Effect of drop impact energy on contact resistance of anisotropic conductive adhesive film joints
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B. Ralph Faculty of Technology and Information Systems, Brunel University, Uxbridge, Middlesex UB8 3PH, United Kingdom (Received 8 December 2003; accepted 20 February 2004)
The contact resistances investigated in this study of anisotropic conductive adhesive film joints using Au/Ni bumps and flexible substrates are found to be increased by the drop impact energy and also by the combined effect of heat/humidity and the impact energy. The samples humidified at 85 °C/85% RH for 384 h, on which impact energy of 50 J was induced, exhibit the most severe results. The contact resistance increases by 700%, which had been about 0.062 ⍀ in the as-bonded condition. The samples without humidification showed a sluggish and gentle increase in contact resistance with induced drop impact energy. The contact resistance was found to be increased by 400% after absorbing 90 J energy. Scanning electron microscopy images show particle deformation due to abrasion and friction between the contacting surfaces resulting from the sudden impact. Joints are also observed with no connections, which signify open circuits. Almost 25% of circuits were found open in the samples (after 384 h in a humid environment), which have suffered severe mechanical shock. Breaking of the conductive layer of the particle and exposing the underlying polymeric portion was also observed. I. INTRODUCTION
Anisotropic conductive film (ACF) consisting of conducting particles in an adhesive matrix provides both attachment and electrical interconnection between electrodes. ACFs are widely used for high-density interconnections between liquid crystal display (LCD) panels and tape automated bonding as a replacement for traditional soldering or wire bonding technology. ACF has the advantage of low-temperature assembly, high-density interconnections, fluxless bonding, and low fabrication cost.1–4 Therefore, ACFs are also expected to increase in use as new interconnect materials in semiconductor packaging that requires smaller and thinner dimensions and very high density interconnections. Their application on flexible substrates in such cases as smart cards, disk drives, and driver chips for LCDs have attracted much interest and widespread use.5 ACFs are adhesives films with anisotropic properties induced by dispersing 0.5 ∼5 vol% of conducting
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Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2004.0245 1662
http://journals.cambridge.org
J. Mater. Res., Vol. 19, No. 6, Jun 2004 Downloaded: 18 Mar 2015
particles into thermoplastic and thermosetting polymer matrices. The dependence of the volume fraction of conducting particles on the conductivity is strongly related to directions in the film. That is, a film filled with a low volume fraction of conducting particles gives a through film (z axis) resistance of less than 1 ohm while providing high resistance in the X–Y plane.6 By controlling the volume fraction of conducting particles, anisotropic conductivity can be imparted to the adhesive film. In a thermo-mechanical system, th
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