Aluminum with 1% magnesium can be drawn into fine wire with similar strength as aluminum with 1% silicon. Al-Mg gives satisfactory bonding and is superior to aluminum with 1% silicon in fatigue failures. It also shows superior ultimate strength after high temperature exposure. Despite this, use has remained less prevalent compared to Al-Si, which has become widely accepted.

Copper bonding wire. Copper ball bonding has seen more recent use, and in particular, copper ribbons have received considerable attention. Copper is economical, has excellent heat and electrical conduction – hence smaller wires possible – and is resistant to sweep during plastic encapsulation.

Copper to copper bonding is possible, but the major issue remains bondability. Since copper oxidizes readily, bonding in inert atmosphere is needed, which somewhat negates the cost advantages of copper bond wire.

Cu-Al intermetallic growth rate is lower than Au-Al, and provided the initial bondability issue is addressed via inert gas, copper offers better reliability than gold wires to aluminum pads with 0.4-4.0 mil wire diameters. However, copper is harder than silver and aluminum (Table 1) and risks die cratering or pad metalization damage – a harder pad metalization is required for copper wire bonding. Currently, copper use is limited to mainly high-end use (CSPs, QFNs); use in COB applications is limited.

Metallurgical Systems

Gold wire-gold plated pads. A mono metal system, the Au-Au bond is extremely reliable. It is not subject to interface corrosion; there is no intermetallic formation and no bond degradation. Even a marginal Au-Au bond performs well despite time and temperature. As mentioned, gold is best bonded with heat. Cold ultrasonic Au-Au bonding is possible but less reliable. Thermosonic bonding is preferred and most common; however, thermocompression bonding is possible. Bonding is highly affected by surface contamination. Au-Au bonding is used in high-end COB applications and within the IC industry for applications such as SiP and MCMs. Plasma etching is extensively used to improve surface activation in high-end processes. Gold plating thickness for the bond pad metalization is target application (reliability) dependent. For high reliability bonding, an ultra pure, soft gold (hardness 60-80 knoop) in the range of 30-100 microinches is typical.

Soldering vs. bonding on gold pads. Gold offers excellent coplanarity, fine features and high-density circuits; it is essential in high-frequency applications. Gold can withstand multiple reflows, has good corrosion protection and excellent wetting. The main issues with gold are cost and a hard-to-manage plating process.

ENIG. Many industry segments have shied from ENIG (electroless nickel immersion gold) as a surface metalization for fear of “black pad” defect. With the prevalent use of the ENP (electroless nickel phosphor) process, which co-deposits phosphor in the nickel plating, the impact of phosphor content in the ENP plating on bond reliability and solderability has been the subject of several studies.⁴ Traditionally, 6-8% phosphor content has been used, but some recent studies show otherwise – up to 10-12 % phosphor content producing a more favorable, corrosion-resistant nickel surface morphology, an important variable, making it less prone to attack by the subsequent gold plating layer or the eventual soldering process.⁴

In the author’s experience and from a metallurgical standpoint, a controlled⁵ and specified ENIG process offers an excellent surface finish for soldering (with SnPb and Pb-free solders) and wire bonding, but the ideal thickness requirements for the gold film are quite different for the two methods. For soldering purposes, gold must be a dense (not porous), thin film, whereas for bonding, it should be thick, pure “soft” gold. The general structure favored for soldering applications is a “dense” gold of around 2-4 microinches over an underlying nickel layer of 200-240 microinches.^4,5,6,7

MIL-QQ-N-290A specifies 200 microinches of nickel between the copper and gold layer as a copper diffusion barrier. IPC-4552 specifies a minimum gold thickness of 0.05 µm [2 microinches] for (statistical) process variability. It also cautions on possible appearance of black pad when immersion gold thickness approaches 0.25 µm [10 microinches]. The gold layer acts as a sacrificial film in protecting the underlying metal, and needs to be sufficiently thin to be consumed by the tin during soldering, with the resulting AuSn IMCs dispersing into the bulk solder fillet. The bond is thus formed to next layer – nickel or copper, as the case may be – which must be active, lest it defeat the gold’s role. Soldering to thicker gold films meant for bonding applications results in a large percentage of AuSn IMC precipitating at the solder joint to pad interface, at the risk of solder joint embrittlement and low cycle life.

