Effect of vacuum on high-temperature degradation of gold/aluminum wire bonds in PEMs

Gold/aluminum wire bond degradation is one of the major failure mechanisms limiting reliability of plastic encapsulated microcircuits (PEMs) at high temperatures, It is known also that oxidative degradation is the major cause of failures in epoxy composite materials; however, the effect of oxygen and/or vacuum conditions on degradation of PEMs has not been studied yet. In this work, three groups of linear devices have been subjected to high-temperature storage in convection air chambers and in a vacuum chamber. Electrical characteristics of the devices, variations of the wire bond contact resistances, mass losses of the packages, and thermo-mechanical characteristics of the molding compounds were measured periodically during the testing. The results showed that high-temperature storage in vacuum and air conditions changed thermo-mechanical characteristics of molding compounds in a similar way; however, the failure rates and the degree of wire bond degradation for parts stored in air was significantly larger than for parts stored in the vacuum chamber. A mechanism of Au/Al wire bond failures in PEMs at high temperatures, the role of oxygen, and non-linearity of the degradation rate at a certain critical temperature above the glass transition temperature of molding compounds are discussed.