Diffusion model to derate moisture sensitive surface mount ICs for factory use conditions

Abstract

Recent industry specifications JESD22-A112 and IPC-SM-786A have identified a set of moisture sensitivity levels for classification of moisture/reflow sensitive devices. The established test conditions are meant to represent average worst case factory environmental conditions that a plastic packaged surface mount IC can safely be exposed to after opening of the protective dry-bag. The exposure times imposed by the level classifications relate specifically to the equivalent testing condition, e.g., a Level 3 device must be assembled within 168 hours for a factory environment of 30/spl deg/C/60%RH. No equivalent exposure times are addressed by the specifications if the factory ambients are not at 30/spl deg/C and 60%RH. This paper addresses the issue of equivalent exposure times for varying temperature-humidity conditions that a factory could maintain. A "derating" procedure has been developed that adopts a first principles approach for solution to moisture diffusion into plastic packages. 1-D and 3-D mathematical and computer generated solutions for moisture diffusion were obtained for plastic packaged ICs. Critical moisture concentrations achieved during JEDEC/IPC testing conditions at internal interfaces were calculated and used as a criterion for the "derating" procedure. Based on this approach, an equivalency table can be developed for any combination of factory temperature and humidity conditions.