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 > > --------------------------------------------------- > Technet Mail List provided as a service by IPC using LISTSERV 1.8e > To unsubscribe, send a message to [log in to unmask] with following text in > the BODY (NOT the subject field): SIGNOFF Technet > To temporarily halt or (re-start) delivery of Technet send e-mail to [log in to unmask]: SET Technet NOMAIL or (MAIL) > To receive ONE mailing per day of all the posts: send e-mail to [log in to unmask]: SET Technet Digest > Search the archives of previous posts at: http://listserv.ipc.org/archives > Please visit IPC web site http://www.ipc.org/contentpage.asp?Pageid=4.3.16 for additional information, or contact Keach Sasamori at [log in to unmask] or 847-509-9700 ext.5315 > ----------------------------------------------------- --------------------------------------------------- Technet Mail List provided as a service by IPC using LISTSERV 1.8e To unsubscribe, send a message to [log in to unmask] with following text in the BODY (NOT the subject field): SIGNOFF Technet To temporarily halt or (re-start) delivery of Technet send e-mail to [log in to unmask]: SET Technet NOMAIL or (MAIL) To receive ONE mailing per day of all the posts: send e-mail to [log in to unmask]: SET Technet Digest Search the archives of previous posts at: http://listserv.ipc.org/archives Please visit IPC web site http://www.ipc.org/contentpage.asp?Pageid=4.3.16 for additional information, or contact Keach Sasamori at [log in to unmask] or 847-509-9700 ext.5315 -----------------------------------------------------