Effect of high thermal stability mold material on the gold-aluminum bond reliability in epoxy encapsulated VLSI devices

Cresolic epoxy novolac resins brominated by specially tailored brominating agents to impart a high C-Br bond energy have provided encapsulants with enhanced thermal stability. By isothermal bakes at temperatures from 190 to 250 degrees C, decomposition times of the experimental encapsulants were found to be three to four times longer than of a state-of-the-art commercial material. The presence of halogenated organic residues among some of the experimental materials was found to cause increased gold-aluminum wire-bond failure through degradation of the intermetallic. These residues were byproducts of resin synthesis, which were eliminated by modification of the chemistry and processing. After such modification, the halogen-induced failure time was found to be prolonged by as much as 80% compared to a commercial resin. The apparent activation energy of bond failure was 0.8 eV, which was found to equal that of diffusion of organic halide through the polymer matrix in an aqueous environment, as determined by aqueous ion extraction. High thermal stability of the C-Br bond in the resins as well as purity of the material from halogenated organic residues was found to be crucial for superior reliability of aluminum metallization and gold wire bond in epoxy plastic-encapsulated VLSI devices.