BGA X ray Inspection: head in pillow defects

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.
Process Sciences (PSI) is a process engineering resource center and solutions company. OEMs, contract manufacturers, and hardware development teams rely on PSI for circuit assembly rework and repair services, X-ray inspection, failure analysis, SMT process qualification, and more. Outsourcing these services to PSI allows our clients to focus on designing and producing reliable electronics, which has made Process Sciences a sought after resource in the circuit assembly and semiconductor industry since 1994.
This X-ray inspection and video capture were performed on one of PSI's three Nikon X-ray stations.