Underfill material requirements for reliable flip chip assemblies

Underfill encapsulation has been demonstrated to be necessary to improve the temperature cycle performance of solder joint interconnections and enhance the reliability of flip chip assemblies. An in-depth understanding of underfill materials is a challenge for packaging engineers to develop underfill materials with functional requirements for applications in IC packaging. This paper investigates the material physical properties observed in a bisphenol/amine underfill system using imidazole as the catalyst and silica as the filler. Typical underfill formulations, comprising epoxy resins, hardeners, silica fillers and catalyst were considered. The impact of the material properties on the reliability of flip chip assemblies was discussed. Test vehicles using 16×7 mm silicon flip chips assembled and encapsulated with different underfill materials on daisy chain PCBs were subjected to a temperature cycling test at -40°C air/20 min to 125°C air/20 min. It was found that underfills with high adhesion strength, low CTE and high modulus exhibited high reliability in temperature cycling tests