Can a via be placed directly under a bump?
Yes, a via can be placed directly under a bump, but care must be taken to ensure that the via is tented (covered with solder mask) in order to prevent solder from wicking into the hole during the reflow process, and to prevent voltage clearance issues due to exposed copper in proximity to the die. A non-filled via can cause the die to tilt or not connect the bump to the pad properly. The recommended via design is a micro-via with a 6mil hole diameter and an annular ring diameter not to exceed the width of the pad and MUST be filled with either a non-conductive or conductive filler. For general tips on device layout on a PCB, go to Optimizing PCB Layout.