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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