Maintaining structural integrity is an important issue for advanced flip chip ball grid array (FC-BFA) packages especially due to the use of Pb-free solder replacing the Pb-contained solders and the large package size induced mechanical vulnerability. For such packages, the mechanical bending causes fracture in the interconnecting plastic PWB. To determine the critical load becomes a challenge to assembly manufacturers since the conventional testing methods based on monitoring electric continuity of the daisy chained solder joints fail to detect the PWB de-bonding near the interconnect.This thesis researches a new method that uses the Fiber Bragg Grating sensor to detect the interconnect fracture failure of 55 mm lead-free FC-BGA under four-point bend test. Experimental setups and procedures are studied, a proper test configuration is determined and a series of testing performed. The validity of the new method in monitoring the solder joint de-bonding is proved through analyzing the test results, which shows that the FBG sensors are sensitive to capture the onset of the fracture in different cases.