Gold bonding wire. Gold ductility provides the flexibility needed for good loop formation. Gold has excellent electrical and thermal properties, and its inert state (gold is a noble metal) makes it well suited to the bonding process. Gold wire is widely used in both ball and wedge bonding applications, producing reliable bonds to aluminum and gold surface metalization.

Needs for flexibility, loop retention and bondability must be balanced. Pure gold wire is too soft and is usually stabilized with dopants such as beryllium (around 5-10 ppm) or copper (around 10-30 ppm). Based on purity, gold wire is classified into various ranges from 99% (2N gold) to 99.99 % (4N gold). Beryllium-doped gold wire is 10-20% stronger than copper-doped and hence better suited for high-stress applications such as high-speed automated bonders. Gold wire is typically supplied in an annealed condition to prevent unwanted breakoffs during initial bond formation.

Table 2 compares properties of 4N versus 2N gold as developed by one supplier for “ultra stiff” gold wire bonding applications.³ It is interesting to note the differences in the minor and the major dopant levels (none in the case of 4N wire).


Surface cleanliness and contamination are critical to bond strength. The bonding metalization surface finish is critical to prevent bond tool capillary clogging. Some tradeoffs with gold bonding wire are:

• Cost.
• Requires expensive bonding tools – ceramic for ball bonding, titanium carbide for wedge bonding.
• Requires high-temperature bonding, a possible source of die stress.
• Limitations in heating the (organic) substrate metalization.

Although gold wire bonds can be made without substrate or wire heating, reliable gold bonding requires heating the bonding metalization and wire to a high temp. Ball bonding requires temperatures of 220-250°C, while reliable gold wedge bonding can be accomplished at around 150°C.

Given the form factor, high thermal conductivity and uniform thermal mass, heating a metal lead frame or die pad (for example, preplated copper lead frames as used in IC manufacturing) is relatively easier compared to organic substrates. Organic PCB substrate variables – lower thermal conductivity, complex thermal mass distribution, size, CTE, whether the PCB is populated or unpopulated – make it less than ideal to create localized heating at the substrate pads. The process may slow throughput or lead to higher stress conditions, marginalizing the overall assembly reliability.

Al-Si bonding wire. Small diameter aluminum wire is often used for wedge bonding and offers relatively good fatigue resistance. As is the case with gold, pure aluminum is too soft to draw into small wires, so it is alloyed with about 1% silicon to provide the desired properties of load and elongation.

Lightweight Al-Si wire is very reliable but is much less expensive than gold. It is used extensively with low-cost wedge bonding tools (tungsten carbide) and, because Al-bonding can be performed at room temperature, it is well suited to organic substrates and therefore an ideal default choice in most low-to-mid-cost COB applications.

As Figure 4 shows, 1% silicon exceeds the solid solubility limits of silicon in aluminum at room temperature. Silicon precipitation can cause stress and possible wire fracture and is one of the failure mechanisms associated with heel cracks.


Small diameter Al-Si wires are usually heat-treated (partially annealed) to disperse silicon uniformly. Large diameter wires are heat-treated before and after final drawing.

Al-Mg bonding wire. Aluminum alloying with magnesium is an alternate to alloying with silicon and offers certain advantages. Figure 5 shows the Al-Mg phase diagram. Magnesium solid solubility in aluminum is better than the 1% maximum of silicon dissolved in aluminum.

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