Vibration and strain from Pb-free solder can crack traces near PBGAs.

This month we feature one recent issue on broken circuit traces on an area array design. A large plastic BGA was found with open circuit connections on the corners of a package. The assembly reportedly failed in the field in a telecommunications application produced in medium volume. Nondestructive x-ray examination confirmed broken traces leading to the corner termination pads. The PBGA was mounted on a standard 0.63" epoxy-glass substrate and soldered with Pb-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 (a condition known as pad cratering) because of mechanical shock or high Tg laminate use, the failure could occur more quickly.

An alternate possibility may be the impact of vibration on the joint interfaces: Pb-free joints are stronger and more rigid, so strain may be transferred to the traces. Closer examination of the broken trace, the quality of the original connection, trace/pad and analysis of the operating environment is required. This type of failure has been seen on flexible circuits due to the strain of flexing.

These are typical defects shown in the National Physical Laboratory’s interactive assembly and soldering defects database. The database (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.