Creep and microstructural development in P91 weldments at elevated temperature

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, with each bead consisting of a columnar region, a coarse-grained region and a fine-grained region (the latter two regions resulting from heat-treatment of the weld bead by deposition of subsequent beads). The columnar regions exhibited high hardness whereas the coarse and fine grained regions exhibited lower hardnesses. SEM imaging revealed that the precipitate distribution throughout the weld was somewhat inhomogeneous, due to inadequate mixing in the weld pool during welding, leading to segregation and liquation effects. Examination by TEM revealed a fine martensitic structure with a distribution of chromium carbides, in addition to Mn-rich inclusions. Anisotropy of microstructure was assessed by metallographic examination on planes with normals parallel to and perpendicular to the welding direction. Creep tests on this material were performed, with the stress axis both parallel and perpendicular to the welding direction. Anisotropic creep behaviour was observed and correlated with the microstructural anisotropy. Failure life is significantly longer when uniaxial creep stress is parallel to the welding direction. The columnar regions of the weld were observed to be creep-strong with a low strain to failure whereas the coarse and fine grained regions were observed to be creep-weak with a higher strain to failure. Microstructural variations within weldments as a function of time and temperature have also been investigated. Specimens were aged at five temperatures between 760°C and 650°C for up to 12000 hours. At all exposure temperatures, the parent metal showed little change in terms of fine (subgrain) microstructure and hardness. Significant degradation of the weld metal microstructure was observed. This consisted of recrystallisation, emanating from the weld bead boundaries; in some cases, the recrystallised areas made up approximately 40 % of the metallographic section. The hardness of the recrystallised regions was typically 170 kgf mm-2, whereas that of the non-recrystallised areas was 240 kgf mm-2. TEM examination of the weld metal showed significant change, in the form of transformation of fine martensitic lath structure to larger, more equi-axed subgrains. Creep tests of aged crossweld samples showed accelerated minimum strain rates and reduced failure lives. It was also observed in crossweld specimens creep-tested at three stress levels between 70 MPa and 93 MPa that the failure location moved from the fine-grained HAZ to the parent at the highest test stress. The HAZ failures exhibited extensive cavitation restricted to the HAZ, and low failure ductility. The high stress parent metal failure, on the other hand, showed high ductility and extensive voiding and grain deformation within the parent metal microstructure. An assessment of the effect of strain on microstructural evolution has been made. This is deemed significant, and strain is believed to accelerate precipitate coarsening and martensite recovery processes

A non-destructive study of crack development during thermal cycling of Al wire bonds using x-ray computed tomography

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 datasets were obtained prior to and after cycling. Cracks could be seen emerging from the extreme ends of the bonds when imaged after 105 cycles. Subsequent cycling lead to the advancement of these cracks toward the centres of the bonds. In addition, damage developed within the interior of the bonds; these also grew with increase in number of cy¬cles, and merged with existing cracks. Virtual cross-sections have been analysed to quantify the rate of damage build up

Predicting lifetime of thick Al wire bonds using signals obtained from ultrasonic generator

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 the classification method is capable of accurately predicting bond quality, indicated by bonded area measured by X-ray tomography. Samples classified during bonding were subjected to temperature cycling between -55 °C and +125 °C, and the distribution of bond life amongst the different classes was analyzed. It is demonstrated that the as-bonded quality classification is closely correlated with thermal cycling life and can, therefore, be used as a nondestructive tool for monitoring bond quality and predicting useful service life

Interdiffusion coefficients of binary multiphase systems with consideration of variation in Molar volumes

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 concentration and/or Molar fraction at the interfaces of the different phases

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

Time-efficient sintering processes to attach power devices using nanosilver dry film

