Research on package reliability of the fiber optic hydrophone based on SEM

As underwater acoustic sensor, the fiber optic hydrophone needs to be used in harsh marine environment for decades. A reliable packaging structure of the fiber optic hydrophone is critical. On one hand, the package structure needs to have a good sound transmission performance to achieve high-precision underwater acoustic detection. On the other hand, it also needs to ensure enough strength to resist hydrostatic pressure and insulate sea water. Therefore, the combination designed of polyurethane packaging and metal matrix is adopted, and the interface was bonded by adhesive. Obviously, the bonded strength of interface is one determined packaging reliability factor, and the proper thickness of adhesive is the guarantee of bonded strength. In other words, if the thickness is not properly controlled, the bonded strength will not enough, and the packaging interface will become the weak links. So, effective measurement and characterization method is the basis of thickness control. In this paper, a research on package reliability of fiber optic hydrophone based on scanning electron microscope (SEM) has been present. In details, two sample preparation methods of breezing method and grinding method were introduced, further the thickness of the adhesive was measured by SEM. The results show that both of two methods can measure the thickness with submicron accuracy, and the comparison test of the same sample shows that two methods have good consistency, and the result is about 12 microns which is a lower than the required adhesive thickness of 25 microns, this will result in the adhesive strength cannot meet the reliable requirements, and became a weak links of reliability. Further analysis also show that, methods proposed in this paper can accurately measure the thickness of adhesive in submicron scale and effectively find the key process defects. This method will help to improve the packaging reliability of the fiber optic hydrophone.