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.
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.
Access from the University of Nottingham repository:http://eprints.nottingham.ac.uk/36483/1/Reliability%20of%20thick%20Al%20wire%20A %20study%20of%20the%20effects%20of%20wire%20bonding%20parameters%20on %20thermal%20cycling%20degradation%20rate%20using%20non-destructive %20methods.pdf Copyright and reuse:The Nottingham ePrints service makes this work by researchers of the University of Nottingham available open access under the following conditions. This article is made available under the Creative Commons Attribution Non-commercial No Derivatives licence and may be reused according to the conditions of the licence. For more details see: http://creativecommons.org/licenses/by-nc-nd/2.5/ A note on versions:The version presented here may differ from the published version or from the version of record. If you wish to cite this item you are advised to consult the publisher's version. Please see the repository url above for details on accessing the published version and note that access may require a subscription. AbstractThe effect of bonding parameters on the reliability of thick Al wire bond is investigated. Samples were prepared with 25 different designs with 5 different bonding parameters such as time, ultrasonic power, beginforce, end-force and touch-down steps (pre-compression) with 5 levels. The bond signals of ultrasonic generator were collected during bonding in order to obtain prior quality information of bonded wires. 3D x-ray tomography was then used to evaluate bond quality during passive thermal cycling between -55 °C and 125 °C. Tomography datasets were obtained from the as-bonded condition and during cycling. The results clearly show ultrasonic power, appropriate levels of begin-force and touch-down steps are all important for achieving a well attached and reliable bond. Analysis of the virtual cross-sections indicates a good correlation between the bond signal (i.e. the initial bond quality) and wire bond damage/ degradation rate. An improved understanding of the wire bonding process was achieved by observing the effect of the complex interaction of bonding parameters on the ultrasonic generator signals and degradation rate under thermal cycling.
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.
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