A fluorescence method was adapted to measure moisture-uptake kinetics in films of poly(methyl methacrylate), and data were interpreted in the context of a Fickian diffusion model. The films, 2-60 m thick, were supported on acid-etched microscope slides. They were compared with several freestanding slabs about 1 mm thick. The moisture diffusion in the slabs was Fickian with a diffusivity of 3.2 ϫ 10 Ϫ9 cm 2 /s. The apparent Fickian diffusivity in the films decreased substantially with decreasing film thickness; however, a careful examination revealed that the initial moisture uptake was governed by a thicknessindependent diffusivity for a wide range of film thicknesses.This suggested that the appearance of non-Fickian behavior originated within about a micrometer of the buried interface, possibly as a result of water accumulation beneath the film or slight thickness variations. Moisture uptake in the thickest films was more rapid than in the slabs, most likely because of residual thermal stresses.
A recent adaptation of the collisional cooling technique which permits gas-phase spectroscopy from 5 K to > 20 K is described. The new apparatus was used to measure pressure broadening parameters and cross sections for the the J = 0-1, K = 0 rotational transition of CH3F broadened by helium from 5 to 21 K. The cross sections show a general upward trend with decreasing temperature ranging from 55.9 A? at 20 K, to 114.0 A?, at 5 K. This compares with a 295 K cross section of 48.8 A? While an accurate CH3F-He potential surface is not presently available for calculating theoretical cross sections, the rise in cross section at low temperature can be attributed to the dominance of resonant collisional processes at very low energy. Although resonant structure in many systems (e.g., CO-He, H(D)CI-He) appears to be smoothed out by Boltzmann averaging, there is experimental evidence that at least one resonance may survive the thermal average in the CH3F-He system.
Unique component handling issues can arise when an assembly factory uses highly-moisture sensitive surface mount devices (SMD's). This work describes how the distribution of moisture within the molded plastic body of a SMD is an important variable for survivability. JEDEC/IPC [1] moisture level rated packages classified as Levels 4-5a are shown to require additional handling constraints beyond the typical out-of-bag exposure time tracking. Nitrogen or desiccated cabinet containment is shown as a safe and effective means for long-term storage provided the effects of prior out-of-bag exposure conditions are taken into account. Moisture diffusion analyzes coupled with experimental verification studies show that time in storage is as important a variable as floor-life exposure for highly-moisture sensitive devices. Improvements in floor-life survivability can be obtained by a handling procedure that includes cyclic storage in low humidity containment. SMD's that have exceeded their floor-life limits are analyzed for proper baking schedules. Optimized baking schedules can be adopted depending on a knowledge of the exposure conditions and the moisture sensitivity level of the device.
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