Bob,

You should definitely be concerned with popcorning based on the storage
procedure you describe. The only long term safe storage is to keep parts
inside a sealed dry bag with fresh desiccant (not just any plastic bag but
one that meets the minimum specification for Water Vapor Transmission Rate
(WVTR) as described in the IPC/JEDEC standard). The humidity indicator is
only there to identify whether the bag was properly sealed during shipment
from the supplier. It does not guarantee in any way that your components are
kept dry enough.

To illustrate this point, a current revision to the IPC/JEDEC standard
includes a proposed limit of 90 days for parts that are kept in a dry
cabinet at less than 10%RH. This is because if the parts are kept in this
condition for a long enough time, they will eventually reach saturation at
10% and this may very well exceed the critical level of moisture and create
internal defects during reflow.

I suggest that you revise your internal procedure based on the joint
IPC/JEDEC standard J-STD-033 : Standard for Handling, Packing, Shipping and
Use of Moisture/Reflow Sensitive Surface Mount Devices. This document is a
free download at www.jedec.org.

As far as the oven is concerned, you will find the table for bake cycles in
the same document. As long as you can maintain 125C for 48 hours that should
work.

Francois Monette
Cogiscan Inc.
50 De Gaspe, Suite A5
Bromont, Quebec, Canada, J2L 2N8
Tel : (450)534-2644
Fax: (450)534-0092
www.cogiscan.com


> -----Original Message-----
> From: TechNet [mailto:[log in to unmask]]On Behalf Of Bob Walker
> Sent: Wednesday, May 30, 2001 8:22 AM
> To: [log in to unmask]
> Subject: Re: [TN] Moisture Sensitive Components
>
>
> Does anyone have any recommendations for a small, inexpensive oven to bake
> IC's with? We're an ASIC design group and build/rework our own
> prototype PCB
> assemblies in small quantities and have decided that we need to be more
> cautious with how we deal with large IC's after they have been
> removed from
> the vacuum sealed bag and prior to reflowing them on PCB's.
>
> After the IC's have been initially removed from the bag, I keep any unused
> IC's in a plastic bag with the original bag of dessicant and humidity
> indicator which never seems to show any excessive amount of exposure to
> moisture. But these parts may not be put down on boards for a couple of
> months and a co-worker feels that they may still be absorbing enough
> moisture to be a potential candidate for "popcorning" even though the
> humidity indicator shows otherwise.
>
> A suggestion was made to just buy a "toaster oven" since we only
> deal with a
> few IC's at a time. Any comments, pro or con, with regards to this? Thanks
> in advance for any feedback.
>
> Regards,
> Bob Walker
> Sandgate Technologies Inc.
> [log in to unmask]
>

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