This guideline is actually in the IPC/JEDEC standard J-STD-033, May 1999 (although it is not spelled out very clearly, thus the current confusion in the industry).
The technical reason is that moisture diffusion is a very slow process. Once parts have been exposed to ambiant conditions, the existing moisture gradient will continue to diffuse towards the center of the package nearer the die/leadframe critical interfaces, where is it more likely to create damage during reflow.
Recent findings have clearly indicated that the time spent in dry storage is just as important as the prior floor exposure for highly moisture sensitive components (level 4-5a). An excellent technical paper on this subject was recently published (Handling of Highly-Moisture Sensitive Components - An Analysis of Low-Humidity Containment and Baking Schedules, R.L.Shook and J.P.Goodelle, Lucent Technologies, 1999 ECTC). As example cited illustrated that a PLCC device classified Level 5 (normally 48 hours floor life) will actually exceed the critical moisture level at the interface, after having only been exposed for 16 hours, followed by 70 hours of dry storage.
Of course it is still good practice to place parts in dry storage. The drier environment will slow down the moisture absorption process and if the parts are left in a dry environment for a long enough period of time, the process will reverse and the components will start to dry again. This is equivalent to a room temperature bakeout.
If the exposure time is very small then the trapped moisture is still near the surface of the package and it can be removed in a relatively short timeframe at room temperature. The IPC/JEDEC standard J-STD-033 specifies that a minimum duration in dry storage of 5X the prior exposure time for previously dry parts having been exposed less than 8 hours will re-set the floor life clock. (Rule 7.1.2 short duration exposure).
In other words, once you exceed 8 hours of exposure time, for any level of components, the clock will keep on ticking until you exceed the maximum specified floor life. The only option at this point is to completely bake the components. By the way another common error is to use the "old spec" bake conditions which were 24 hours at 125C or 192 hours at 40C. The current standard specifies default conditions of 48 hours at 125C or 68 days at 40C (if the parts are in low temp carriers such as tape and reel)...To compensate for this increase however, the new standard includes a table to optimize (reduce) the duration of bake based on the sensitivity level and component body thickness.
I guess it's a complicated answer to a simple question...but life is never that simple, is it
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Hi everyone
I am in the process of reviewing our storage ...
- Jan 16, 2001
by
Mike
No we do not use such a storage unit.
We say our preferre...
- Jan 16, 2001
by
davef
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