Bonding Wires and typical metallurgical systems

Wires and typical metallurgical systems:

Gold and aluminum are the commonly used wire materials, in addition, copper and silver have also been used. Bonding these wires to different pad materials produces different metallurgical systems.

1. Wires usually used in wire bonding

Gold wire:

Gold wire is used extensively for thermocompression bonding and thermosonic bonding. In producing the gold bonding wires, surface finish and surface cleanliness are the critical issues to ensure the formation of a strong bond and to prevent clogging of bonding capillaries. Pure gold can usually be drawn to produce an adequate breaking strength (ultimate tensile strength of the wire) and proper elongation (ratio of the increase in wire length at rupture to the initial wire length given as a percentage) for use as bond wire.

Ultrapure gold is very soft, therefore small amounts of impurities such as 5-10 ppm by weight of Be or 30- 100 ppm by weight of Cu are added to make the gold wire workable. Be-doped wire is stronger than Cudoped wire by about 10-20% under most conditions, thus advantageous for automated thermosonic bonding where high-speed capillary movements generate higher stresses than in slow or manual bonders.

Aluminum wire:

Pure aluminum is typically too soft to be drawn into a fine wire. Therefore, aluminum is often alloyed with 1% Si or 1% Mg to provide a strengthening mechanism. At room temperature, 1% silicon exceeds the solubility of silicon in aluminum by a factor of 50, which leads to silicon precipitation. The number and the size of the silicon precipitates are dependent on the cooling rate from higher temperatures.

Slower cooling rates result in more precipitation and large nonuniform silicon nodules, while faster cooling rates do not allow sufficient time for silicon precipitation resulting is uniformly dispersed nodules. Silicon grain size can affect wire ductility, the second phase can become a potential nucleation site for fatigue cracks.

Aluminum alloyed with 1% magnesium can be drawn into a fine wire that exhibits a breaking strength similar to that of Al-1% Si. The Al-1% Mg alloy wire bonds satisfactorily and is superior to Al-1% Si in resistance to fatigue failure and to degradation of ultimate strength after exposure to elevated temperatures. These advantages of Al-1% Mg wire occur because the equilibrium solid solubility of Mg in Al is about 2% by weight, and thus at 0.5-1% Mg concentration there is no tendency towards second-phase segregation as isthe case with Al-1% Si.

Copper wire:

Recently, copper-ball bonding to IC metallization has received considerable attention primarily because of their economy and their resistance to sweep (tendency of the wire to move in the plane perpendicular to its length) during plastic encapsulation. The major problem for this system is the bondability. Copper is harder than gold and aluminum, which can lead to cratering or pushing the metallization aside. Therefore a harder metallization is required. In addition, the ball bonding must be performed in an inert atmosphere as copper oxidizes readily.

2. Metallurgical systems

In wirebonding process, different pad metallizations are used, depending to the production requirements. Therefore, different metallurgical systems can be formed with different reliability behaviours.

The typical metallurgical systems are:

Au-Au system:

Gold wire bonded to a gold bond pad is extremely reliable because the bond is not subject to interface corrosion, intermetallic formation, or other bond-degrading conditions. Even a poorly welded gold-gold bond will increase in strength with time and temperature. Gold wire welds best with heat although cold ultrasonic Au-Au wire bonds can be made. Either thermocompression or thermosonic bonds are easily and reliably made. Thermocompression bondability, however, is strongly affected by surface contamination.

Au-Al system:

Au-Al welding system is the most commonly used in wirebonding process. However, this bonding system can easily lead to formation of Au-Al intermetallic compounds and associated Kirkendall voids. The formation can be accelerated with the temperature and time of the operational life. There are five intermetallic compounds that are all colored: Au5Al2 (tan), Au4Al (tan), Au2Al (metallic gray), AuAl (white), and AuAl2 (deep purple). AuAl2 can initially form in the interface between gold and aluminum during bonding process even at room temperature and could transform to other Au-Al compounds depending on the temperature, time and bonding configurations. Therefore, this system often presents a problem in reliability of the bonds.

