The effect of underfill epoxy on warpage in flip-chip assemblies

The thermally-induced warpage of both a real flip-chip thermosonically bonded assembly and a simulated tri-layered assembly was investigated. It revealed the warpage of the assemblies was dominated by the forces applied by the underfill epoxy rather than the solder joints. The roles the underfill epoxy and solder joints played in causing warpage did not change even when the assembly had 196 solder joints under a 5.8 mm×5.8 mm chip. Mechanical properties of epoxy depend on the curing and the glass transition temperatures, and these characteristic temperatures clearly divide the warpage levels into two distinctive regions. When the maximum temperature the assembly was exposed to was less than the glass transition temperature (T_g), the warpage of the assembly was characterized by the curing temperature. When the maximum temperature the assembly was exposed to was higher than T_g, the warpage was characterized by T_g regardless of how high the temperature was. The distinctive deformation curves with sub-micron repeatability are reported for the first time. Depending upon the different characteristic temperatures of an assembly, e.g., 80°C for curing and 130°C for T_g, the warpage and the Von Misses stress each could increase by as much as a factor of two. Such an increase could affect device reliability for RF packages and alignment for optoelectronic packages