Cu discs and 9 layers of the TIMs. The results obtained indicate that the specific thermal resistan...

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 i...

A fixed-grid source-based numerical method has been developed to simulate the diffusion-controlled growth of Cu6Sn5 and Cu3Sn intermetallic compounds (IMCs) and other many layers of IMCs. Data fittings of measured thicknesses of the IMCs to the simulated results can be further employed to determine the interdiffusion coefficients for the IMCs. Comp...

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 the...

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...

This paper reports the effect of the addition of 1 wt% Al into 100Sn, 96.5Sn–3.5Ag (SA) and 95.5Sn–3.8Ag–0.7Cu (SAC) solders, on growth rates of intermetallic compound (IMC) layers between the solders and a Cu substrate. During reflow at 260 °C under 2%H298%N2 forming gas for 5–120 min, the most pronounced reduction in IMC growth was observed in th...

Historically, each discipline in power electronics has been pursued independently, and as a result, individual product classes, such as power modules, passive components, thermal management systems, gate drives and converters, have been developed without full consideration of the inter-component interactions. This serial approach to design and manu...

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 depende...

This paper concerns the reliability of ultrasonically bonded high purity thick aluminium wires at elevated temperature. To date, the evolution of the microstructure of wire bonds during thermomechanical exposure and its influence on reliability have not been fully characterised and understood, particularly as they pertain to thermal cycling regimes...

A thin interlayer of pure Sn foil has been sandwiched between two pieces of Cu foil base metal and reflowed at 260, 300 and 340°C, under 2%H2/98%N2 forming gas, for 5 – 480min. We report here: (i) the interfacial microstructures to show Cu6Sn5 scallops and Cu3Sn columnar crystals; (ii) the pronounced difference in the thicknesses of the Cu6Sn5 and...

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. The thermal resistance of potential thermal interface materials (TIMs) has been measured as a function of contact pressure and applied power with the samples aged under pr...

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 simplificati...

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...

The kinetics of the interfacial reaction of a thin layer of Sn sandwiched between two pieces of Ag foil has been investigated at temperatures of 260 °C, 300 °C and 340 °C. A time dependence of the form t1/n with n = 3 was obtained for the kinetics of both the consumption of the Sn remaining and the thickening growth of the Ag3Sn scallops formed bet...

A transient liquid phase (TLP), in which a liquid layer is formed and subsequently solidifies, and other diffusion-controlled phase changes are generally associated with moving phase-change interfaces. Both fixed and variable grid discretization models have been formulated to investigate these diffusion-controlled problems. However, all numerical e...

Cold sprayed Sn–Cu coatings approximately 40 and 25 μm in average thickness were deposited on aluminium and direct bonded copper (DBC) substrates respectively. Both a statistical analysis of coating thickness and a roughness analysis of the coating/substrate interface and the coating surface were carried out for the as-sprayed coatings using scanni...

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. An innovative vertical integration scheme for power semiconductor devices has been presented, and...

The adoption of SiC devices as a viable technology depends crucially on maximising the potential advantages of the material. This is best achieved by the adoption of co-design techniques in which the optimisation of the SiC device is performed in parallel to that of the package and the overall application. This paper considers suitable techniques f...

A numerical modelling method for the analysis of solder joint damage and crack propagation has been described in this paper. The method is based on the disturbed state concept. Under cyclic thermal-mechanical loading conditions, the level of damage that occurs in solder joints is assumed to be a simple monotonic scalar function of the accumulated e...

In this paper, we report on the effects of high temperature bonding, post-bond annealing and extended temperature range thermal cycling on the reliability of ultrasonically bonded Al wires. 99.999% pure Al wires of 375μm diameter were bonded to Al coated silicon dies at temperatures of 25°C, 100°C and 200°C, employing the same bonding parameters. G...

With the constant drive for smaller, more cost effective power electronic systems offering enhanced performance and reliability, Aluminium (Al) ribbon bonding has become an attractive alternative to the well established thick Al wire bonding. Improved electrical performance and assembly cost reduction are the main expected benefits. Although ribbon...

This research concerns weldments in P91 steel and their creep behaviour. Its scope covers three main topics: the microstructure and creep response of the (i) weld metal, (ii) parent metal, and (iii) the effect of extended thermal exposure and creep on the weldments. Microstructural examination of the weld metal revealed an inhomogeneous structure,...

Welds form an integral part of most power and chemical plant structures. At elevated temperature, the service lives of these structures are often governed by the creep behaviour in localised regions of the welds. Efforts have been made to understand the characteristic features of the creep stress distributions in and the deformation behaviour of we...