Stacy,

There will always be pro and con discussion on benefits of baking bare boards
prior to solder assembly.  The "prebake" numbers you showed looked good.  The
above 100 degree C will get the moisture gaseous and the duration will drive
it out of the substrate.  The word being substrate.  Generally, prebake
drives moisture out of the fiberglass weave of board dielectric.  Polyimide
substrate is particularly prone to moisture absorption; to such an extent
flex circuit suppliers usually enclose a "PREBAKE WARNING" with all
deliveries [warning: prebake prior to soldering].  This is the first I have
heard of solder mask entrapment of moisture.  It is usually the combined Z
axis expansion that delivers delamination (we hate that word).  The issue
with solder mask may be one of "Popcorning" where less than optimum "tenting"
design of vias is causation
to micro explosions (akin to volcanic eruptions) that will lead to
delamination (ugly word again) of solder mask to substrate.  The bond of mask
to substrate may be so great that the fiberglass weave is actually upset.  In
any case any board with the ill fated design inadequacy is destined to be
problematic.  Prebake may lessen propensity but in itself not eliminate it.

Lastly, boards needs to be populated ASAP/RUSH after prebake [here come the
cards and letters for those whom have not pressed delete yet] there are
studies that show 80% of moisture is reintroduced within 20 minutes of bake.
A sort of inverse 80/20 rule.  I have seen all kinds of prebake and storage
including nitrogen filled storage bags.  The best thing is to have constant
room and humidity controlled assembly area.  If you are going to prebake do
it prior to assembly... hmmm, both sides.

Just a little insight it is a very hot topic.

Boston Brad
781 858 0783

---------------------------------------------------------------------------------
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 e-mail to [log in to unmask]: SET Technet NOMAIL
To receive ONE mailing per day of all the posts: send e-mail to [log in to unmask]: SET Technet Digest
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
---------------------------------------------------------------------------------