Wedge bonding tools for fine-pitch applications: connecting leads to die involves precise tool selection.

Wire bonding is a process that creates an electrical connection between a die and a substrate or lead, typically using gold or aluminum wire. Wedge bonding is a specific type of wire bonding that uses a wedge-shaped tool to create the welds. The design of the wedge tool has changed very little over

the past decade. The wire is fed at an angle through the back of the wedge. This angle is typically 30[degrees] to 60[degrees] and is application-dependent. Some applications require a higher feed angle because of package clearance issues. Some deep access applications require a 90[degrees] feed angle. In this configuration, the wire is fed (Figure 1) through a hole in the shank of the wedge tool.

[FIGURE 1 OMITTED]

A wedge has a number of features that contribute to the quality and consistency of the wire bonds and loops. A thorough understanding of the wedge geometry is required for selecting a suitable wedge for any application. Figure 2 shows a detailed view of some of the critical features of a wedge-bonding tool.

[FIGURE 2 OMITTED]

Bond length is calculated using the formula:

BL = 2/3FR + BF + 2/3BR

where

FR = front radius

BR = back radius

W = width of the wedge

[THETA]H = hole diameter

H = wire feed angle (30[degrees] through 90[degrees])

BF = bond foot length

BL = bond length

VR = vertical relief

T = total length of the wedge

Special considerations should be taken before selecting a wedge tool for an application. Here are some of the critical design features that drive wedge selection.

Wedge material. The wire material plays an important role in determining the wedge. A tungsten carbide wedge is used for aluminum wire, and a titanium carbide wedge is used for gold wire. The wrong material will cause premature wear of the wedge tool, resulting in poor welds. The latest developments in bonding tools for fine-pitch applications focus on implementation of new materials such as cermets, void-free carbides, and ceramics to increase long-term stability and to avoid interactions with wire and pad materials.

Bond pad pitch. Pad size and pad pitch play a significant role in selecting the wedge. Typically 100% of the bond is required to be on the pad. Bond width, bond length and pad pitch should be considered before selecting the wedge. (Bond pad pitch less than 50 [micro]m is considered fine pitch.)