Degradation of wire bonds (WBs) is one of the major factors limiting reliability of plastic encapsulated microcircuits (PEMs) at high temperatures.  Use of PEMs in military and aerospace applications requires extended and thorough evaluation of encapsulating materials and reliability of packages in harsh environments.  However, the effect of environmental conditions on characteristics of molding compounds (MCs) and reliability of wire bonds has not been studied sufficiently to date

In this work, two types of PEMs in QFP-style packages have been stored in different environments at temperatures from 130C to 225C for up to 4,500 hours in some cases.  To assess the effect of oxygen, the parts were aged to 198C in air vacuum chambers.  The effect of humidity was evaluated during long-term highly accelerated temperature and humidity stress testing (HAST) at temperatures of 130C and 150C.

Thermo-mechanical and thermo-gravimetrical analyses were used to evaluate the effect of environment on characteristics of molding compound.  Contact resistances of wire bonds and their mechanical strength were used to monitor degradation of wire bonds throughout the testing.  Correlation between degradation of MC and WB failures has been analyzed.  The effect of environmental conditions on accelerating factors of WB failures has been assessed, and the mechanism of wire bond degradation due to the presence of moisture and oxygen is discussed.