Thermo-mechanical reliability of sintered all-Cu electrical fine pitch interconnects under isothermal fatigue testing benchmarked against soldered and TLP-bonded SnAg3.5 joints

Cu sintering is one of the emerging technologies in the field of micro- and power electronics where operating temperatures higher than 150°C are required. At these temperatures, solder joints reach their limits due to high homologous temperatures. Hence, Cu sintered joints can serve as a substitute for these soft solder joints, being advantageous also with respect to thermo-dynamic stability, fatigue resistance, electrical conductance and cost. This paper addresses failure analysis of sintered (neck-based) All-Cu electrical interconnects (NEI) along with soldered SnAg3.5 and transient liquid phase bonded (TLPB) specimens which form an SnCu intermetallic (IMC) and are used in a homogenous Si-Si flip chip assembly for fine pitch interconnects. The SnAg3.5 solder serves as a benchmark for the NEIs and the TLPB joints. All the flip chip specimens were free of underfill material. The test samples were assembled on spring steel substrates using a Silicone-based adhesive for a low stress bond and then put under isothermal accelerated fatigue tests using 4-point bending at low-homologous temperatures (R.T.). The reliability investigation involves monitoring of electrical resistance as a failure indicator for interconnect fatigue. A failure criterion at 20% increase in resistance is defined to establish a correlation between the experimental failure times and resistance. The fatigue behaviour of the joints was also studied using Finite Elements analysis (FEA). The focus of the modelling was towards the behaviour of the critical joint. Cross-sections were prepared and analysed using optical microscopy and SEM to investigate the failure mode and mechanism.