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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