An analysis case on the crack failure of CBGA solder joints

Ceramic ball grid array (CBGA) solder joints were considered to crack after a series of environmental tests, including thermal cycling, random vibration and high temperature operational life test. In this paper, the failure analysis on the crack of ceramic ball grid array (CBGA) solder joints is revealed with the aid of 3D X-ray inspection, microsection technique, optical microscope (OM) and scanning electron microscope (SEM). The result of 3D X-ray shows that voids were found in the CBGA and no obvious cracks in joints were detected. By employing micro-section technique, micro-cracks were found in several CBGA solder joints with aid of OM and SEM. The result of micro-section of CBGA solder joints also reveals that the solder balls were made of eutectic tin-lead alloys. The OM and SEM images of cross-section of CBGA solder joints demonstrated that significant coarsening of solder structure was found near the cracks. In the CBGA solder joints, Pb-rich phase was well distributed into the Sn-rich. The size of Pb-rich phase in the crack area was larger than that in other area of the solder joints. As the size of microstructure increased, the interfaces between different phases decreased, which would weaken the mechanical structure of the solder joints, leading to crack failure eventually. It can be inferred that the crack of CBGA solder joints was related to thermal-mechanical fatigue. Generally, the thermal expansion of ceramic is much smaller than that of PCB substrate, which led to large thermal mismatch during the thermal cycling test, causing the thermal mechanical fatigue crack in the CBGA solder joints. The intermetallic compound (IMC) at the soldering interface and the shape of solder joints were also observed by SEM. The IMC showed a continuous morphology with proper thickness, and the solder joints kept a normal shape. In this paper, the cross section of plastic ball grid array (PBGA) solder joints on the same printed circuit board (PCB) were also observed by OM and SEM. However, no thermal mechanical fatigue cracks and coarsening of solder structure were found. It was due to that the coefficient of thermal expansion of PBGA substrate is similar to that of PCB substrate, and that the thermal mismatch between the PBGA and PCB was much smaller than that between the CBGA and PCB. Hence, thermal-mechanical fatigue cracks were found in CBGA solder joints, while the PBGA solder joints remained intact. This paper also reveals that the height of solder joint can significantly influent the reliability of the CBGA joints under the thermal cycling load.