Limor sent this wonderful little document my way after we had some problems with a 0.4mm pitch footprint I made, just to see if we could improve the paste layer and reduce the bridging a bit: Root Cause Failure Analysis of Printed Circuit Board Assemblies. Despite the organization not being the best — a clearer separation of problems from potential solutions/recommendations would have been helpful — there’s a lot of useful information in here.  It’s often easier to understand the kinds of problems you encounter working with fine-pitch devices looking at photos.  Unfortunately, X-ray images of things like ‘ball-drop’ with BGA reflow aren’t easy to come by, nor post-mortem, sawed-in-half BGA packages, etc.  If you’re fascinated by the many ways boards can die a premature death, this is a goldmine of info.