Larry,
Another major issue is often overlooked for double side boards with moisture
sensitive components on both sides, as in this case.
The key point is that the first reflow does not bake the moisture out of the
components. Moisture diffusion inside a plastic package is a very slow
process and a reflow cycle is much too fast to have any significant impact.
The net result is that the components on the first side continue to absorb
moisture after they are reflowed on the board, in addition to what they had
already absorbed prior to placement. Of course, you need to make sure that
the board will go back through the second reflow before any of the
components, one the top and bottom side have reached the limit of their
floor life.
The outcome is that for double-sided boards you have to account for the
overall exposure time, from the time that all parts were originally taken
out of their dry bags, until the final reflow is complete. This means that
whatever system or procedure you had in place for controlling
moisture-sensitive components must be extended to track partially assembled
boards and their remaining floor life in addition to the remaining floor
life of components prior to placement.
The above requirements are clearly defined in the joint IPC/JEDEC standard
J-STD-033, section 8.4.4 : "If more than one reflow pass is used, care must
be taken to ensure that no moisture sensitive components, mounted or
unmounted, have exceeded their floor life prior to the final pass".
I hope this information is helpful,
Regards,
Francois Monette
Cogiscan Inc.
50 De Gaspe, Suite A5
Bromont, Quebec, Canada, J2L 2N8
Tel : (450)534-2644
Fax: (450)534-0092
www.cogiscan.com
>Dear TechNet,
>My company has been asked to bid on assembling a board that would have
>two BGAs on the topside and two more BGAs mirrored on the bottomside.
>That is, they will be directly across from each other on both sides. I
>never done them on both sides. So far, everything that I have done has
>been on one side only. Has anyone done them on both sides before?
>
> I am figuring xray will be a problem as after the second side is
>placed, because the pads/balls will mirror each other on both sides.
>Would I be able to tell the difference between the two BGA connections?
>Suggestions?
>
>Will I need to epoxy the first side BGAs so they would not drop off
>during my second reflow? Wouldn't I need to epoxy them after first
>reflow so that the balls could collapse?
>
>Thanks,
>Larry Koens
>SMT Manufacturing Engineer
>E.I. Microcircuits
>Mankato, MN
***************
---------------------------------------------------------------------------------
Technet Mail List provided as a free service by IPC using LISTSERV 1.8d
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 delivery of Technet send the following message: SET Technet NOMAIL
Search previous postings at: www.ipc.org > On-Line Resources & Databases > E-mail Archives
Please visit IPC web site (http://www.ipc.org/html/forum.htm) for additional
information, or contact Keach Sasamori at [log in to unmask] or 847-509-9700 ext.5315
---------------------------------------------------------------------------------
|
