The aluminum-silver or Al-Ag system is commonly used in thick-film hybrid technologies, to bond an aluminum wire onto a thick-film silver alloy (with Pt or Pd)  bonding pad on the leadframe. The Al-Ag phase diagram is very complex, involving many different intermetallic phases. Al-Ag bonds have a tendency to degrade as a result of excessive interdiffusion between Al and Ag.  They also tend to oxidize in the presence of moisture. These drawbacks make Al-Ag systems less popular than other metallurgies for wirebonding.

Al-Ni wirebonding systems are generally reliable, but may present some bondability problems because of the tendency of the nickel surface to oxidize quickly.  Thus, wirebonding should occur immediately in an inert atmosphere after the pads are nickel-plated.  Another alternative is to clean the pads or subject them to sandblasting just prior to bonding.  Some substrate manufacturers deposit a very thin layer of gold over the nickel surface to prevent it from oxidizing.

Cu-Al and Cu-Au Systems

Copper-aluminum or Cu-Al systems and copper-gold or Cu-Au systems generally pertain to the bonding of copper wires onto aluminum or gold bonding pads or substrates.

As discussed on Page 1, the Cu-Au system is quite vulnerable to void formation at high temperatures, and requires bonding surface cleanliness to ensure bonding reliability.  In a Cu-Al system, void formation due to intermetallic growth is not much of an issue compared to a Cu-Au system.  However, a Cu-Al system produces more intermetallic compounds, one of which is the brittle CuAl2 phase. Excessive growth of this brittle intermetallic compound, which is accelerated by temperature, lowers the over-all shear strength of the bond. Cu-Al systems are also vulnerable to corrosion in the presence of chlorine and moisture.

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