Effect of moisture and temperature on Al-Cu interfacial strength

In power management semiconductor industry, the most comprehensively used metals in microelectronic packages include copper, aluminum, nickel, gold and silver. When different metals contact to each other, intermetallic compound (IMC) will form at the interface. Under different conditions, IMC may vary to show very complicated characteristic and composition, some Moderate IMC will increase interfacial strength and form reliable bond, but as the brittle intermetallic compound layer thickness increases, the joint contact resistance will increase and the bonding strength will decrease. Excessive IMC growth acts as a major cause for bonding failure such as discontinuous connection, even crack or bond lift. Especially when a package undergoes reliability test of moisture and high temperature, IMC growth will accelerate much faster than in the ambient environment. This paper mainly studies the IMC growth and interfacial adhesion of Al wire bond on Cu based leadframe with Bare Cu surface, Ni plated surface and Ag plated surface. The interfacial adhesion force is measured by bond shear and wire pull test at initial condition, after temperature cycling (TC, -65degC to 150degC), Autoclave (AC, Ta=121degC, relative humidity =100%, Pressure=15 psi), High Temperature Storage (HTS, 150 degC), simulating several typical package storage and usage conditions. The failure mechanism is studied from the fracture surface characteristics and cross section analysis after different aging time. In Autoclave environment with moisture and temperature effect, Al and Cu form an electrochemical couple. Because Al has low electrode potential and is more active, Al acts as anode to lose electrons and corroded in the galvanic corrosion. Further more, in HAST (130C/85%RH, 80% of rated BV) test, the effect of electrical bias to galvanic corrosion is analyzed.