Poor registration appears when component leads don't line up properly with solder or pads or when solder balls on a BGA don't align properly with pads. The computer enhances the areas of misalignment. Solder voids within a solder joint show up as light spots on an x-ray image. Optical inspection and electrical tests cannot find this type of defect. Although a joint that contains a void may operate properly, the void can indicate process problems and may lead to a later failure.
Voids result from heating volatile compounds trapped inside the solder.
Poor solder contact shows up when solder doesn't flow properly between a component's contact and a PCB pad. Poor contact also results from a cold-solder joint in which a reflow process failed to heat the solder enough to make it properly flow between the pad and the contact. Open connections occur when a contact and the solder on the corresponding pad do not touch. For example, the solder balls at one end of a BGA melt properly and make good contact, but the balls at the other end do not and make no connection. Some x-ray systems can detect the unconnected balls, but some cannot tell the difference between a whole solder ball and one that wets the BGA contact and the pad.