Au-Cu system:

Bonding gold wires to bare copper lead frames can cause the formation of three ductile intermetallic phases (Cu3Au, AuCu, and Au3Cu) with overall activation energies of 0.8 to 1 eV. The formation of these intermetallic compounds can decrease the bond strength at higher temperatures (200-325oC) as a result of

Kirkendall voiding. The degradation is apparently dependent on the microstructure, weld quality, and impurity content of the copper. Cleanliness of the bonding surface is extremely important to ensure good bondability and reliability in Cu-Au systems. In addition, if polymer material is used for die attach, the polymer must be cured in an inert atmosphere to prevent oxidation.

Au-Ag system:

The Au-Ag wire bond-system is very reliable for very long times at high temperatures. This bond system does not form intermetallic compounds and does not exhibit interface corrosion. Gold-wire bonds to silverplated lead frames have been successfully used in high production for many years. Bondability problems can be caused by contaminants like sulfur. Thermosonic Au-Ag bonding is usually performed at high temperature (approximately 250oC) which dissociates thin silver-sulfide films thus increases bondability of silver.

Al-Al system:

The aluminum- aluminum wire bond system is extremely reliable because it is not prone to intermetallic formation and corrosion. Aluminum wire on aluminum metallization weds best ultrasonically, although a thermocompression bond can be produced by high deformation.

Al-Ag system:

Aluminum wire bonded to a silver-plated lead frame is often used in thick-film hybrids (usually in alloy form with Pt or Pd). The Ag-Al phase diagram is very complex, with many intermetallic phases. Kirkendall voids can occur in this metal system, but typically at temperatures higher than the operating range of the microcircuits. In practice, Ag-Al bonds are seldom used because of their tendency to degrade due to interdiffusion and to oxidize in the presence of humidity. Chlorine is the main driving element of the corrosion process. Aluminum wires with large diameters are routinely bonded to Pd-Ag thick-film metallization in automotive hybrids. However, the bonding surface must be prepared by washing with solvents, followed by careful resistivity-monitored cleaning in deionized water. Then the hybrids are covered with a silicone gel for further protection.

Al-Ni system:

Al-Ni bonds using large diameter, >75 mm, aluminum wires are less prone to Kirkendall voiding and galvanic corrosion, thus more reliable than Al-Ag or Al-Au bonds under various environments. This system has been used in high production on power devices and high-temperature applications such as aircraft turbine blades for over fifteen years. In most cases, the nickel is deposited from electroless boride or sulfamate solutions, which results in reliable bonds. However, phosphide electroless nickel solutions that co-deposit more than 6 or 8% of phosphorous can result in both reliability and bondability problems. The main difficulty encountered when bonding to nickel plating is bondability rather than reliability due to nickel surface oxidation.

Thus,packages should be bonded soon after they are Ni-plated, protected in an inert atmosphere, or chemically cleaned before bonding. Changing bonding machine schedules, such as impacting the tool-wire-plating with the ultrasonic energy applied, can improve bondability to slightly oxidized nickel surfaces. Various surface preparation techniques (such as sandblasting) are sometimes applied before or after Ni plating to increase bondability.

Cu-Al system:

Copper wire can be bonded to both gold and aluminum substrate. Au-Cu system has been discussed before. For Cu-Al system, there exist five intermetallic compounds favoring the copper-rich side. Thus, there is the possibility of various intermetallic failures similar to those of Au-Al system. However, intermetallic growth in Cu-Al bonds is slower than in Au-Al bonds. The intermetallic growth in Cu-Al bonds does not result in Kirkendall voiding bur lowers the shear strength at 150-200oC due to the growth of a brittle CuAl2 phase. In the temperature range 300-500oC, bond strength significantly decreases with the increase of the total intermetallic thickness. The rate of Cu-Al intermetallic formation relies on the ambient atmosphere composition.

For example, the copper-aluminum bond system is adequately reliable as long as some oxygen is present in the package because Cu oxide prevents or inhibits the growth of void-like grooves under the bond. However, the presence of Cl contamination and water can cause corrosion of the aluminum metallization containing copper-aluminum intermetallics.

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