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March 1997

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Subject:
From:
[log in to unmask] (Gary Peterson)
Date:
Tue, 4 Mar 1997 14:48:52 -0700
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     Nalin Jain asked whether it is necessary to bake an assembled board 
     before PBGA rework, and if yes what the recommended temperature & 
     humidity condition would be.

Gordon Davy responded with issues associated with damaging the board that the
PBGA is soldered to.

Another concern is damage to the removed PBGA due to moisture absorption
in the package, itself.

We must remember that the PBGA construction consists of an overmolded laminate 
substrate (usually BT resin) to which the silicon is mounted on one side
and solder-balls are mounted on the other.  I ALWAYS bake assembled boards
prior to hot-air removal of PBGAs if I plan to save the PBGA (I am not certain
that they are electrically dead).  I ALWAYS bake the PBGA I plan to put back
on the board (unless I'm certain it has not absorbed moisture).  PBGAs are
classified according to moisture sensitivity just like other packages (e.g.
Level 1 - Level 6).  I don't believe there are any Level 1 PBGAs on the market
(Level 1 implies that you won't get PBGA delamination, popcorning, when
soldering/desoldering the part even if you haven't kept the assembly dry).

To be scientific about it, one would need to know the diffusion rates of
the overmold plastic and the BT resin substrate.  My rule-of-thumb baking
procedure is:  24 hours at 125 degrees C ... I start with a cold oven and let
the parts come up to temperature with the oven.  Although I doubt you could
heat the assembly fast enough in an oven to popcorn any parts even if the oven
were up to temperature when you put the assembly into it.  Check the PBGA
package for moisture sensitivity Level X and baking instructions for drying
parts that have been subjected to moist environments out of the sealed package.

Gary P.
---
                                  Gary D. Peterson
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