Whole field vapor pressure modeling of QFN during reflow with coupled hygro-mechanical and thermo-mechanical stresses

Abstract

A comprehensive and integrated package stress model is established for QFN (Quad Flat Non-lead) packages with consideration of the effects of moisture diffusion, heat transfer, thermo-mechanical stress, hygro-mechanical stress, and vapor pressure induced during reflow. The critical plastic materials, i.e. mold compound and die attach, are characterized for hygroswelling and moisture properties. The moisture absorption during preconditioning at JEDEC Level 1, and moisture desorption at various high temperatures are characterized. The vapor pressure modeling applies the micro-mechanics approach, the Representative Volume Element (RVE), with consideration of the micro-void effect. The vapor pressure can be calculated based on the local moisture concentration after preconditioning. Results show that the vapor pressure saturates much faster than the moisture diffusion, and a near uniform vapor pressure is reached in the package. The vapor pressure introduces additional strain of the same order as the thermal strain and hygro strain to the package. Vapor pressure-induced expansion is directly related to the vapor pressure distribution, rather than the moisture distribution.