Peter,
All PBGA's warp to some degree DURING reflow. The larger the BGA, the more
pronounced the warp. Consider for a moment, the CTE at Tg of the plastic
encapsulant (or glob top), and the CTE at Tg of the BT laminate. The
plastic encapsulant has a higher CTE at Tg than the laminate. This causes
the BGA to act like a bimetallic strip of metal. It will bend at reflow.
If this is a proto BGA, you can work with the BGA supplier to change the
size of the encapsulant. If this BGA is already in full production, you'll
have to deal with the problem. One work around is to shim the corners of
the BGA with defect stickers, preventing the collapse from being so severe.
However, be careful not to cause an open with the center balls if the BGA is
so equipped.
I have seen this problem quite often, and I believe the biggest problem is a
reverse warpage from the PCB during rework that exacerbates the problem. A
rework station does not heat the entire PCB all the way up to reflow
temperature, only a small area at the rework site is at reflow temperature.
Because the laws of physics demand that area to expand due to the CTE, it
has to expand somewhere. If that small area can not expand the entire PCB,
it will expand by curving.
On very touchy BGA's with respect to corner shorting, sometimes the reflow
oven can be used as a band aid to get the BGA successfully attached.
I wish you the best of luck!
Ryan Grant
-----Original Message-----
From: Peter Lee [mailto:[log in to unmask]]
Sent: Monday, September 09, 2002 2:16 PM
To: [log in to unmask]
Subject: [TN] BGA rework - excessive collapsing
Technet,
We are having problems replacing 35mm BGA, 1.27mm pitch on a OK industry
BGA rework station. The resulting X-ray revealed a solder bridge at one
corner.
There was some warpage on the BGA but not significant. PCB was
reasonably flat. Thermal profiling was done to ensure all temp. were
optimum. I did notice the BGA balls collapsing quite a bit (at least
half of the ball height) after rework.
Anyone has suggestion on how to minimize the chance of bridging? Is it
common for BGA of this size? We don't have the capability to paste the
board for BGA rework.
Rgds,
Peter
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