Fracture toughness of the molten zone of resistance spot welds
A methodology for measuring the fracture toughness at crack initiation and the crack extension resistance of the molten zone of resistance spot welds under Mode I loading has been developed. The cross tensile test of U-shaped specimens was modified by crack growth monitoring and stress intensity factor determination. The resulting values of fracture toughness at crack initiation are independent of the nugget diameter and of the base material mechanical properties. The crack extension resistance seems to depend on base material mechanical properties and nugget diameter. Mixed cleavage + ductile mode associated to medium values of fracture toughness (54–90 $$\text{ MPa }\,\text{ m }^{0.5}$$) suggested a ductile to brittle transition behaviour. The relatively low fracture toughness (55–59 $$\text{ MPa } \,\text{ m }^{0.5}$$) associated to full interfacial ductile failure was quantitatively related to the high number density of small particles in the molten zone. This study opens the possibility to apply the local approach to fracture under monotonic loading to interfacial failure of resistance spot welds.
Closed-form structural stress and stress intensity factor solutions for spot welds under various types of loading conditions
The theoretical framework and closed-form stress intensity factor solutions in terms of the structural stresses for spot welds under various types of loading conditions are presented based on elasticity theories and fracture mechanics. A mechanics description of loading conditions for a finite plate with a rigid inclusion is first presented. The loading conditions of interest are the resultant loads on the inclusion with respect to the center of the inclusion in a finite or infinite plate and the surface tractions on the lateral surface of a finite or infinite plate. The surface tractions on the lateral surface of the plate can be decomposed into a load-balanced part and a self-balanced part. The load-balanced part is statically in equilibrium with the resultant loads acting on the inclusion. The self-balanced part can be represented by the resultant loads on the lateral surface of the plate. The resultant loads on the inclusion and the self-balanced resultant loads on the lateral surface are then decomposed into various types of symmetric and anti-symmetric parts. Based on the stress function approach and the Kirchhoff plate theory for linear elastic materials, closed-form in-plane stress, moment and transverse shear force solutions are derived for a plate with a rigid inclusion subjected to various types of resultant loads on the inclusion and various types of resultant loads on the plate lateral surface. Based on the J integral for a strip model, closed-form analytical stress intensity factor solutions for spot welds joining two sheets of equal thickness are derived in terms of the structural stresses around a rigid inclusion in a plate under various types of loading conditions. The closed-form solutions presented in this paper are used as the basis to develop new analytical stress intensity factor solutions for spot welds in various types of specimens presented in a subsequent paper.
Failure analysis for poor solderability of Au surface of PCB
Nonwetting is one of the common problems and is always rejected in electronic assemblies. ENIG pads of a PCBA exhibited nonwetting after lead-free reflow soldering. With the aid of stereomicroscope, SEM&EDS and XRF, a series of analysis was performed. And after visual inspection, surface and cross section analysis, thickness measurement and solderability test, it can be inferred that oxidation of plating surface was the main reason for nonwetting, and corrosion in Ni plating caused an adverse impact on solderalibity.