Mixed-mode fracture analysis of brittle intermetallic compound in solder joint

Intermetallic compound (IMC) layer formed between solder materials and copper is typically the fracture sites for most of the solder joints subjected to type and mode of loading. Based on industrial failure case of solder joints, mechanical loading has resulted with mixed mode fracture pattern which the micro cracks growth initiated at the IMC layer. The complex fracture IMC behaviour is presently unexplained by theory to define the individual and overlapping stress shielding and stress amplification of micro cracks. This study aims to investigate the fracture behaviour of IMC solder joint based on Kachanov theory of interaction. Initially, two finite element (FE) models were developed for IMC solder joint to find the fit width and length value of the IMC layer, which validated with Brown & Srawley and Srawley model. Then, the single edge micro crack IMC model is embedded into full solder joint model, a series of von-Mises stress analysis is conducted to identify the critical stress concentration area before the fracture parameter e.g. stress intensity factor (SIF) and strain energy release rate (SERR) are evaluated. In IMC model establishment, FE codes are developed using ANSYS APDL software to execute the FE modelling of solder butt joints IMC layer failure based on displacement extrapolation method (DEM) and J-integral. By reliable agreement of fracture behaviour with Brown & Srawley and Srawley model, the new IMC solder joint model is continued for different parallel and coplanar micro cracks behaviour simulation to evaluate the individual and overlapping stress shielding and stress amplification subjected to shear and tensile loading. The micro cracks interaction is elucidating based on Kachanov theory of crack interaction in relating to Brown & Srawley and Srawley model. The results display that soldering material become less tough if greater crack length is present in the joint. The thicker IMC has slightly reduced the SIF on the crack tip but the change from solder to IMC layer decrease the solder joint fracture toughness. The interaction between multiple edge cracks in solder ball was carried out to quantify the effect of shielding and amplification on the crack driving force based on stress singularity and strain energy release rate approach. Larger length of macro-crack is found to provide shielding to the smaller crack or micro-crack. The greater distance between the two parallel edge cracks will reduce the shielding effect. When two co-linear cracks are closer together, the stress increases and demonstrated the amplification effect. Besides, the effect of inner crack length and vertical distance are highlighted. It is found that when compare both shielding and amplification, the distance between the two crack-tip must be shorter to give better effect. Location of multiple cracks include both soldering bulk and IMC layer were discussed. It also reveals that both approaches become unstable when both crack-tip are near together. The effect of mix-mode shielding and amplification between parallel and co-linear cracks demonstrates that the amplification become dominant if more crack tips are array in co-linear pattern, while shielding is in prevailing when the configuration of the very near crack tips all are in stack mode. More simulation should be carried out in future to understand the effect of difference shape and orientation of cracks under both static and dynamic loading.