BGA X-RAY INSPECTION

In this video are three separate X-ray inspections of BGA's, each revealing a distinctive solder joint defect called "head-in-pillow," so named for its resemblance to a head resting on a pillow. The defect is usually easy to spot using off-axis X-ray. 'Head-in-pillow' (HiP) defects occur under BGA devices when the solder sphere and the paste underneath it do not flow together, that is, they fail to bond chemically and mechanically. Direct causes of HiP fall into two basic categories, either the solder ball and paste are not in contact at the time of reflow, or there is insufficient flux activity to overcome surface oxides. (Or both.) Lack of physical contact might be due to alignment problems, lack of coplanarity of board or part, or variation in solder volume. Insufficient flux activity might be due to improper storing or handling of paste, too long a reflow profile, or excessive oxidation. And in turn, each of these problems can have multiple underlying causes.

In this video, one solitary sphere has failed to form a bond with the pad at the board interface. The defect comes into focus at about the 0:19 mark. Notice how the solder sphere in the center of the screen is round, whereas all of its neighbors have a gumdrop shape. Two factors contribute to the general shape of these spheres, 1) there is warping of either board or part, increasing the stand-off height, and 2) the pads on the board are wider than the pads on the part. Nothwithstanding their atypical gumdrop shape, all these joints are properly formed. However the open sphere in the center is not in contact with the board pad, and shows no evidence of wetting.