BGA's can be problematic even for experienced assembliers with professional equipment, as the BGA device is covering the pads and solder area, it can be difficult to observe good quality soldering, and also the device itself, may not settle evenly or (flat) if the thermal profile is not correct, you can experience "outgassing" or "void"; which is where a bubble of gas is formed and is located inside the soldered area, so even if you have tested a BGA and it passes, there may be very weak solder joints which may fail in the near future, also briging (shorts) are also difficult to find without x-xay eqipment. If lead free solder is not cooled down fast enough after reflow it can form large grainy solder joints, again working at test, but very weak and will fail sometime, probably soon.
Repairing these faults will lead you having to re-ball the device if it is not damaged and if can be re-used, and this also has a few quirks within the process. The introduction of lead free soldering also made soldering generally more demanding, as to achieve good solderability you nornally need to take a board and components over 120 degrees C, this can be tough on the components themselves which can have a max temperature of only 150 deg C. the upshot of this is that you have to have tight controls over most processes now-er-days, especially when it is applied to BGA's.
Removing BGA's alone can be a nightmare, especially with fine pitch, and thin and delicate pads..
(falls off soap box)
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QUOTE=billt;571352]I'll never understand why BGA is such a stigma on the bigger chips. OK, you can't really do them yourself at home in your kitchen, so why not have a pro assemble them like Mike does?[/QUOTE]
