Design of moisture vapor pressure sensor for popcorn failure analysis in molding compound

Abstract

Moisture absorbed by the hygroscopic polymers like molding compound and die-attach vaporizes during reflow lead to a high vapor pressure inside the electrical components lead to failure in the electrical device, named as popcorn failure. Popcorn failure in plastic encapsulated microcircuits has been a critical issue for electronic device reliability. Researches have been conducted on investigating the failure mechanism. Among all the factors that contributed to the failure, vapor pressure is one of the primary sources of stress that causes crack of the molding compound and delamination between critical interfaces. Numerous publications demonstrate the vapor pressure evolution and contributing factors with mathematical models, simulations, and tests. However, direct measurement of vapor pressure is not yet reported. This thesis presents the design and fabrication verification of a pressure sensor to measure the vapor pressure evolution in moisture-containing polymers at reflow temperatures. The specifications and requirements are extracted from the failure mechanism reported in the literature. A touch-mode capacitive pressure sensor with in-situ doped poly-SiC is designed to measure vapor pressure from atmospheric pressure to 8 MPa under reflow temperature up to 300 °C. Simulation on touch mode capacitive readout is performed to verify the design parameters and provide an estimation of device performance. The fabrication process is designed and conducted with several methods for crucial steps along with different structure dimensions to investigate an optimal solution. A complete fabricated device is achieved with measured initial capacitance from 12.3 to 26.7 pF for different sizes of diaphragms. The deviation between the measured results and simulation results due to fabrication problems is analyzed. Possible causes and solutions of problems that occurred in fabrication and measurement, such as unexpected upward bending of the diaphragm, leakage current between the capacitor electrode plate, and uniformity of SiC layer fabrication, are discussed. Problem correction, device optimization, complete characterization, and experiments for vapor pressure measurement in molding compound remain as future work.