Brad,

Your advise and insight are greatly appreciated!!!

We make memory module.  The PCB material is FR4, 4-6
layers.

1. If we do not "prebake", what would be the
consequences?
2. Does the 80/20 rules apply to the memory PCB board
(especially the Immersion/Flash Gold)?
3. If the boardhouse prebake and then vaccum seal the
product, is it neccessary for SMT to prebake it prior
to soldering if they open then process the PCB
immediately?  Our SMT room environment is 20-30C and
35-60% Humidity.

Again, your advise is greatly appreciated.

Stacy
----------------------
--- [log in to unmask] wrote:
> 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
>
>
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