Thanks Francois:
                        it is a good article. The key is to limit exposure to less than 8 hours then store parts in a dry environment, in between processes,.for level 4 and 5 devices,  or avoid these types of devices..
                Regards,
                Ramon

> -----Original Message-----
> From: Francois Monette [SMTP:[log in to unmask]]
> Sent: Friday, March 19, 2004 1:29 PM
> To:   [log in to unmask]
> Subject:      Re: [TN] Subject: double sided reflow and MSDs
>
> Hi Ramon,
>
> Yes you are right, when placed in a dry atmosphere the moisture diffusion
> process will first slow down and eventually it will reverse. The real issue
> is how long it will take to do this. Immediately after being placed in dry
> storage the moisture gradient that was already absorbed keeps moving slowly
> towards the center of the package which is completely dry initially. It
> takes a lot of time before the concentration at the die surface reaches a
> maximum level and the process starts to reverse. During that period of time
> the moisture concentration at the die surface may temporarily exceed the
> critical level. This is why the floor life clock does not always stop in dry
> storage.
>
> After a long period of time in the same environment the moisture
> concentration inside the component will reach an equilibrium with the
> outside RH level. Keep in mind that for highly-sensitive components (level 4
> and higher), a 10%RH environment may be enough to exceed the critical level
> even if the parts have not been exposed to ambient conditions prior to dry
> storage. Ref : J-STD-033A, section 5.3.2.1 Dry cabinet at 10% RH : "...These
> dry cabinets should not be considered a MBB...Storage of SMD packages in
> these dry cabinets should be limited to a maximum time per table 7-1..."
>
> If you are interested in understanding more of the technical bakground
> behind all the rules of the joint standard, I highly recommend the following
> paper that was written by Rick Shook from Lucent :
> http://www.aecouncil.com/Papers/aec8.pdf.
>
> Regards,
>
> Francois Monette
> Cogiscan Inc.
>
>
> -----Original Message-----
> From: Dehoyos, Ramon [mailto:[log in to unmask]]
> Sent: March 19, 2004 10:32 AM
> To: TechNet E-Mail Forum.; Francois Monette
> Subject: RE: [TN] Subject: double sided reflow and MSDs
>
>
>
>         Hi Francois:
>                         Wouldn't the law of diffusion allow the moisture absorbed inside the
> component start to reverse and come out when the component is placed in a
> dryer atmosphere? Gases must balance throughout the environment they are in.
> Moisture will diffuse into a component for as long there is more of it
> outside than inside.
>                         Regards,
>                         Ramon
>
> > -----Original Message-----
> > From: Francois Monette [SMTP:[log in to unmask]]
> > Sent: Friday, March 19, 2004 9:37 AM
> > To:   [log in to unmask]
> > Subject:      [TN] Subject: double sided reflow and MSDs
> >
> > Hi Joe,
> >
> > You raise some good questions. There are many important factors to take
> into
> > account with this issue :
> >
> > 1. The aqueous wash is not really a concern, unless some of your
> components
> > stay wet for many days. The moisture diffusion process inside a component
> is
> > a very slow process and overall exposure time is the most critical
> element.
> > Ref : J-STD-033A, section 5.4.4. Multiple Reflow Passes : "For cavity
> > packages in which water may be entrapped, water clean processes after the
> > first reflow can be an additional source of moisture..."
> >
> > 2. The real problem is based on the fact that the MSDs continue to absorb
> > moisture from the ambient air between the first and second side reflow.
> > Again because of the short timeframe associated with the reflow cycle,
> there
> > is no significant drying effect during the first reflow and the floor life
> > clock just keeps ticking. Ref : J-STD-033A, section 5.4.4. Multiple Reflow>
> > Passes : "The floor life clock is NOT reset by any reflow or rework
> > process". Somehow you have to carry over the remaining floor life of each
> > component prior to placement to the partially assembled boards.
> >
> > 3. Systematic baking after the first reflow would simplify your tracking
> > procedure but you still have to control the exposure time of the boards
> > until final reflow. The main concern associated with this solution is the
> > additional cycle time and lead oxidation caused by the bake process. You
> > have to insure that all the MSDs mounted on the board will not exceed the
> > maximum cumulative bake time allowed. Ref: J-STD-033A, section 4.2.7.1
> > Oxidation Risk : "...the cumulative bake time at a temperature greater
> than
> > 90C and up to 125C shall not exceed 48 hours..."
> >
> > 4. The solution of storing PCBs in a dry box until 2nd reflow is not a
> safe
> > solution either. Again, based on the physics of moisture diffusion,
> whatever
> > amount of moisture was previously absorbed will continue to diffuse inside
> > the component, even in dry storage, and may eventually exceed the critical
> > limit at the die interface. This is why the floor life clock does not
> > necessarily stop when previously exposed components are returned to dry
> > storage. Ref : J-STD-033A, section 4.1 Post Exposure to Factory Ambient :
> > "Placing MSD packages, which have been exposed to factory ambient
> conditions
> > for greater than one hour, in a dry cabinet or dry pack does NOT
> necessarily
> > stop/pause the floor life clock..."
> >
> > As you can tell there is no simple shortcut out of this. I invite you to
> > read the following article published by Delphi Automotive explaining how
> > they handle the issue of double-side reflow and MSD Control.
> > www.cogiscan.com/documents/delphiSMTAI.pdf
> >
> > I hope this is helpful,
> >
> > Francois Monette
> > Cogiscan Inc.
> >
> >
> > Date:    Thu, 18 Mar 2004 17:30:56 -0800
> > From:    "Macko, Joe @ IEC" <[log in to unmask]>
> > Subject: double sided reflow and MSDs
> >
> > Fellow Techs,
> >
> > Some new designs have moisture sensitive devices (MSDs) such as BGAs on
> both
> > sides of the pwb.  A lot of effort is put into making sure that the MSDs
> are
> > properly packaged, not opened until ready for use so they stay within the
> > allotted floor life, etc..  Some MSDs are level 5 sensitivity which only
> > have a 24 hour floor life.
> >
> > What I would like to hear about is how assemblers handle MSDs when they
> are>
> > already mounted on 1 side of the board, maybe aqueous washed and then see
> > another reflow cycle when the other side of the board is
> assembled/reflowed.
> > Are the MSDs/boards rebaked which adds a lot of cycle time but maybe
> > necessary?  Assembled within the allotted floor life for the most
> sensitive
> > MSD - probably the most ideal situation?  Not washed until both sides are
> > reflowed which doesn't make sense when using water soluble flux?  Or,
> stored
> > in a dry-box until ready for the 2nd side reflow - which should work.
> >
> > Also, what impact does aqueous cleaning have on a mounted MSD?  Does it
> > saturate the MSD with moisture requiring a bake out?
> >
> > I look forward to hearing how other users manage this problem.  thanks
> >
> > joe
> >
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