Hi Ramon,
The old weight gain meaurement technique came up short because it was based
on the the assumption that the moisture is evenly distributed inside the
component. In real life this is almost never the case and moisture ingress
takes the form of a gradient that is either slowly moving in or out of the
package, depending on the changes in ambient conditions. It was also
determined that the location and concentration of that gradient is really
the critical factor in terms of causing internal damage during reflow.
The new moisture diffusion model is simply based on good science that has
been validated with experimental studies. I suggest that you read the
following paper if you want more information on this subject :
"Handling of Highly-Moisture Sensitive Components - An Analysis of
Low-Humidity Containment and Baking Schedules." by Shook and Al., Lucent
Technologies, ETCT 1999.
Many of the key rules and tables that are found in J-STD-033 are based on
the above research.
This paper can be downloaded from the MSD Knowledge Base at
www.cogiscan.com. Click on the MSD logo, then click on "List of publications
and other references" and scroll down the list by chronological order.
Regards,
Francois Monette
Cogiscan Inc.
-----Original Message-----
From: Dehoyos, Ramon [mailto:[log in to unmask]]
Sent: Tuesday, August 26, 2003 16:01
To: 'TechNet E-Mail Forum.'; 'Francois Monette'
Subject: RE: [TN] Dry Cabinet
Francois:
I have been exposed to the weight charts only. I am surely
interested in knowing about the moisture diffusion models. It sounds
interesting.
Regards,
Ramon
> -----Original Message-----
> From: Francois Monette [SMTP:[log in to unmask]]
> Sent: Tuesday, August 26, 2003 9:23 AM
> To: [log in to unmask]
> Subject: Re: [TN] Dry Cabinet
>
> Bonjour Yannick,
>
> Here are a few additional considerations relative to dry storage :
>
> Make sure that you understand exactly what kind of benefit you will get
> from
> your dry storage solution. In the contract manufacturing business, many of
> your customers will expect that you comply with the guidelines from the
> latest IPC/JEDEC standard J-STD-033A. This document contains precise rules
> on how to account for dry storage. In summary the clock of exposure time
> does not necessarily stop when previously exposed components are returned
> to
> dry storage, whether it be a dry cabinet, Nitrogen box or re-sealed dry
> bag
> with desiccant. The actual effect of dry storage on the remanining floor
> life will vary based on the type of component (MS level), the type of dry
> storage (RH level) and the complete history of exposure and subsequent
> storage.
>
> I also suggest that you be careful when reading the marketing data
> provided
> by some of the vendors of dry storage solutions. In many cases their
> conclusions are based on experimental data using a weight gain measurement
> technique. This is not consistent with the current industry standard which
> is based on a much more accurate moisture diffusion model. I can provide
> more details and technical references if you are interested.
>
> Regards,
>
> Francois Monette
> Cogiscan Inc.
> Tel : 450-534-2644
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