Wirebond Metallurgical Systems


The wirebonding process of semiconductor packaging consists of forming a metallurgical bond between the bond wire and the bonding substrate. The bonding substrate can be a bond pad on the die or a metal pad on the lead frame, package, or circuit substrate.  The metal wire and the metal bonding substrate constitute the metallurgical system of the wirebonding process.  There are many different wirebonding metallurgies used in the microelectronics industry, the common ones of which are presented below. Each of these systems has its own distinguishing characteristics, so proper choice of the wirebonding metallurgy in relation to the application is important.


Gold-Aluminum System


Gold-aluminum or Au-Al metallurgy is the most widely used system in the semiconductor industry, being employed primarily in thermosonic bonding of plastic packages for the formation of the bond between a gold wire and an aluminum bond pad on the die.


Au-Al thermosonic bonding is usually done at an elevated temperature (at about 250 deg C) to promote intermetallic formation between the Au ball bond and the Al bond pad.  During bonding, Au and Al interdiffuse into each other, forming five different intermetallic compounds that exhibit different colors, i.e., white, tan, purple. The combination of these colors in various proportions can produce many other observable colors. The interdiffusion mechanism between gold and aluminum is quite active and thus inclined to form voids, such as those formed in the aluminum bond pad when too much aluminum has diffused into the gold ball.  These voids can lead to bond fractures that cause ball bond lifting, which is a major reliability issue in Au-Al wirebond systems.


Gold-Silver System


The gold-silver or Au-Ag system is another metallurgy commonly seen in semiconductor applications. It is used primarily in the formation of a wedge or fish-tail bond between the gold wire and the silver-plated lead finger of the leadframe in most leadframe-based plastic package technologies today.  In fact, it is usually the 'second bond' of Au-Al thermosonic bonding, the 'first bond' being the Au gold ball bond onto the Al bond pad itself. 


The Au-Ag system has been proven to be very reliable even at elevated temperatures, since it does not form excessive intermetallic compounds nor is it vulnerable to corrosion issues. Bondability problems caused by contaminants (such as sulfur), however, may be an issue.  Fortunately, Au-Ag second bond formation is done at an elevated temperature (about 250 deg C), which tends to remove silver-sulfide films, enhancing the bondability of the silver pad in the process.


Aluminum-Aluminum System


Aluminum-aluminum or Al-Al systems are used primarily in hermetic packaging, for bonding aluminum wires onto the aluminum bond pads of the die. The Al-Al system is also not prone to intermetallic formation and corrosion, making it a reliable wirebonding metallurgical system as well. Aluminum wire on aluminum bond pad is usually done ultrasonically at room temperature, although a thermo-compression bond can be created through high deformation.


Gold-Gold System


The gold-gold or Au-Au system is an extremely reliable wirebonding metallurgical system, since it is not prone to interface corrosion, intermetallic formation, and other mechanisms that degrade the bonding. In fact, even non-ideal formation of gold-gold bond welds can further develop and increase in strength with time and temperature. Au-Au bonding is usually performed at elevated temperature by thermocompression or thermosonic means, although cold ultrasonic Au-Au wire bonding can also be achieved. Au-Au bonds formed by thermocompression, however, is sensitive to surface contamination.


Gold-Copper System


Gold-copper or Au-Cu bonding metallurgy is usually employed in bonding gold wires to bare copper lead frames. Gold wire-copper leadframe bonding produces three ductile intermetallic phases (i.e., Cu3Au, AuCu, and Au3Cu), which tend to form voids at high temperatures. These voids degrade the bond and lowers its reliability.  Cleanliness of the bonding surface is therefore imperative in gold-copper systems to ensure reliable bonding.


Al-Ag System


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.


The presence of corrosive contaminants, primarily chlorine (Cl), and moisture can result in serious corrosion problems in an Al-Ag system. The potential risk of corrosion in the field is usually addressed by using large-diameter aluminum wires and thick silver alloy bonding sites. Needless to say, the bonding surface must likewise undergo cleaning (usually with a good solvent wash and DI water rinse) prior to bonding. For further protection against corrosion, moisture-resistant silicone gel may be used to encapsulate the hybrid device.


Kirkendall voiding can occur in an Al-Ag system, but may not be a major concern most of the time because it typically occurs at temperatures higher than the operating temperature range of IC's.


Al-Ni System


Aluminum-nickel or Al-Ni systems are used in the wirebonding of power devices or devices used in high-temperature applications.  It is also used in bonding chips directly onto various special substrates.  This metallurgical system usually consists of a large diameter aluminum wire and nickel bonding pads deposited through electroless means.


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.


See also:   Wirebonding Bond Lifting Package Failures




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