THE RELATIONSHIP OF LIFE PREDICTION FOR QUAD FLAT N0-LEAD PACKAGE (QFN )PACKAGES USING 4 POINT BEND TEST AND THERMAL CYCLE TESTING
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In the structural design of the Printed Circuit Boards, the stiffness modulus of the involved Fr4 and Cu layers play an important role. The 4 point bend test is one of the few test methods which is included in the JEDEC standard for the Monotonic Bend characterization for board level interconnects. The study in the past has shown that the damage mechanism of these two testing methods is similar. And also, correlation had been drawn by observation of failure mechanism through Scanning Electron Microscope (SEM). The purpose of this study is to create a methodology for co-relating 4 point bend test and thermal cycling and to understand the failure` displacement on the solder joint. The printed circuit board assembly with the devices and solder joints aligned in the same direction was subjected to cyclic loading conditions and their behaviour under the stress was analysed. The test component contains daisy-chain connection to allow in-situ continuity monitoring of the device board level interconnects during the test. The daisy chain link in the outermost package is connected to a corresponding Printed Wiring Board(PWB) daisy chain link and electrically monitored. The electrical resistance was monitored to realize the amount of resistance the board can withstand before the solder joint starts to fail. It has been noticed from the experiment that the increase in displacement leads to decrease in the fatigue life of the solder joint. Among the many environmental accelerated testing methodologies for assessing reliability of electronic systems, thermal cycling is the most commonly used for the characterization of devices as well as interconnections. Therefore, the assembly was subjected to thermal cycling simulations to estimate the number of cycles that would be needed for the accumulation of plastic work (energy density) in the critical solder joint. This will give us the methodology for correlating the 4 point bend test and thermal cycling.