Tamir,

I am not sure if I fully understand the problem that you describe, however
here are a few typical problems related to PBGA moisture sensitivity during
rework :

1. All PBGAs are moisture sensitive, this means that if they have been
exposed to ambient conditions too long prior to reflow, they are susceptible
to warpage as well as internal cracks and delaminations. The warpage can
cause solder opens on the perimeter of the parts and shorts in the center.
The internal defects can cause electrical failure at electrical test or
reliability problems in the field.
2. When you want to re-use BGAs after rework, you should make sure that the
board is properly baked prior to hot air removal. 24 hours might not be
sufficient, depending on the actual body thickness of the parts you are
dealing with. (see bake table in J-STD-033). The default bake cycle is 48
hours at 125C.
3. The same comment is applicable if you have other parts on the board that
have absorbed too much moisture prior to the additional reflow cycle. This
is also true when you are dealing with double-side reflow with moisture
sensitive components on both sides. In any case you have to know how long
the parts have been out of their dry bag prior to the last reflow.
4. Before you replace new components, you should make sure that they are dry
enough. Often times, components used for rework are kept unprotected for an
undetermined period of time before being used. In this case you create a new
defect in the rework process.

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

Date:    Wed, 30 May 2001 17:21:01 +0300
From:    Tamir Ben Shoshan <[log in to unmask]>
Subject: BGA's failure during rework process.

Hi,

The board's type I am dealing with is 3.8 mm thick with 28 layers made of
GETEK raw material. The BGA's rework process had been defined as followed:
Baking the boards for 24 hours at 125C, remove the defected component with
the BGA's rework station, clean the component's area, paste printing, locate
the component on the pads and pass it through the reflow oven under the same
profile definition as the first assembly (because of a big board size- 17" X
18" and thickness we found out that this process is better then local
profile).
On each board 6 BGAs should be replaced (version upgrade). After replacing
the BGAs with the above process few other BGAs components were fail in away
that implementing force on the component makes it work properly. The X ray
inspection didn't show anything wrong with any of the solder joint. After
replacing the suspected components the boards pass all the functional test.
The failed components were plastic BGA.
Does any one familiar with that phenomena, I appreciate any help or advise.

Best Regards,  Tamir.

------------------------------

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