The Effects of Isothermal Aging on the Mechanical Behavior of Underfill Encapsulants

Abstract

Underfill encapsulants are widely used in the microelectronics industry to improve the reliability of electronic components assembled to printed circuit boards. In this work, the effects of isothermal aging on the stress-strain and creep behaviors of underfills have been experimentally characterized. A novel method has been developed to fabricate underfill uniaxial test specimens so that they accurately reflect the encapsulant layer present in flip chip assemblies. Samples were subjected to various aging temperatures and aging times before testing. The measured experimental data demonstrated that large changes (up to 100X) occur in the mechanical properties of the underfill. Empirical and constitutive models have also been developed that describe the aging effects on the mechanical properties of the material. Furthermore, the effects of underfill cure temperature and JEDEC MSL preconditioning on underfill mechanical and strength properties, as well as flip chip assembly reliability, have been explored. The failure mechanisms in the flip chip assemblies were studied using CSAM, X-ray and SEM analysis. The results clearly indicate the advantages of the higher curing temperature. These included improved mechanical properties, superior thermal cycling fatigue life, and enhanced resistance to detrimental effects from moisture exposure and solder reflow.