What look like open BGA joints might be tricks of the light.

Figure 1 shows a ball grid array solder joint after reflow soldering. The joint appears optically to be open, as the edge of the pad and the base of the ball are visible. In-circuit testing did not confirm any fault on the component, and x-ray showed perfect joints.

This is most likely just an optical issue; flux residues are around the base of the ball and the pad. This caused a reflection: a trick of the light rather than a defect. Optical inspection of BGA terminations requires good practical training. IPC, SMTA and SMART Group offer a BGA Inspection and Defect Guide CD-ROM on the subject, which may help train staff.

When a defect is suspected during optical inspection, always try different lighting and nondestructive inspection techniques before any destructive examination or rework.

These are typical defects shown in the National Physical Laboratory’s interactive assembly and soldering defects database. The database (http://www.defectsdatabase.npl.co.uk), available to all Circuits Assembly readers, allows engineers to search and view countless defects and solutions, or to submit defects online. 

Dr. Davide Di Maio is with the National Physical Laboratory Industry and Innovation division (npl.co.uk); defectsdatabase@npl.co.uk.