Investigation of Pad Cratering in Large Flip-Chip BGA

Anurag Bansal, Cherif Guirguis and Kuo-Chuan Liu
Cisco Systems, Inc., San Jose, CA

Electronics assemblies with large flip-chip BGA packages can be prone to either pad cratering or brittle intermetallic (IMC)failures under excessive PCB bending. Pad cratering cracks are not detected by electrical testing or non-destructive inspection methods, yet they pose a long term reliability risk since the cracks may propagate under subsequent loads to cause electrical failure.

Since the initiation of pad cratering does not result in an instantaneous electrical signature, detecting the onset of this failure has been challenging. An acoustic emission methodology was recently developed by the authors to detect the onset of pad cratering [1, 2]. The instantaneous release of elastic energy associated with the initiation of an internal crack, i.e., Acoustic Emission (AE), can be monitored to accurately determine the onset of both pad cratering and brittle intermetallic (IMC) failures.

In this study, the AE technique is used to systematically investigate pad cratering in a daisy chain 40 x 40 mm Flip-Chip BGA (FCBGA) package with lead-free SAC305 solder balls and 1 mm ball pitch. AE sensors have been attached to a fourpoint bend test vehicle to determine the onset of either pad cratering or brittle IMC failures. A two-dimensional AE source location method has been used to determine the planar location of failures on the test board. The test matrix is designed to investigate the effects of normal or diagonal strain orientation, NSMD or SMD PCB pads, and single or multiple reflow cycles. Physical failure analysis has been performed to correlate the test results with failure modes.

Results from this study demonstrated that monitoring Acoustic Emission during board bending tests is an effective
methodology to detect the initiation of pad cratering and partial brittle IMC cracks at the BGA joints. While the utility of the AE method was previously demonstrated to detect pad cratering [1, 2], this study shows that even brittle fracture at the IMC layers may be partial in nature and may not cause instantaneous electrical failure. The conventional approach of using electrical tests will therefore significantly overestimate PCB bending strain limits for board handling and test operations.

Using the AE method, the mechanical bending strain limit was determined for a 40 x 40 mm FCBGA package with a full array of NSMD pads and SMD pads at the corners. The effects of PCB pad design, component orientation, and number of reflows were investigated in detail. The failure mode was highly dependent on the PCB pad design. With a full array of NSMD pads only pad cratering failures were observed. With the use of corner SMD pads, the failure mode shifted to a combination of brittle IMC fracture and pad cratering.

In contrast with full NSMD pads, the use of corner SMD pads resulted in significantly lower electrical failure strains. Tests with 0° and 45° component/strain orientations, tested after 1X or 3X reflows, showed that the propensity for pad cratering is higher if the bending strain is oriented along the component diagonal and if the boards are subjected to multiple reflow cycles. The propensity for brittle IMC fracture is also higher after multiple reflows, but the effect of strain orientation was insignificant.