You need to do the math for the package size and ball size/style you're working with.

From applications where I've done the math it had to be tighter than .75% (at most .5%). 
However, don't put any stock in that SWAG -- I don't know the specifics of your assembly and I'm not full time in SMT anymore. 

For worst case, figure how much deflection there can be at max bow/twist across your package footprint then look at paste height and see if all your balls will even make contact with the solder paste. 

For less than worst case,  consider the effect of bow/twist on the finished solder column shape and solder joint reliability. 
That is to say, even if the balls are all making contact with the solder paste, give some thought to what your solder joints will look like at the area of greatest separation between ball and board. 
In those areas of greatest package to pwb separation, your BGA solder column will be more elongated and may not be reliable.

cheers,
Dwight
Sr. Staff QE
"If I was any good at this engineering stuff how'd I wind up in QA?"


At 10:03 AM 6/27/2002 -0700, Mcmaster, Michael wrote:

I'd consider a BGA to fall under the surface mount spec.