Hi John!
As Daan has said, normally there shouldn't be any reason to use an adhesive on bottom-side BGA's unless they're really heavy, or your reflow oven has a vibration in in the conveyer, or jerky movement.
In fact, as I learned yesterday at the seminar I attended at APEX by Jean-Paul Clech (Solder Joint Reliability of BGA Assemblies), the BGA solder joint reliability is actually improved slightly from a double-sided reflow.
The stand-off of a bottomside BGA increases slightly from gravity as the solder goes liquid during the second reflow cycle...increased stand-off is a good thing, it helps make the solder joint more compliant. Something else I learned at the seminar that helps with reliabilty, albeit small, is to use a 2% silver solder.
But, if you want to play things real safe, I attended another conference this morning by Dr. Wayne R. Johnson about Corner Bonding CSP's. You print paste,
dispense glue dots at the 4-corners of the device (it was a Loctite 3615 I believe...), place the part, then cure/reflow. It was shown that by doing this the reliabilty of a CSP was improved 3-fold surviving a drop test from a CSP that wasn't corner bonded. The question was asked if the same improvement would be realized with a BGA device, and the answer was expected to be yes, even though testing was only done with CSP's.
The finer points when doing this is to concentrate on dot size and location. You want a dot size large enough to contact the corner of the part well, but not so large as to have the adhesive creep into the solder balls at the corners. You want the location of the dots to be positoned so that the corner of the device is located in the center of the dot. It was stated that the epoxy did not interfere will ball collapse, and that the nice thing is that rework a part is not a problem.
So, you could use adhesive dots if you really want to (but you probabably don't need to), and improve the reliabilty at the same time.
One last note about corner bonding, the improvement in reliabilty was ONLY for drop tests (a 6-foot drop onto a concrete floor), there hadn't been any tests done for thermal cycling, and he didn't want to comment about any sort of improvement in that regard since there hadn't been any testing done...
-Steve Gregory-
Any secrets to mounting BGA's on both sides of an assembly?
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