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E.H. Wong MicroSystems, Modules & Components Lab., Institute of Microelectronics, Singapore 117685 (Received 11 September 2004; accepted 18 February 2005)

Non-conductive adhesive (NCA) flip chip interconnects are emerging as attractive alternatives to lead or lead-free solder interconnects due to their environmental friendliness, lower processing temperatures, and extendability to fine pitch applications. The electrical connectivity of a NCA interconnect relies solely on the pure mechanical contact between the integrated circuit bump and the substrate pad; the electrical conductivity of the contact depends on the mechanical contact pressure, which in turns depends to a large extend on cure shrinkage characteristic of the NCA. In addition, to reduce the cost and increase the output, NCA is usually ramped up to 200 °C and cured for 1 min during the assembly process. However, fast cure reaction poses a great challenge for the accurate measurement of cure shrinkage. In this paper, to precisely determine the cure shrinkage at high temperature, cure shrinkage was first measured at lower temperatures with slow reaction rate by means of thermomechanical analyzer and then extrapolated to high temperatures. With the increase of cure temperature, the maximum of degree of cure will increase, but the maximum cure shrinkage reduces due to the expansion of materials at higher cure temperature. Furthermore, the slopes of the linear relationship between the cure shrinkage and the degree of cure at different cure temperatures after gelation were found to be similar and independent of the cure temperature. The cure shrinkage from gel-point to complete curing was determined to be 4.275%.

I. INTRODUCTION

As electronic packaging requirements are driven toward smaller, higher density, and lower cost solutions, flip chip polymeric interconnects that provide an environmentally friendly solution for interconnections by replacing lead-based solder balls are well positioned to meet these challenges.1–3 As an alternative interconnection technology to solder, non-conductive adhesive (NCA) offers many advantages such as easier processing, lower cost, and extendability to fine pitch applications. The NCA flip chip interconnects are formed through mechanical contact between the bump on the silicon chip and the pad on the substrate that enables the electrical connectivity between them (Fig. 1). The quality of the electrical contact depends solely on the magnitude of the mechanical contact pressure: high mechanical contact leads to low electrical contact resistance. A high mechanical contact pressure is critical to enabling a low

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Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2005.0159 1324

http://journals.cambridge.org

J. Mater. Res., Vol. 20, No. 5, May 2005 Downloaded: 11 Apr 2016

electrical contact resistance, which is induced by the cross-linking shrinkage and thermal strain during the curing and cooling process. The magnitude of the mechanical contact pressures depends on three factors: The a