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2010
Conference Paper
Titel
A detailed investigation of the failure formation of copper trace cracks during drop tests
Abstract
The development cycle of new products can be dramatically reduced if exact lifetime models are at hand. This requires the precise knowledge of the failure modes and the failure position under all test and service conditions. In case of dynamic mechanical loads like drops of BGA modules, broken copper traces at the PCB side are more and more often observed to be the ultimate failure effect. However, straightforward FEM simulations have shown unrealistic high stress and strain results not matching experimental observations which prove that a realistic representation of this failure is not trivial. A comprehensive physical investigation of the failure formation combined dye and pry tests, electron backscatter diffraction EBSD and 3-D X-ray tomography analyses. The results revealed the complex nature of the dynamic failure propagation. In addition to the ultimate fracture of the copper trace cracking of the IMC on top of the PCB pad was detected. Both failure modes are initiated during the drop event at different times and propagate with different speeds. Both failures are mutually interacting with each other with no single mode alone dominating the failure evolution. Hence, lifetime modeling needs to account for both failure modes in order to capture the copper trace stress realistically. Numerical simulations have been conducted investigating the experimental findings. A small flaw already reduces the stress level at the pad/solder interface substantially. The IMC crack is found to be initiated first reaching a stable length at the half pad diameter. In this condition, the copper trace is stressed at a lower level accumulating plastic strain enough to ultimately create the trace fracture in a realistic number of cycles. A very realistic 2nd level interconnection model has been created based on the combined failure mechanism found by comprehensive physical failure analysis, and the numerical simulations. It now allows lifetime prediction of BGA assemblies under drop test conditions with high accuracy and reliably. Hence, it will be able to replace expensive experiments by fast numerical assessments in the design optimization phase of product and technology development.