Determining thermal and mechanical properties of polyimide using the DIC method

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ABSTRACT Polyimide (PI) is widely used in the aer ospace and microelectronics industry and engineering due to its excellent electric and mechanical properties; however, the high thermal expansion of the PI causes the problem on the thermal expansion mismatching. A new type of PI with P-Phenylene diamine as diamine monomer is proposed to reduce the thermal expansion. The coefficient of thermal expansion (CTE) and the Young’s module of this PI films are studied in this paper using DIC method. The CTE of the new type PI films is about 21 10 /u  6  , which is greatly reduced. The successful results demonstrated the feasibility of DIC method to meas ure the thermal and mechani cal properties of films. Keywords o PI, DIC method, CTE, mechanical properties, Young’s module 1. INTRODUCTION Polyimide (PI) is an aromatic macromolecule polymer material containing imine in the main chains. PI are typically chosen to fit in these high temperature applications as the coating materials and insulators for electronic parts due to their good thermal stability, high planar ization, easy process, and high glass transition temperature. In the past decades, the PI films with low dielectric constant, excelle nt mechanical properties and outstanding thermal stability have attracted great interest in the aerospace and microelectro nics industry and engineering. [1-3] The coefficient of thermal expansion (CTE) of the PI film is a critical parameter for its using. As the PI films exhibit high thermal expansion and the thermal expansion mismatch is a major source of failure in microelectronic structures, the application of the PI film s is limited [4]. Recently, attempts have been focused on minimizing thermal expansion of PIs [5-8], many kinds of inorganic/PI nano-composites have been studied. In order to evaluate the properties between the nano-composites and the pure PI, the CTE and the mechan ical properties (Young’s module) should be studied. The traditional methods for measuring CTE and mechanical properties are often suitable for bulk specimens [9-11]. These methods are difficult to use to measure the CTE and mech anical properties of films, b ecause of the small-scale of the films. Therefore, some novel methods are performed to measure the CTE and mechanical properties of film specimens [12-15]. There are some limitations for using the methods mentioned in the literatures, such as complex testing process, vibration isolation, and so on. In order to avoid these limitations, the DIC method is proposed to measure the CTE and mechanical pr operties of PI films. In this paper, the full field deformation induced by the thermal expansion and the mechanical loading within the observing area of the material are measured by digital image correlation (DIC) method [1  ,1 ], which is a non-contact, full-field surface deformation measuring method. By using this method the CTE and Young’s module of a new type of PI films have been obtained. In the following, we will first present the principle of the DIC method. Then, the measuring experiment is introduced in detail. Finally some discussion about the testing result is proposed. E-mail address: xiehm@mail.tsinghua.edu.cn Tel: +86-10-62792286, Fax: +86-10- 62781824