IEEE TRANSACTIONS ON COMPONENTS AND PACKAGING TECHNOLOGY, VOL. 27, NO. 3, SEPTEMBER 2004 499 Abstract— Hygroscopic swelling behavior of mold compounds is analyzed by a novel experimental procedure using a whole-field displacement technique. Large variation in moisture content at the virtual equilibrium state is observed, while the coefficient of hygroscopic swelling is shown to not vary significantly. An investigation on an actual package is also performed to determine the hygroscopic swelling mismatch strains at the chip/mold compound interface. The results are compared with the thermal expansion mismatch strains at the same interface and reveal much higher hygroscopic swelling mismatch strains. The hygroscopic strains must be considered for reliability assessment when a package is subjected to environments where the relative humidity fluctuates. Index Terms – Chip/mold compount interface, humidity, hygroscopic swelling behavior, mold compounds, plastic encapsulated microcircuit (PEM). I. INTRODUCTION A plastic encapsulated microcircuit (PEM) consists of a silicon chip, a metal support or leadframe, wires that electrically attach the chip’s circuits to the leadframe, and a plastic epoxy encapsulating material, or mold compound, to protect the chip and the wire interconnects [1]. The mold compound is a composite material made up of an epoxy matrix that encompasses silica fillers, stress relief agents, flame-retardants, and many other additives. In spite of many advantages over hermetic packages in terms of size, weight, performance, and cost, one important disadvantage of PEMs is that the polymeric mold compound absorbs moisture when exposed to a humid environment. Moisture absorption is caused by the polymer-water affinity action. The action occurs due to the availability of hydrogen Manuscript received December 1, 2003; revised February 17, 2004. This work was supported by the CALCE Electronic Product and Systems Center, University of Maryland. This work was recommended for publication by Associate Editor L.T. Nguyen upon the evaluation of the reviewers’ comments. Eric Stellrecht is with the Mechanical Engineering Department, DRS Electronic Warfare and Network Systems, Inc., Buffalo, NY 14225 USA (e- mail: estellrecht@drs-ewns.com). Bongtae Han and M.G. Pecht are with the CALCE Electronic Products and Systems Center, Department of Mechanical Engineering, University of Maryland, College Park, MD 20742 USA. Digital Object Identifier 10.1109/TCAPT.2004.831777 bonding sites along the polymer chains that constitute the molding compound [2],[3]. Two distinct states of water have been identified to exist within a polymeric material. The first, called “free” or “unbound” volume, is attributed to water molecules that group in voids in the material. The second, called “bound” volume, describes water molecules that form hydrogen bonds with the polymer chains [2]-[6]. It has been shown that the swelling efficiency [2], defined as the ratio of hygroscopic volume of expansion to the volume of absorbed liquid water, is less than one. This implies that not all of the absorbed moisture contributes to the swelling of the mold compound, and it has been speculated that only the bound volume contributes to hygroscopic swelling. More specifically, the polar water molecules form hydrogen bonds with the hydroxyl groups in the mold compound and disrupt inter-chain hydrogen bonding. These water molecules effectively increase the inter-segmental hydrogen bond length and collectively cause the polymeric material to swell [2],[6],[7]. Hygroscopic stresses arise in an electronic package when the mold compound and other polymeric materials swell upon absorbing moisture while the adjacent non-polymeric materials, such as the lead frame, die paddle, and silicon chip, do not experience swelling. The differential swelling that occurs between the mold compound and non-polymeric materials leads to hygroscopic mismatch stresses in the package [2],[8],[9]. This study characterizes the hygroscopic swelling properties of five different types of mold compounds by a novel experimental procedure. The procedure utilizes a real- time whole-field displacement measurement technique called moiré interferometry, to conduct extremely accurate measurements. The technique is used subsequently to investigate the deformations of an actual package, caused by the mismatch in hygroscopic swelling. The hygroscopic deformation is compared with the thermal deformation and its implications are discussed. II. HYGROSCOPIC SWELLING A. Experimental Method An experimental procedure to document hygroscopic swelling in mold compounds using moiré interferometry is described here. Additional details concerning the experimental procedure can be found in [9]. Characterization of Hygroscopic Swelling Behavior of Mold Compounds and Plastic Packages Eric Stellrecht, Bongtae Han, Member, IEEE, Michael Pecht, Fellow, IEEE