Reliability assessment of a high CTE CBGA for high availability systems

High availability network switches and routers employ high power dissipation ASIC's that operate at near 100% utilization in a wide range of end-use environments. Ceramic ball grid array (CBGA) packages are typically employed to address these extreme thermal loading conditions. Unfortunately, these ceramic packages exhibit a high coefficient of thermal expansion (CTE) differential when compared to the FR-4 printed circuit board. This large CTE mismatch results in a CBGA solder joint reliability that is significantly lower than that of a plastic ball grid array (PBGA) package. Various suppliers have developed high-CTE ceramic materials to be used as substrates for CBGA packaging. This high-CTE ceramic material has a CTE that is more closely matched to that of FR-4 and hence, improves the solder joint reliability of the package when subject to thermal loading conditions encountered during field life. This study assessed the solder joint reliability of a 42.5 mm(2) high-CTE CBGA for use in a high availability network backbone router. A numerical technique based on fracture mechanics and energy methods was used to predict reliability under accelerated temperature cycling (ATC) conditions and is compared with experimental data. Excellent correlation was found between the numerical prediction and experimental data. Subsequently, the field life of the component was estimated based on Norris-Landzberg empirical relationships used to calculate the appropriate acceleration factor between the reliability test conditions and the actual product operating environment. The solder joint reliability of the high-CTE CBGA component was also compared with various ceramic and plastic packages. Finally, the solder joint reliability of this high-CM CBGA as a function of PCB thickness was investigated via finite element methods. The industry trend has been to increase performance, or functional density, by increasing the number of signal layers in the board to enable routing, thereby increasing the overall board thickness. Hence, an understanding of the impact of the board thickness on the component reliability is of significant interest.