Phenolic Resin Surface Restructuring upon Exposure to Humid Air: A Sum Frequency Generation Vibrational Spectroscopic Study Xiaolin Lu, †,‡ Jianglong Han, † Nick Shephard, § Susan Rhodes, § Alex D. Martin, ‡ Dawei Li, †,‡ Gi Xue,* ,† and Zhan Chen* ,‡ Department of Polymer Science, Nanjing UniVersity, Nanjing, People’s Republic of China, 210093, Department of Chemistry, UniVersity of Michigan, 930 North UniVersity AVenue, Ann Arbor, Michigan 48109, and Specialty Chemicals Business, Materials Science Technology Platform, Dow Corning Corporation, 2200 W. Salzburg Road, Midland, Michigan, 48686 ReceiVed: June 20, 2009; ReVised Manuscript ReceiVed: July 27, 2009 Epoxy and phenolic resins are extensively used for modern microelectronics, for example, as packaging materials. Humidity may greatly alter or degrade their function and application, leading to failure of the device. A nonlinear optical laser technique, sum frequency generation (SFG) vibrational spectroscopy, was used to investigate the molecular surface structures of the epoxy and phenolic resins after exposure to humid air. It was found that the adsorbed water molecules at the phenolic resin surface can induce substantial surface restructuring. The surface phenyl groups were reoriented closer to a perpendicular position to the surface after exposure to humid air from a more parallel position in air. Epoxide group surface restructuring was not observed. 1. Introduction Encapsulating materials are universally used to protect the microelectronic units inside. Upon curing in the packaging process, an encapsulating material positions and stabilizes a microelectronic unit onto the metal leadframe to form an electronic device with an independent function. However, for storage and transportation of the packaging materials a normal shelf life ranging from days to months before curing in the packaging process is needed. During this period, exposure to the atmospheric environment can lead to diffusion of water into the materials. 1-3 Although the amount of the absorbed water may not be substantial, as will be shown later, negative effects on the stability and reliability of the electronic unit caused by such absorbed water should not be neglected. The temperature for reflow soldering after packaging has increased greatly because of the prohibition of the use of lead as soldering materials by RoHS. 4 Therefore, to avoid delami- nation of the interface, the interfacial adhesion between the encapsulating material and the metal leadframe must be strong enough to resist the water vapor pressure during the reflow soldering process. In addition, the absorbed water can also reach the interface between the encapsulating material and the metal leadframe. The corrosion of the metal leadframe surface may occur as a result of the chemical reaction involving water at the interface. Both the water vapor pressure and the metal corrosion at the interface at elevated temperatures can destroy the interface between the encapsulating material and the metal leadframe finally leading to the failure of the electronic device. This may be a more pronounced problem with the shrinkage of modern microelectronics, when packaging materials are required to handle the high frequency, power, and operating environment requirements of global interconnect. Extensive research has been performed to study the water absorption to the bulk of epoxy materials, 5-16 which are widely used as a main component of packaging or encapsulating materials for microelectronic devices. Many different analytical techniques have been used in such research, including Fourier transform infrared spectroscopy (FTIR), 5-11 differential scanning calorimetry (DSC), 12,13 dielectric relaxation spectroscopy (DRS), 8,14 dynamical mechanical analysis (DMA), 14,15 and simulation. 16 These studies indicate that at least two kinds of absorbed water molecules exist inside the epoxy bulk: free water molecules and water molecules hydrogen bonded to the polar groups, such as ether, amine, and hydroxyl groups. To the best of our knowledge, no systematic molecular level study has been performed to understand how epoxy or other resin (e.g., phenolic resin) surfaces/interfaces respond to humid air or water moisture. We believe that the studies on such surface/interface structures are important because the epoxy or phenolic resin surface responding to water moisture is the first step of the water diffusion into the material bulk. Also, the interfacial structure of packaging materials composed of epoxy and phenolic resins ultimately determines whether delamination or debonding will occur, leading to the failure of the device. This initial study investigates how water absorption can alter the surface structures of packaging materials. More detailed research about the water effect on the delamination of the buried interface between packaging materials and metal leadframes or dies will be carried out in the future. Recently, a second order nonlinear optical technique, sum frequency generation (SFG) vibrational spectroscopy, has been developed into a powerful tool to study molecular structures at surfaces and interfaces with a submonolayer specificity. 17-61 Under the electric dipole approximation, the SFG process is forbidden for centro-symmetric materials, but is allowed at the surfaces and interfaces due to breaking of the inversion symmetry. 17-60 Therefore, SFG can be used to selectively investigate molecular surface/interface structures. SFG has been successfully applied to study polymer surface structures and their * To whom correspondence should be addressed. E-mail: zhanc@ umich.edu; xuegi@nju.edu.cn. Fax: 734-647-4685. † Nanjing University. ‡ University of Michigan. § Dow Corning Corporation. J. Phys. Chem. B 2009, 113, 12944–12951 12944 10.1021/jp9058092 CCC: $40.75  2009 American Chemical Society Published on Web 09/09/2009