The prediction of long term solder joint reliability, (SJR), of microelectronic devices and packaging solutions continues to challenge the microelectronic packaging industry, particularly with the introduction of lead-free materials, the push for higher performance (frequency/speed/thermal) and lower unit cost. High performance packages are generally custom designed and therefore have minimal industry data on configuration specific reliability performance. In this application, the package substrate coefficient of thermal expansion, (CTE), was closely matched to the die resulting in a relatively large CTE mismatch between the package and organic PCB. In addition, the package RF and thermal performance requirements required this particular solution to be configured as a “cavity down” perimeter ball array with a large central ground pad to electrically couple the package to the PCB. Given the package's unique design requirements and CTE mismatch, even modest daily temperature swings of 20°C usually found in a controlled or “Central Office” environment could have an adverse impact on the interconnect reliability.
This study provides an overview of the solder joint reliability assessment methodologies performed for a custom design lead-free, high performance RF package as part of the requirements to demonstrate compliance to product specifications. SJR life predictions were made for varying package BGA configurations using a multi-tiered approach using constitutive material models, thermo-mechanical finite element simulations, and material specific fatigue models. Empirical accelerated life testing was performed and a life prediction obtained through modeling was validated. Finally, statistical failure distributions were fit to empirical data and discussed in the context of absolute solder life predictions of small fractions unit failures, (100ppm).
