Analysis on cracking blind vias of PCB for mobile phones

Abstract

Microvia interconnection technology, which mainly includes blind via and buried via, has been put forward to meet the trends of “faster, lighter, reliable” in electronic industry. The technology plays a crucial role in the development of low cost and high density printed circuit board (PCB). However, as the cost for raw materials, like copper, keeps increasing, one defect blind via will lead to not only failure of the whole board but great economic loss as well. The defect causes and mechanisms have been diversified due to the complexity of process, which can be traced back to material property, microvia design, manufacturing process and service environment etc.. This paper focuses on systematic failure analysis of blind vias on PCB for novel mobile phones. A series of modern analytical instruments and methods were used to observe defects of the vias. Meanwhile, focused ion beam (FIB) was adopted, for the first time, both at home and abroad, to obtain the microstructure of copper grains in the cracking interface. Characterization techniques such as FT-IR, TGA and SAM were also performed to inspect thermal stability of the bare boards. The results have shown that obvious cracking occurring along the interface of different plating layers is the main cause of open circuit; and that both sulfur impurity on wall of the vias related to incomplete desmear process and inappropriate location of the vias concerned with circuit design predominantly account for blind vias cracking. It has been revealed that the reason of the cracking generation is concerned with the sulfur segregation and the formation of brittle Cu x S (1≤x≤2). Moreover, the influence of warpage on the reliability of the via has also been mentioned. Based on these defaults, improvement countermeasures and suggestions have been addressed in the paper, which are of significant value for reference to the safe reliability and structural integrity of PCB products during manufacturing and services.