Victor, here are a few more elements of information to answer your question.
Both the temperature and relative humidity on the manufacturing floor have a significant impact relative to the rate of diffusion of moisture inside a plastic package and the resulting maximum floor life. This limit will either decrease or increase if the ambient conditions are above or below the 30C/60%RH.
The IPC/JEDEC standard J-STD-033 (available for free at www.jedec.org) includes a table to take this into account. (Re. Table 5. Derating due to factory environmental conditions). Unfortunately moisture diffusion is a complex physical phenomenon and the de-rating table reflects this situation. The de-rating factor depends on the actual classification of the component (level 1 to 6) and the package body thickness. The impact of relative humidity is very significant. For example, a thin Level 3 package (TSOP, TQFP), which is safe for 7 days at 30C/60%RH, will actually exceed the safe limit after only 1 day at 70%RH.
It is a very good practice to use dry box on your manufacturing lines as Dason suggested. However, you must be aware that putting previously exposed parts in dry storage does not mean that they are completely safe. Initially, the previously absorbed moisture will continue to diffuse towards the center of the package where is is more likely to create damage during reflow. A recent paper by Shook and Goodelle from Lucent cited an example that a PLCC device classified Level 5 (normally 48 hours floor life) will actually exceed the critical moisture level after having only been exposed 16 hours, followed by 70 hours of dry storage. This is why the IPC/JEDEC standard does not allow any provision to stop or re-set the clock of exposure time once the parts have been exposed more than one hour (except for a complete bake or for a very specific condition of room-temperature desiccation as defined by the rule for short-duration exposure).
I have done a lot of research lately on the subject of Moisture Sensitivity and would be glad to answer any of your questions on this subject.
Regards, Francois.
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