Julien,

I would not conclude that your problem is not related to moisture
sensitivity just because you keep your parts in dry cabinets (or dry bags
for that matter).

Contrary to popular belief, the moisture diffusion process does not stop
when previously exposed components are returned to dry storage. The moisture
previously absorbed will continue to diffuse towards the center of the
package and may eventually exceed the critical level at the die interface
while the parts are in dry storage.

An excellent paper on this subject has been published by Lucent (Shook & Al.
Handling of Highly-Moisture Sensitive Components - An Analysis of
Low-Humidity Containment and Baking Schedules, ECTC 1999). A practical
example showed that a PLCC level 5 (normally 48 hours maximum floor life)
will actually exceed the critical level after only 16 hours of initial
exposure followed by 70 hours of dry storage. Of course the drier
environment will slow down the moisture diffusion process. Eventually the
moisture gradient will reverse and the part will start to dry again (back in
the safe range after roughly 250 hours of dry storage in the above example)

Unfortunately this phenomenon is quite complex and it is critical to take
into account prior exposure and the time spent in dry storage to determine
when a component can be reflowed safely. (Ref. Joint industry standard
J-STD-033, free download at www.jedec.org)

In order to verify if moisture is causing a problem you can simply bake your
components prior to reflow (48 hours at 125C, ref. Bake table 2 in
J-STD-033) and see if the problem goes away. Even if you do not have
catastrophic failures like popcorning, you should be concerned with the
internal package defects (partial cracks and delaminations) that will be
caused by reflowing or reworking components that have absorbed too much
moisture. These are insidious latent defects that can easily pass electrical
test and result in early life failures in the field.

Francois Monette
Cogiscan

--- Julien Bouchard <[log in to unmask]>
wrote:
> Hi everybody,
>         Two week ago, we got a big problem of BGA
> that do not work after the
> rework.  Almost all BGA fail after the rework.
> THe BGA is a 27mm x 27mm standard BGA,.  We have two
> reworks machine, an
> air-vac DRS24 and air-vac DRS22.  The problem only
> occur on the DRS24, which
> is the newest machine !  We modify the profil to
> lower the temperature, but
> my problem is that when i reach around 215 C in the
> join of the bga, the top
> of the bga body is around 235C, which is more than
> the recommanded
> temperature for BGA by the manufacturer.
>
> The profil we use is like :     preheat : 2 min at
> 60% of the flow and 250 C
>  the join reach around 165 C)
>                                         soak : 1 min
> at 50% of the flow and 290 C   ( the join reach
> around 180
> C )
>                                         ramp : 1.5
> min at 60% of the flow and 330 C  ( the join reach
> around
> 215 C )
>
> The BGA are keep in a dry cabinet to redust moisture
>
> Does is seem good ?  I not sure of the balance
> between longer time and lower
> temperaturer versus shorter time and hotter
> temperature. Anyone have
> experience with this.  Also, when you mesure the
> temperature, which ball you
> use : the inner or the outter ? The inner ball seem
> to be more hot than the
> other one. Also, for the people who have air-vac
> machine DRS22 and DRS24,
> have you problem of stability with the DRS24,
> compare to the old DRS22.
>
> Thank you.
>
> Julien Bouchard
> Matrox - Process Engineer
>
>

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