A new detection method for the onset of PCB pad cratering

Abstract

The conversion to lead free and halogen free materials have raised several new assembly and reliability concerns such as pad cratering, dynamic resilience of the assembly and so forth. The pad cratering cracks typically are observed in the laminate under the solder connect pads, and occur during the board level reliability tests due to mechanical stresses. So far, pad craters have not been detectable by electrical testing or nondestructive inspection methods. In the past, the gross failures were detected only if a change in electrical resistance was observed during a board level reliability test. The characterization of pad cratering resistance has been problematic as it is difficult to rely on the in-situ monitoring schemes during board level reliability tests. The initiation of pad cratering may not have an electrical signature. Therefore some studies have highlighted the concern of how to detect the onset of pad cratering in a board-level testing methodology. In this paper, the new detection method of acoustic emission (AE) was introduced for the initiation of pad cratering on PCBs. An acoustic emission methodology is used to investigate the pad cratering in a daisy chain 45 × 45 mm Flip-Chip BGA (FCBGA) package with lead-free SAC305 solder balls. The four point bend test vehicle was used to which AE sensors were attached, to detect the onset of pad cratering. A two-dimensional AE source location method has been used to determine the planar location of failures on the test board when the acoustic event took place. The bend test results, in conjunction with failure analysis, have shown that in comparison with conventional electrical resistance monitoring, the acoustic emission method is indeed an effective methodology to detect the onset of pad cratering failures. The electrical failure of daisy-chain nets was detected at significantly higher strain level during the bend test. However, the failures recorded at lower strain levels with the acoustic emission revealed the PCB pad cratering failure mechanism and its initiation path. This board level test methodology with acoustic emission method may be used to evaluate the propensity of different materials and packages to pad cratering, and also to improve back-end manufacturing processes without using daisy chain test vehicles.