This month we feature one recent issue on broken circuit tracks on an area array design. A large Plastic Ball Grid Array PBGA was found with open circuit connections on the corners of a package. The assembly had reportedly failed in the field in a telecommunications application produced in medium volume. Non destructive x-ray examination confirmed broken tracks were leading to the corner terminations pads. The PBGA was mounted on a standard 1.6 mm fibre glass substrate and soldered with lead-free solder.
The probable cause may be expansion and contraction of the package and board during local high temperature operation. If the pad surface had separated from the board, (Pad Cratering), due to mechanical shock or use of high Tg laminate the failure may occur more quickly. An alternative possibility may be the impact of vibration on the joint interfaces, the lead-free joints are stronger and more rigid so strain may be transferred to the tracking. Closer examination of the broken track, the quality of the original connection, track/pad and analysis of the operating environment will be required.
This type of failure has been seen before on flexible circuits due to flexing but on a tin/lead assembly. The NPL database is available to readers to search or submit problems online with a possible solution at
http://defectsdatabase.npl.co.uk/
