Nanomechanical Characterization of Single Micron-Sized Polymer Particles J. Y. He, 1 Z. L. Zhang, 1 H. Kristiansen 2 1 NTNU Nanomechanical Lab, Department of Structucal Engineering, Norwegian University of Science and Technology (NTNU), 7491 Trondheim, Norway 2 Conpart AS, 2013 Skjetten, Norway Received 1 October 2008; accepted 10 December 2008 DOI 10.1002/app.29913 Published online 14 April 2009 in Wiley InterScience (www.interscience.wiley.com). ABSTRACT: The mechanical characterization of single micron-sized polymer particles is very important for understanding the anisotropic conductive adhesives inter- connection. In this article, a nanoindentation-based flat punch method was employed to investigate the mechani- cal properties of single polymer particles. A diamond flat tip, instead of a commonly used sharp tip for indentation, was specially designed to deform single polymer particles. The maximum applied load is 10 mN and the linear load- ing/unloading rate is 2 mN/s. Two types of amorphous polymer particles were examined. The polymer particles display significantly different stress–strain behaviors. The material responses at different strain levels were analyzed and compared. A particle size effect, the smaller the diam- eter, the harder the particle, on the compression stress– strain behavior, was observed. V V C 2009 Wiley Periodicals, Inc. J Appl Polym Sci 113: 1398–1405, 2009 Key words: amorphous; crosslinking; mechanical properties; stress; strain INTRODUCTION Polymer particles have received much attention in materials science and pharmaceutical and chemical industries because of their novel characteristics, such as the strong adsorption capability, the surface reac- tive ability, and the large specific surface area. Examples include carriers for biomolecules in the biomedical field, 1 the reinforced composite in light concrete, 2 and the toughening phase in the high- impact polymer materials, 3–6 etc. Recently, there is a renewed interest in exploiting polymer particles to- ward use in the manufacturing of electronics and microsystems. One example is the use of metal- coated polymer particles in the anisotropic conduc- tive adhesives (ACA), in which the typical size of particles is from 3 to 10 lm. The metal-coated poly- mer particles have potential advantages in terms of reduced package size, of being lead-free, and by reducing manufacturing cost. The substitute of compact metal particles with metal-coated polymer particles improves the compliance of the intercon- nection and hence enhances the reliability of the as- sembly. 7–12 The electrical characteristics as well as the reliability of the interconnection are partly deter- mined by mechanical performance of polymer par- ticles. There is also a significant interest for larger metal-coated polymer particles with the diameter of 50 to several hundred microns for use in ball grid arrays and chip scale packaging. In these applica- tions, the added compliance is expected to improve the reliability of the interconnect. There is also a cru- cial advantage in terms of reduced environmental impact, by reducing the amount of heavy metals. 13–15 Therefore, the knowledge of mechanical properties of single-polymer and metal-coated polymer par- ticles is of great interest for many potential applications. Most studies on polymer particles have been focused on synthetic methods and processes. The lit- erature concerning mechanical properties of single particles is relatively sparse. However, mechanical characterization of single particles possesses chal- lenges due to the inherent complexity of the spheri- cal geometry as well as the large deformation involved. In the past, the mechanical behavior and electrical resistance of ACA assemblies were typically meas- ured through grouping a number of particles (typi- cally several hundreds) between two polished silicon chips. 8,16 Mostly, ACA assemblies were designed to determine the mechanical and electrical contact properties of the interconnect component including metal-coated polymer particles. The effect of elastic recovery on the electrical contact resistance, the deg- radation mechanism, and the reliability of ACA Journal of Applied Polymer Science, Vol. 113, 1398–1405 (2009) V V C 2009 Wiley Periodicals, Inc. Correspondence to: Z. L. Zhang (zhiliang.zhang@ntnu.no). Contract grant sponsor: Research Council of Norway (through NANOMAT KMB Project); contract grant number: NANOMAT-169737/S10. Contract grant sponsor: Conpart AS and Invitrogen AS.