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 > > > > --------------------------------------------------- > > 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 > ----------------------------------------------------- --------------------------------------------------- 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 -----------------------------------------------------