Monotonic and cyclic mechanical reliability of metallization lines on polymer substrates
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Andreas Klugb) NanoTecCenter Weiz Forschungsgesellschaft mbH, Weiz A-8160, Austria
Emil J.W. List-Kratochvil Institut für Physik, Institut für Chemie & IRIS Adlershof, Humboldt-Universität zu Berlin, Berlin 12489, Germany
Megan J. Cordilla) Erich Schmid Institute of Materials Science, Austrian Academy of Sciences and Department of Material Physics, Montanuniversität Leoben, Leoben A-8700, Austria (Received 30 January 2017; accepted 20 March 2017)
Mechanical stability of Ag and Cu printed and evaporated metallization lines on polymer substrates is investigated by means of monotonic tensile and cyclic bending tests. It is shown that lines which demonstrate good performance during monotonic tests fail at lower strains during a cyclic bending tests. Evaporated lines with the grain size of several hundreds of nanometers have good ductility and consequently good stability during monotonic loading but at the same time they fail at low strains during cyclic bending. Printed lines with nanocrystalline microstructure, in contrast, demonstrate more intensive cracking during monotonic loading but higher failure strains during cyclic bending. Apart from the grain size effect, the effect of film thickness on the saturation crack density after cyclic bending is also demonstrated. Thinner films have higher crack density in accordance with the shear lag model.
I. INTRODUCTION
Electronics developed for flexible applications, such as rollable displays, wearable sensors, circuitry or flexible batteries,1–5 need to have the ability to stretch and bend without failure. This attribute is in contrast to rigid electronic counterparts which mostly fail through electrical degradation or thermal-mechanical instabilities, such as electromigration, crack formation, or interface delamination. Much research has gone into determining failure mechanisms and fatigue lifetimes of materials used in flexible electronics. Methods include in situ tensile straining to study crack and deformation evolution,6–8 stress–strain development with X-ray diffraction (XRD),8–11 and interface adhesion measurements.12–14 The electrical behavior can also be examined with tensile straining with two- or four-point-probe resistance geometry incorporated into the grips15–17 or with wires attached to the sample 18,19 for tensile straining. Contributing Editor: Erik G. Herbert a) Address all correspondence to this author. e-mail: [email protected] b) Present Address: AVL List GmbH, Hans-List-Platz 1, A-8020 Graz, Austria, [email protected] DOI: 10.1557/jmr.2017.121
The advantage of in situ resistance measurements is the precise measurement of the crack onset strain (COS), or failure strain, of brittle films and lines, such as transparent conductive oxides or brittle metals. The COS can be considered the threshold for stretching of flexible technologies. Not only the COS of brittle materials can be determined with in situ resistance measurements during straining, but also the electrical degradation with continued strain and any resistance recovery during the unloadin
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