No abstract

Chip scale packages (CSPs) are widely used in miniaturized electronic systems, and the performance advantages and reliability of single layer packages have been well studied for consumer applications. Three dimensional stacked modules allow for further miniaturization while retaining manufacturability and performance advantages, but they are not yet as widely used or investigated as single layer packages. Of particular concern is the survivability of a board mounted, stacked module under board level drop impact and shock loading, which may soon be of great importance in consumer, industrial and military applications. A CSP stack module is fabricated by stacking several CSPs to achieve high-density 3D systems without increasing the package's footprint. But it also introduces more probabilities of mechanical failure including drop impact-induced malfunction in solder interconnections. In order to evaluate the survivability of a stacked package, we have developed a model for a two level CSP stack module attached to a FR-4 board following the JEDEC standard. With this assembly undergoing drop impact in the 0º orientation (horizontal drop) under two types of impulse loads, we discuss the results of board-level impacts theoretically and numerically. Our results suggest that (1) the solder balls at the first CSP level are the determining factor in package reliability, (2) the first CSP level is most susceptible to damage and failure for the horizontal drop, and (3) damages in the higher level solder joints are due to a combination of vibration induced and stress wave induced stresses.

Riding chip scale packages (CSPs) in the z-direction enables high-density 3D electronic packaging in which the shorter, through-interposer interconnections can provide faster signal transmission and integrity. Successful applications of such 3D stacking packages require a better understanding of their mechanical responses to and reliability under various loading conditions. In this paper, we present analysis results for failure mechanisms of 3D packaging by (1) simulating detailed mechanical response of the critical joints to a board-level drop impact and identifying the possible failure modes and mechanisms of 95.5Sn4AgCu (SAC) solder joints in 3D stacked-CSP modules under drop impact, and (2) building a theoretical framework that estimates impact-induced stresses in the critical solders. The results suggest the following: (1) Stresses predicted by the theoretical models are on the same order of magnitude as numerical results. (2) Both the theoretical and numerical results show that stresses fluctuate at a higher frequency and are about one order of magnitude smaller in the 90° orientation drop than in the 0° drop. The latter implies that all the stack-like 3D packaging reacts dynamically "stiffer" in the 90° drop scheme. This explains why the 0° drop scheme is the most critical test; if a specimen can survive from the 0° drop test, it should survive a drop test in any other orientation. (3) The FE results display uneven deformation among the solders of the stack CSP module, in particular in the 0° orientation drop. We deduce that the unequal deformation among the solders, due to differential flexure between the board and the packages, is the main cause for such high stresses in the critical solders in the 0° drop. Solders carry impact load more equally and react stiffer in the 90° orientation drop, resulting in much smaller stresses.