IEEE TRANSACTIONS ON COMPONENTS, PACKAGING AND MANUFACTURING TECHNOLOGY, VOL. 1, NO. 10, OCTOBER 2011 1533 Effect of Temperature on Water Molecules in a Model Epoxy Molding Compound: Molecular Dynamics Simulation Approach Seung Geol Lee, Ji Il Choi, Wonsang Koh, Seung Soon Jang, Jongman Kim, Member, IEEE, and Gene Kim Abstract— The effect of temperature on the distribution and transport of water molecules in a model epoxy molding com- pound (EMC) system is investigated using atomistic molecular dynamics simulation with 4 and 7 wt% water content at various temperatures, such as 298, 323, 353, and 373 K. The thermal expansion of the hydrated model EMC was evaluated as 1–5% of its dried volume with increasing temperature. The spatial distributions of the amine groups and hydroxyl groups are not significantly affected by temperature due to the crosslinked topological constraint. The correlation of these functional groups with water molecules was not affected by temperature due to their hydrophilicity. In contrast, it is observed that the water phase is expanded with increasing temperature, which is more distinct as a function of water content. The temperature effect on the water diffusion was clearly observed: the diffusion coefficient became larger with increasing temperature. The activation energy for the water diffusion via a hopping mechanism was 21.9 kJ/mol (0.23 eV) and 21.2 kJ/mol (0.22 eV) for the 4 wt% and the 7 wt% water contents, respectively, which infers that the water transport is more facilitated with increasing water content because the water structure of the water phase in the model EMC is more developed. Index Terms— Diffusion, epoxy, epoxy molding compound, molecular dynamics, packaging, simulation, water absorption. I. I NTRODUCTION E POXY molding compounds (EMC) have been widely used for electronic packaging applications since it has several good performance attributes, such as strength-to- weight ratio, thermal stability, dielectric constant, and process- ability [1]. However, the moisture uptake of EMC (up to 7 wt%) [1]–[3] has been considered as a serious problem, affecting the product reliability via mechanical breakdown and delamination [2]–[4]. One of the reliability concerns is Manuscript received July 7, 2010; revised April 15, 2011; accepted June 13, 2011. Date of publication September 15, 2011; date of current version October 12, 2011. Recommended for publication by Associate Editor A. Chandra upon evaluation of reviewers’ comments. S. G. Lee, J. I. Choi, and S. S. Jang are with the School of Materials Science and Engineering, Georgia Institute of Technology, Atlanta, GA 30332 USA (e-mail: seunggeol.lee@gatech.edu; jiil.choi@gatech.edu; Seung- Soon.Jang@mse.gatech.edu). W. Koh is with the School of Physics, Georgia Institute of Technology, Atlanta, GA 30332 USA (e-mail: gtg047j@mail.gatech.edu). J. Kim is with the School of Electrical and Computer Engineer- ing, Georgia Institute of Technology, Atlanta, GA 30332 USA (e-mail: jkim@ece.gatech.edu). G. Kim is with the Cookson Electronics Assembly Materials Group, Suwanee, GA 30024 USA (e-mail: gkim@cooksonelectronics.com). Color versions of one or more of the figures in this paper are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/TCPMT.2011.2160343 associated with hygroscopic swelling, which is the expansion of a material due to moisture absorption [7]–[9]. EMCs’ have a capability of absorbing the moisture in humid environ- ments, and the swelling mismatch between different materials in microelectronic devices (such as die, metal lead frames, etc.) induces hygroscopic stresses in the package [8]. This hygroscopic stress increases the effect of thermal stress dur- ing high temperature operation that induces vaporization of the absorbed moisture and abrupt expansion in the package, which leads to electronic package failure such as interface delamination, voiding and cracking [7]–[11]. Although the deteriorating influence of moisture uptake on EMC has been effectively reduced for reliable performance during operation of electronic devices [12], the nature of such moisture in EMC, especially the distribution and diffusion of moisture through EMC has not been thoroughly investigated, which would be partially due to the lack of appropriate experimental tools capable of characterizing the relevant events occurring in the nanometer scale. To obtain good reliability, it is crucial to understand the mechanism of water absorption, transportation, and interaction between water-EMC in the electronic package at a fundamental level. In this situation, simulation techniques, especially a full atomistic molecular dynamics (MD) simulation approach would provide useful information in understanding the behav- ior between the EMC and the absorbed water molecules. Unfortunately, there have been very few systematic studies on the mechanism of water transportation in EMCs at the molecular level. For instance, the diffusion of small molecules, such as water and oxygen, in an epoxy polymer network structure was discussed in a computer simulation study done by Yarovsky and Evans [13], and the interface of EMC with copper was investigated through MD simulation to investigate the water diffusion in another recent publication by Fan and co-workers [14]. However, there is no report on the detailed structural information of the distribution of water molecules around specific polymeric structural units. Since hydrophilic hydroxyl and amine groups in EMCs play an important role in the absorption of moisture [15], it is important to investigate the distribution of water molecules around the hydrophilic groups in EMCs at various temperatures. Jang and his coworkers have been using such MD simula- tion methods for various topics in materials systems includ- ing water diffusion in hydrated polymeric membranes for fuel cells [6]–[19] and mechanical properties of polymeric 2156–3950/$26.00 © 2011 IEEE