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 combined parameters of sintering temperature 250 °C, sintering pressure 10 MPa and sintering time 5 s were selected as the benchmark process to attach 2 mm × 2 mm × 0.5 mm dummy Si devices. Then the effects of either the sintering temperature (240 to 300 °C), time (1 to 9 s) or pressure (6 to 25 MPa) on the porosity and shear strength of the sintered joints were investigated with 3 groups and a total of 13 experimental trials. The average porosities of 24.6 to 46.2% and shear strengths of 26.1 to 47.7 MPa are comparable with and/or even better than those reported for sintered joints using wet nanosilver pastes. Their dependences on the sintering temperature, time and pressure are further fitted to equations similar to those describing the kinetics of sintering processes of powder compacts. The equations obtained can be used to not only reveal different mechanisms dominating the densification and bonding strength, but also anticipate the thermal-induced evolutions of microstructures of these rapidly sintered joints during future reliability tests and/or in service

Packaging/assembling technologies for a high performance SiC-based planar power module

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-less SiC-based power module with low parasitic inductance; (ii) demonstrated feasibility and reliability for the sintering of Ag nanoparticles and flexible printed circuit board as alternative joining and interconnect technologies which have been selected to assemble the designed power module; and (iii) preliminary construction of the designed module and electrical switching test of the constructed module

Reliability and Characterization of Nanosilver Joints Prepared by a Time-Reduced Sintering Process

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 time of a few seconds. The reliability of the sintered attachments were evaluated against Pb5Sn solder joints under constant temperature swing power cycling conditions over the range 50 to 200 °C. The thermal performance and microstructural changes of the sintered joints were monitored and evaluated non-destructively at regular intervals using transient thermal impedance and X-ray computed tomography. The results show that sintered joints with higher shear strengths (>50 MPa) and lower porosities (<25 %) tend to maintain their thermal performance up to 100k power cycles before gradual degradation occurs. Sintered joints with intermediate shear strengths (20 to 40 MPa) and with corresponding analogous porosities (35 to 51 %) also demonstrated comparable power cycling behavior; exhibiting a progressive decrease in effective thermal conductivity with increasing cycles. The evaluated lifetime of sintered joints with the highest shear strengths were found to be at least double those for the lower shear strength joints, and up to fourteen times those of a Pb5Sn solder die attachment. Even the most porous sintered joints exhibited lifetimes appreciably longer than a Pb5Sn die attachment. Degradation in thermal resistance was seen to correlate with observed microstructural changes, with a dependence on initial sintering parameters

Comparative Thermal and Structural Characterization of Sintered Nano-Silver and High-Lead Solder Die Attachments During Power Cycling

13.5 mm × 13.5 mm sintered nano-silver attachments for power devices onto AlN substrates were prepared at 250 ºC and a pressure of 10MPa for 5 minutes and compared with Pb5Sn solder joint die attachments under constant current power cycling with an initial temperature swing of 50-175 ºC. Both the effective thermal resistance and microstructural evolution of the samples were monitored using transient thermal impedance measurement and non-destructive X-ray computed tomography at regular power cycling intervals. The results showed a gradual increase in the effective thermal resistance of the Pb5Sn solder joints from 0.047 to 0.133 K/W from zero to 41k power cycles, followed by a rapid escalation to 0.5018 K/W at 52k cycles. This was accompanied with the formation and development of oblique cracks within the Pb5Sn die attachments until delamination occurred at the solder/device and solder/substrate interfaces. By contrast, the effective thermal resistance of the sintered Ag joints remained almost constant at 0.040 K/W up to 116k power cycles. This was explained in terms of thermally induced continuation of densification of the sintered structure and the formation and development of networked vertical cracks within the sintered Ag die attachments, some of which further extended into the Cu tracks of the AlN substrate

Shear strength of die attachments prepared using dry nanosilver film by a time-reduced sintering process

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 investigated using uniform design and single factor experimental trials. The shear strength data series for each of the experimental trials was statistically analyzed to indicate any deviation from normal distribution. The resulting shear strength values were regressed or fitted, and further analyzed by polynomial and kinetic-like equations to estimate the relationship between the shear strength and sintering parameters. The regression analysis for shear strength may not only be used for explaining mass transportation mechanisms, but also for identifying the proposed manufacturability of the time-reduced sintering process