Epoxy Degradation Induced Au-Al Intermetallic Void Formation in Plastic Encapsulated MOS Memories

Previous high temperature aging studies have shown that the presence of epoxy in integrated circuit packages degrades the strength and contact resistance of Au-Al bonds [1-3]. Cl impurities in the resin or Br from the flame retardant or both are associated with this phenomenon. High temperature aging experiments were performed with Au wire bonded, Al metallized 4K NMOS Static RAMs and epoxy molding compounds in two types of DIPs. The initial study focussed on the effect of the flame retardant by using the epoxy with and without flame retardant in the cavities of epoxy sealed ceramic DIPs. The apparent activation energy for the degradation was determined by aging devices molded in DIPs using the standard flame-retardant formulation at three temperatures. The Cl and Br content of the epoxies was measured by neutron activation analysis and water extraction. Electron and ion microprobe analysis showed that Cl and Br are present in the bond zone with the Au-Al intermetallics. The median life at 180°C was only a factor of three higher without the flame retardant, even though the Br content was 600 times lower, suggesting that the Cl plays a major role. The apparent activation energy for the bond degradation in the flame-retardant molding compound is ~0.8 eV, within experimental error of that for pure Au-Al intermetallic formation [4] and that for the breakdown of the brominated flame retardant determined by water extraction. The water extraction results also indicate that the chlorinated impurities are less thermally stable than the brominated flame retardant.