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Humidity of SMT component storage environment.

Mike

#4989

Humidity of SMT component storage environment. | 16 January, 2001

Hi everyone

I am in the process of reviewing our storage of SMT components. At the moment they are pretty much on open shelves in the factory environment.

We endevour to reseal components labled 'moisture sensitive' in their original packaging, but this is not always reliable. Luckily we have observed no pop-corning by visual inspection or test results , but have seen some intermittant solderability problems with certain termination materials.

I am considering the use of a carousel system to utilise space more efficiently. With this system comes a humidity control option.

Is anyone using this type of system?

The components we receive in packaging for moisture sensitive components indicate danger if humidity is greater than 30%. What should the target be if we are able to control the storage environment? around 25%?

Does humidity controlled storage assist in the preservation of solderability on all components?

Sillier question to finish: where do we now find the archives?

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#4991

Humidity of SMT component storage environment. | 16 January, 2001

No we do not use such a storage unit.

We say our preferred environment is within: * 30%RH / 84�F * 30%RH / 72�F * 70%RH / 78�F * 70%RH / 68�F

J-001C says [words to the effect of]: * At 30%RH and below you�d better make sure your ESD control program is working correctly * Make sure the range of humidity in the assembly area is sufficient for the soldering and assembly materials to function properly. * Consider operator comfort when selecting temperature range and upper humidity limits.

We had wonderful archives, but Brian trashed them when he converted over to the new Forum. Fortunately, he's asked Neil to help him get the Archives back [ha ha ha, funny, eh Brian??? Oh well, there goes my IT department that was going to help me convert over to IE, pooh!!!]

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#5024

Humidity of SMT component storage environment. | 20 January, 2001

Dear Mike,

As you might have noticed from the labels on the dry bags, the maximum floor life is specified by the component manufacturer and it can range from 1 year (level 2) down to a few hours after mandatory bake (level 6). This limit is based on factory ambiant conditions of 30C/60%RH. Of course if the storage conditions are drier, this will slow down the moisture diffusion process and eventually it might start to re-dry the components. (although this is a very slow process)

All the proper rules for handling and storing moisture sensitive components are defined in the new joint IPC/JEDEC standard J-STD-033, which is available for free download at http://www.jedec.org. This includes a table for derating floor life based on factory ambiant conditions. It also includes specific guidelines for safe storage, based on re-sealed dry bags or dry atmosphere cabinets, capable of maintaining 25+/-5C and less than 10%RH. These safe storage conditions allow the floor life clock to remain at zero for previously dry components (having been exposed less than one hour outside the original dry bag).

Take note that contrary to popular belief, it is not acceptable to assume that the floor life clock will be temporarily stopped when components are put back in dry storage following exposure to ambiant conditions exceeding one hour.

It is important to note that when components have absorbed too much moisture prior to reflow, the internal cracks and delaminations that will be created inside the package cannot be detected by visual inspection or electrical test (or even x-ray). Only in extreme cases this might break an internal wirebond connection and result in electrical component failure at test. In most cases these latent defects will go unnoticed in the assembly process and jeopardize the reliability of the product in the field.

I believe that dry storage will have a beneficial impact for solderability but I do not have any specific data on this subject.

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#5040

Humidity of SMT component storage environment. | 22 January, 2001

Please expand on your thinking, when you say ...

>Take note that contrary to popular belief, it is not acceptable to assume that the floor life clock will be temporarily stopped when components are put back in dry storage following exposure to ambient conditions exceeding one hour.

... why doesn�t putting components back in dry storage following exposure to ambient conditions exceeding one hour essentially stop the floor life clock?

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Edger

#5042

Humidity of SMT component storage environment. | 22 January, 2001

Hi! Mike My company is using the electric dry box and it can controlled less than 10% within 2.5 hours if we are not going to open the door. In normal working environment and it still can't control around 30%.

Is it something that you are looking for?

Edger

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#5045

Humidity of SMT component storage environment. | 23 January, 2001

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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