Adhesion of benzocyclobutene-passivated silicon in epoxy layered structures
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Yueguang Wei and John W. Hutchinson Division of Engineering and Applied Sciences, Harvard University, Cambridge, Massachusetts 02138 (Received 6 April 2000; accepted 10 October 2000)
Adhesion and subcritical debonding at interfaces between a silica-filled epoxy underfill and a silicon die passivated by silicon nitride and benzocyclobutene (BCB) layers were investigated. Adhesion was measured in terms of a critical value of the applied strain energy release rate, G (J/m2). Subcritical debond-growth rates in the range of 10−9 to 10−3 m/s were characterized as a function of applied G. Adhesion and subcritical debonding were affected by changes in interfacial chemistry and environment. The surprisingly large effect of adjacent layer elastic properties on interfacial adhesion was demonstrated with simulations of interfacial fracture using a mechanics of materials approach. Interfacial chemistry was modified by using different adhesion promoters, by varying the BCB cure state, and by using different epoxy underfill resins. The effects of environmental variables were studied with temperature- and humidity-controlled environments in order to determine the separate roles of moisture activity and temperature.
I. INTRODUCTION
Demands for faster performance and higher I/O counts for integrated circuits require commensurate increases in the density and complexity of package interconnect structures. Traditional packages are being supplanted by flip-chip solder ball array packages in which the IC and substrate are connected face-to-face through an array of metallized bond pads and solder balls. Such structures contain a number of passivation and solder mask layers together with a polymer underfill intended to assist in thermal management and provide protection from moisture and other environmental species. The underfill is typically an epoxy resin with thermal expansion and elastic properties tailored by the addition of micron-sized silica beads. During fabrication and in use, high thermal expansion mismatch and polymer curing strains may be generated in the packages. The resulting residual stresses must be accommodated by the solder ball array and the layered underfill region. Failure of the package involving delamination of interfaces in the underfill region and metallized bond pads together with cracking of the adjacent mate-
a)
Currently at Sandia National Laboratories, Albuquerque, NM 87185-1415. b) Address all correspondence to this author. e-mail: [email protected] J. Mater. Res., Vol. 16, No. 1, Jan 2001
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rials and solder balls is frequently encountered and poses significant challenges for current flip-chip technologies.1–3 Delaminations may permit moisture to penetrate into the package initiating corrosive processes in the metallized interconnections as well as increasing the residual and thermomechanical stresses carried by the solder balls. A critical issue in determining overall package reliability is therefore the resistance of interfaces within the pa
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