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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