BGAs are difficult to rework as they require a pretty deep knowledge of the original manufacturing process, knowledge of the chemistries involved in the original manufacturing process, the thermal profiles of the various processes as well as an experienced eye in terms of BGA inspection.
In terms of BGA inspection, while the IPC-A-610 standard indicates that the inspection of BGAs can be confirmed from a process point via x-ray, it is imperative that the tools of visual inspection, endoscopic inspection, and x-ray be used for proper BGA inspection.
Visual BGA inspection on the edge of the BGA package and the geometries of the depiction is very limited and is a function of having access to a periphery of the device. Many times the operator will have to be able to articulate the board underneath the microscope. This also asserts that the edge of the board has not interfered with respect to the edges of the device.
Endoscopic inspection is similar in scope and in its limitations to the visual inspection process. The endoscopic mirror is placed in vicity to the edge of the BGA so that BGA inspection can occur. The magnification of the inspection instrument is such that unlike optical inspection, the periphery of the BGA ball interface can be seen. The ball to package interface is important because this part of the BGA inspection can determine if the wetting of the ball to the package is sufficient. The interface of the ball to the PCB also determines what the solder joint of the ball to the board looks like. Here again, the inspector looks at the wetting of the interface of the ball to the board. What is particularly critical is the board corners. If collapse and ball shape are confirmed at the corners in the BGA inspection process then it is generally assumed that the rest of the ball interfaces will be acceptable.
X-ray inspection of BGAs gives the processor many details of the output of the BGA rework or original assembly process. Not only is the ball size and it's uniformity a reflection of the consistency of the reflow process but other parameters can be determined from the x-ray. In addition to the ball collapse uniformity, any shorts can be pointed out through the examination of the x-ray image. The x-ray image in a BGA inspection process can also determine if anomalies such as the flowing of solder down the "dogbone" pattern of the contact to the via is disturbed.