Investigation of underfilling BGAs packages - Thermal fatigue

Reliability of solder joint is one of the most concerns in the electronic packages. The trend of smart electronic devices decreases in size and thickness and demands more and more functionality which has required electronic packages to achieve the higher I/O density within a smaller chip size. Therefore, increased demand for flip chip fine-pitch Ball Grid Arrays (BGA) is inevitable. Fine-pitch BGA pushes the size of solder balls smaller, which has raised more concerns about the reliability of smaller BGA solder balls under mechanical or thermal load. Underfilling BGA has been considered a possible option to enhance the reliability of fine pitch BGA under mechanical vibrations. However, Underfilling BGAs can have an adverse outcome in terms of the thermal effect. Numerically, many researchers have investigated the effect of an underfill layer on the thermal reliability of BGA solder joints by observing accumulative plastic strain during thermal cycling. However, an experimental approach to quantify the amount of plastic strain accumulated after every thermal cycle has not been widely conducted. This work presents a novel experimental procedure to measure the plastic strain of the solder ball under thermal cycling with a two-dimensional digital image correlation (2D DIC) technique. By employing the experimental methodology with the 2D DIC presented in this work, the plastic strain accumulation during thermal cycling has been quantified. The accumulated plastic strain per cycle is one of parameters which helps to assess the impact of an underfilling layer on the thermo-mechanical reliability at second level interconnect.