Method for early detection of PCB bending induced pad cratering

Abstract

The pad cratering phenomenon has caused considerable concern as a type of latent defect that can impact long term reliability. Studies have concluded that the risk for pad cratering is elevated as the industry transitions to lead-free assemblies. In the past, characterization of pad cratering resistance has been problematic because typical board level reliability tests rely on monitoring the electrical resistance of daisy chain components. However, it has been known for some time that the initiation of pad cratering failures does not result in an instantaneous rise in electrical resistance. Therefore several studies have highlighted the need for a board-level methodology to detect the onset of pad cratering. In our recent study a new approach based on monitoring acoustic emission (AE) was introduced. [1] It was shown that in comparison with conventional electrical resistance monitoring, the acoustic method is able to detect the onset of pad cratering failures in board-level mechanical bending tests. This paper builds upon our recent work by evaluating the effects of several variables on the propensity for pad cratering. The tests have been conducted on 1 mm pitch HSBGA, 1 mm pitch FCBGA and 0.8 mm pitch CABGA packages. The effects of strain rate, OSP or ENIG PCB surface finish, and multiple reflows have been evaluated. The results show that the AE method can be used to detect the onset of both pad cratering and brittle solder joint fracture. Based on the measured PCB strain at the onset of failure, Weibull distributions have been used to predict the PCB strain limits.