Effects of hydrothermal aging on the dielectric properties of epoxy molding compounds
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Effects of hydrothermal aging on the dielectric properties of epoxy molding compounds Patrice Gonon1, Alain Sylvestre1, Jérôme Teysseyre2, Christophe Prior2 1 Lab. for Electrostatics and Dielectric Materials. University of Grenoble and French National Center for Scientific Research. LEMD-CNRS, BP 166, 38042 Grenoble Cedex 9, France. (E-mail: [email protected]) 2 Corporate Package Development, ST Microelectronics. BP 217, 38019 Grenoble Cedex, France ABSTRACT We studied the influence of hydrothermal aging on the dielectric properties of epoxy molding compounds used for integrated circuits packaging. Samples were subjected to two different aging (JEDEC 1 and JEDEC 3 procedures, followed by a thermal stress at 240°C for a few minutes). Resistance to aging was studied as a function of crosslinking density. Water uptake increases with crosslinking. Aging leads to a reduction in the dielectric constant and to an increase in the loss factor. Post-mold curing is found to slightly enhance the resistance against hydrothermal aging.
INTRODUCTION Epoxy molding compounds, which basically are epoxy/silica composites, are a class of materials which are widely used for microelectronic packaging. During their service life these materials encounter several environmental stresses (moisture, heat, radiations...) whose influence on package reliability should be carefully evaluated. The dielectric properties of epoxy/silica composites, more especially the dielectric constant (ε') and the loss factor (ε''), are known to be very sensitive to water diffusion. These quantities are usually observed to be increased by water uptake [1-5]. Undesirable consequences are a higher crosstalk, a limited operation towards high frequencies (increase in the dielectric constant), and higher current leakages (increase in the loss). Past studies were mainly concerned with the influence of water absorption on its own, with little attention paid to the effects of subsequent thermal aging. However, during operation materials could be subjected to a thermal stress occurring after water uptake. This is the case for composites used in electronic packaging, where soldering process could be performed after devices have been stored in humid environments. It is the aim of the present work to investigate the effect of moisture absorption and consecutive thermal stress on the dielectric properties of epoxy molding compounds.
EXPERIMENT Samples are industrial-grade epoxy molding compounds. The polymer matrix is a multifunctional epoxy. It contains micrometer-size silica particles (90 wt.%). Molding is performed at 175°C, during 100 s, using an acid anhydride as hardener. After molding the cross-linking density is 89% (as determined by measurement of the residual heat of reaction using differential scanning calorimetry). Samples are square-shaped (28 mm *28 mm) with a thickness d=3.3 mm. For some samples post-mold curing (PMC) was performed at 165°C, during 1.5 hour or 12 hours, leading to crosslinking densities of 93% and 99%.
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