Why do I need to be able to inspect BGAs, flip chips and CSPs at oblique angle views

If you are only able to inspect solder bumps from directly above the sample then the physical bulk (i.e. main mass) of the solder ball will mask any subtle variations that might occur, and be seen, in the interfacial areas between the solder ball and the device, on one side, and the pad on the other. It is these interfacial areas that will tell you if the joint quality is acceptable. By looking at the joint at an oblique angle view, and as important being able to select that angle view at any point around the joint, then the bulk of the solder ball is partially removed from interfering with the investigation of the interfaces, allowing solder ball shape and interfacial variations (as well as voids) to be more clearly seen in the x-ray. If you consider a good BGA solder ball joint to be like a soccer ball sandwiched (and partially squashed) between two flat pieces of wood and then consider the diagrams below, then by only being able to look form the top down will limit the information that can be obtained, compared to having access to oblique angle views. The larger the oblique angle that is possible then the better the separation of the bulk of the ball and the interface becomes.

Using digital x-ray inspection, which has much greater greyscale sensitivity compared with analogue x-ray inspection, then more information is able to be seen within the BGA solder balls. For example, the distinction and differences between interfacial voiding and bulk voiding within the solder joint, as well as being able to distinguish the actual interfaces from the bulk of the solder ball. An example is shown below where the interfaces have been highlighted to aid description. Further information can be found in the following paper presented at the Proceedings of SMTA International, Chicago, September, 2004 - Use Of Digital Imaging As A Process Control Tool.