Aluminum Bond Pad Contamination by Thermal Outgassing of Organic Material from Silver-Filled Epoxy Adhesives

Effects of outgassing from commercial die attach adhesive systems on the mechanical strength of gold aluminum bonds were studied. Bond pad metallizations on unpackaged test dice were exposed to the die attach media under standard cure conditions, and gold bonds were subjected to a steam pressure pot storage. Bond pull strength as well as resistance data were obtained to determine the extent of bond degradation. Bonds exposed to die attach systems containing reactive epoxy diluents were found to cause faster mechanical degradation of the Au/Al bond under steam pressure pot storage as compared with the die attach systems formulated with ester type or other nonreactive solvents. Cleaning the bond pad with N2/O2plasma prior to wire bonding was found to dramatically improve the mechanical strength of the bonds. It was determined that the epoxy reactive diluents in some of the die attach systems could deposit on the bond pad during the cure and cause growth and reinforcement of the native Al2O3film on the pad surface with a rigid self-polymerized organic resin. This could prevent complete exposure of the underlying metal during the wire bonding process and leave debris within the bond. Wire bonds made under such a condition would be chemically vulnerable to attack by the moisture and the ionic species present in a plastic package.