| Printed circuit board (PCB), the secondary substrate in electronic packaging, has been widely used in various electronic products to realize the electrical connection among the components and devices on the board. However, factors like circuit design, material selection, assembly process and service environment can cause failures during surface mounting technology (SMT) or service. There are three most common failure modes in electronic products including signal terminating, signal interruption and signal changing. Therefore, it is necessary to conduct systematic failure analysis in PCBs, which is of great importance in improving reliability and significant value in ensuring the structural integrity of the electronic products.In the first part, the basic knowledge, the classification and causes of failure in the failure analysis in electronic products were introduced. The research background including process and structure of PCB were illustrated. The common defects and characterization methods used in analysis of PCB were systematically concluded, and the research progress in blind via and BGA solder joints was further addressed in the second part. Failure analyses of two different types of PCBs for mobile phones were carried out in the most important section. Through automatic optical inspection (AOI), the defaults in the PCBs were mainly open circuit which was caused by cracking blind via and short circuit which was due to the unknown scatters. About ten kinds of macro and micro testing and characterization methods were adopted including fourier transform infrared spectroscopy (FT-IR), thermogravimetic analyses (TGA), scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS) and focus ion beam (FIB) and scanning acoustic microscopy (SAM) as well as thermal cycling experiment to find out the failure causes.Failure blind via is one of the main causes for open circuit in PCB. Metallographic inspection shows obvious cracking along the interface of different copper layers. Micrograph observation and chemical analysis on the grain boundary have definitely identified that residue sulfur probably resulted from incomplete desmear process and inappropriate chemical compositions of plating solution as well as inappropriate location of the vias concerned with circuit design predominantly accounted for blind vias cracking. Moreover, the influence of warpage on the reliability of the via was noted. Complementarily, the most easily cracking section in the blind via structure was verified by finite element method (FEM) after thermal cycling. Based on the analysis above, the probable cracking mechanism was proposed for the first time from chemical viewpoint, and the microcrack propagation was explained by fatigue crack theory.The quality of Ball Grid Array (BGA) solder joint is of great importance to perform the chips function in semiconductor packaging. A defect solder joint can cause failure in the whole device. Through material inspection, thermal testing, macro and micro observation and chemical analysis as well, the failure behavior of solder joints and the cause of scattered excessive solder balls were systematically studied. It was shown from experiment results that the predominant cause of the failure was improper processing parameters of SMT, to some extent, during reflow soldering process. Moreover, the mismatch between solder paste with poor quality and reflow profile led to a few weak solder joints. Meanwhile, potential risks to other defects like collapse of solder bars, splash of solder balls, tombstone effect and cracking of solder mask were also mentioned in the paper.Based on the problems and risks mentioned above, improvement countermeasures to the prevention of blind via cracking and detailed suggestions for reflow soldering parameters and modification of the solder paste have been addressed so as to ensure the structural integrity and safe reliability of PCB for mobile phones. |
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