Fatigue and Failure Analysis of FBGA Solder Joints in Memory Modules under Vibration

Abstract

As electronic devices become smaller, faster and thinner, integrated circuit (IC) packages with high input/outputs (I/Os) become denser. Fine-pitch ball grid array (FBGA) which is a surface-mounted IC package is ideal for packaging devices with high densities and I/Os and small sizes and weights. FBGAs are widely deployed in space-conscious applications, microprocessors/microcontrollers, dynamic random access memories (DRAMs), and consumer electronics. DRAM memory modules used in personal computers, workstations, and servers are composed of a number of packages mounted on a printed circuit board (PCB). Packages, which are integrated circuits of DRAM, are mounted on PCBs through solder joints. Solder joints are further used to provide the electrical signals between package chips and the PCB. Memory modules are exposed to various mechanical impacts, such as vibration over various frequency ranges, shock or temperature cycling during transportation, operation, installation or handling, which may cause products to malfunction. The reliability and performance of memory modules are standardized by the Joint Electron Devices Engineering Council (JEDEC). Performance and fatigue life of the memory modules mostly depend on the solder joints. Therefore, it is important to understand the dynamic behavior of memory modules, as well as the behavior of solder joints subjected to vibration. Memory modules should also meet the diverse demands of applications. High reliability and performance of solder joints under vibration is demanded by customers and the electronics manufacturing industry to meet consumer needs and JEDEC reliability test conditions. Predicting the fatigue life of solder joints in memory modules under various operating conditions is another important topic in designing the robust memory devices that are demanded by the manufacturing industry. Firstly, this dissertation investigates the failure mechanism of Fine-pitch Ball Grid Array (FBGA) solder joints in daisy chains assembled memory module with double-sided packages due to harmonic excitation by using the experiments and the finite element method. The experimental setup was developed to monitor resistance variation of FBGA solder joints due to the harmonic excitation of the JEDEC standard. A finite element model of the memory module was developed, and the natural frequencies and modes were calculated and verified by experimental modal testing. Forced vibration analysis was performed to correlate the cracked solder joints in experiments with the solder joint of high stress concentration in simulation. Secondly, this research studies the effect of solder pad size on the fatigue life of FBGA solder joints of a memory module with double-sided packages due to harmonic excitation using a global-local modeling technique. A finite element model of the memory module presented by the global-local model was developed, and calculated displacements from the global model were substituted along the boundary of the local model in order to detect vulnerable parts of solder joints under vibration. The effect of solder pad size on the fatigue life of the memory module was investigated using the Basquin equation and Miner’s rule. Fatigue life of the memory modules with different pad size is estimated by using solder material fatigue constants in the literature. It was experimentally verified that reliability and fatigue life of memory module was affected by solder pad size on PCB side. Finally, this study predicts the fatigue life of FBGA solder joints in a memory device with double-sided packages due to harmonic excitation through experiments and the finite element method. The calculated displacements from the global model were substituted along the boundary of the local model with detailed solder joints in order to calculate the stress magnitude of solder joints of the memory device under vibration. The fatigue life of the memory device was predicted using the Basquin equation and the Miner’s rule. The fatigue life of the solder joints in two different memory devices with a daisy chain assembly was measured for experimental validation. It was experimentally verified that the predicted fatigue life of the memory device matches the experimental results with reasonable accuracy under cumulative damage conditions.