Isothermal Cyclic Bend Fatigue Test Method for Lead-Free Solder Joints

Isothermal three-point and four-point cyclic bend fatigue test methods have been developed for Sn–Ag–Cu solder joints. Reported bend tests from the literature were conducted at room temperature (25°C) and there is lack of data for lead-free solder joints. In this study, very-thin quad flat no-lead (VQFN) assembly with Sn–Ag–Cu lead-free solder was tested under three-point and four-point cyclic bending loads at both room temperature (25°C) and high temperature (125°C). The correlation between three-point and four-point bend tests was developed. Two different board surface finishes of electroless Ni and immersion gold (ENIG) and organic solderability preservatives (OSP) were investigated. Bending fatigue resistance of VQFN with OSP finish is slightly better than ENIG finish case. The acceleration factor of failure at high temperature (125°C) is higher than that at room temperature (25°C). Finite element analysis modeling and simulation were performed for different test conditions to investigate the solder joint stress-strain behavior. Volume-averaged energy density was used as a fatigue damage parameter and energy-based bending fatigue models were developed for VQFN with Sn–Ag–Cu solder joint under cyclic bending load at both 25°C and 125°C.