For COB-SMT, then, the ideal practice with gold would be to follow the structure desirable for soldering combined with selectively plated “thick” gold at the bonding sites. This process is costly and used mainly in high-end COB applications. If the product positioning does not justify it, a compromise has to be made, in which case the thin gold (favoring soldering) is chosen; it provides a product life commensurate with product expectation.

DIG. Direct immersion gold is a relatively new process in which gold is plated directly over copper.6 The gold film properties (thickness, porosity, morphology, etc.) apply to DIG as much as to ENIG, however, during soldering as the tin bonds to the copper, forming SnCu IMC, which has been documented to have a higher growth rate than the SnNi IMC layer.⁸ Hence, a lower reliability may be expected compared to ENIG, but it may well meet the lifecycle of many products. DIG field experience is limited in COB applications, but it “appears” well suited and cost-effective.

Gold wire-aluminum die pads. Au-Al is very common in wire bonding for COB applications. However, there are reliability issues over time. It is easily subject to Au-Al IMC layers and Kirkendahl voids. IMC formation accelerates with operating time and temperature and five IMC layers are formed as listed below (Figure 6).

• Au5Al2 (tan color) (some studies suggest Au8Al3).
• Au4Al (tan color).
• Au2Al (metallic gray color).
• AuAl (white color).
• AuA12 (deep purple color).

It is believed that initially AuAl2 forms at the Au-Al interface, then transforms to other IMCs with time and temperature.

Au-Al intermetallic growth. In a controlled ball bond process, Au-Al IMC growth shows relatively planar morphology. IMC initial growth rate is believed to be parabolic, settling to about 3-4 µm over time. At high temperatures (175°C) the aluminum pad converts to IMC; Au5Al2 (gold side) and Au4Al (pad side) predominate in the IMC layer. Au4Al may also grow by consuming Au5Al2 (Figure 6). Au4Al is susceptible to corrosion by epoxy molding compounds used in COB and may also oxidize.

Gold wire-copper PCB pads. Gold bonding to copper pads is possible but rarely used in COB. Three IMC phases are formed: Cu3Au, AuCu and Au3Cu. IMC formation decreases bond strength at high temperature (200 to 325°C). Kirkendahl voiding can occur due to copper migration. Bond strength degradation is dependent on micro-structure, bond quality and impurities in the copper. Cleanliness is extremely important for bondability and reliability when bonding to copper. Use of inert gas (argon) shielding improves bondability and reliability, prevents copper oxidation and is needed for curing polymer die attach material.

Soldering aspects. While copper with OSP would be a good choice for soldering low-end products, OSP coated pads are not considered usable “as is” for wire bonding. The alternatives of copper OSP pads for soldering and selectively plated (gold, silver or nickel) pads for wire bonding, while feasible, again negate the cost advantage and are hard to justify.

Gold wire-silver pads. The Au-Ag is very reliable for long terms at high temperature. Au-Ag does not form intermetallic compounds. Gold wire bonds to silver lead frames or silver plated pads have been successfully used in high-volume production for years. Silver bondability issues are caused by contaminants like sulfur that tarnish the silver plating. Au-Ag high temperature thermosonic bonding is performed at about 250°C and improves bondability by displacing the tin sulphide films. Use of silver is currently not prevalent in COB applications for cost reasons and thermosonic bonding requirement.

Soldering aspects. The immersion silver process is similar to immersion tin, except it uses electroless silver deposits. Immersion silver is being promoted as an alternative to Gold (due to black pad fears with ENIG) – and can be adopted into COB applications. However, silver metalization on solder pads is not free of solder joint reliability issues. Microporosity at the Ag-Cu interface occurring from the silver-plating process (corrosion of the copper surface) has been reported and deemed a reliability threat to solder joints. Silver solderability can easily degrade in contact with sulphur compounds. Thickness depends on two types of chemistry for immersion silver: “thin” silver (min. 0.05 µm ) and “thick” silver (min. 0.12 µm). These thicknesses are expected to guarantee a minimum one-year shelf life. Upper thickness limits for both types of immersion silver were not established in the initial release of IPC-4553. However, immersion silver is not an allowable finish for Class 3. Another reason why silver is being promoted is to eliminate tin whiskers. Costs are similar or marginally more than immersion tin. Silver use is relatively new and not common in the high-volume, cost-sensitive COB segment. Further field experience and volumes are needed to characterize long-term impacts.

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