I have been told to look for open BGA connections, the open ball would appear larger than the others due to the fact that it has not columnized. I have also been told that it will look smaller due to the fact that it has not collapsed. Could anyone shed some light on this for me Any help would be greatly appreciated.
Reading x-rays is an art. All balls should be symmetrical in size and shape. Please provide more information on your question. Why just opens? If the Balls are not consistently the same in size or shape there is a problem. Larger balls could be from mis-print and solder migrated to that site, no necessarily an open..........as a rule of thumb same size and shape is good. inconsistent size and shape is bad. Voids in the center can be acceptable. Voids on outer towards the component or PCB can be bad. Cal
Glenbrook colleagues monitor this site, they can provide more info.
First, x-ray inspection machines have a wide range of capilities. Resolution is a key measure of a machine's ability to "distinguish" features. People say that some x-ray inspection machines have the ability to position the board being inspected at diffent angles that some say aids in finding cracks. [I've never used that style of equipment.]
Second, there's lots of causes of BGA opens [eg, mechanical stress, poor pad plating, poor soldering, secondary solder opens]. A boss might agrue that YOU can devine differences in appearance between each of these balls and a good ball. I wouldn't buy it, if he was my supplier.
We had a similar conversation within the past year [??] on SMTnet. Check the Archives.
If you can find opens with x-ray, you are lucky. Acoustic microscopy is a much more repeatable technique. [Although I've heard some argue that ultrasonic technoques require extensive intrepetation.] The space between the solder on either side of the open make a distinctive white "glow" that requires limited "special skills" to determine opens. It ain't rocket science. Trust me.
X-ray laminography can show opens, but it is likely to be prohibitively expensive in most applications
Taking a different tact, I'd speculate that opens are easier to find with an ERSAscope than x-ray. Mucho cheaper if it truely does work!!
Finally if your boss is correct and your x-ray inspection machine is capable of trouble-shooting BGA opens, your inspection machine supplier should be able to demonstrate that capability. Call 'em up.