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° to 60° and is application-dependent. Some applications require a higher feed angle because of package clearance issues. Some deep access applications require a 90° feed angle. In this configuration, the wire is fed (Figure 1) through a hole in the shank of the wedge tool.

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.

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
   ϴH = hole diameter
   H = wire feed angle (30° through 90°)
   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 µm is considered fine pitch.)

Wire feed angle. Loop consistency is critical, especially in stacked die applications with inner and outer loops. Lower wire feed angles give better loop control, while higher feed angles give less loop consistency. Figure 3 shows the importance of wire looping to avoid shorting. Higher wire feed angles are preferred for deep access application.

Back radius. For applications where tail consistency is critical, a sharper back radius is required. However, sharper back radii can result in heel cracking. A tradeoff in the back radius is critical for achieving uniform tailing and avoiding heel stress after the first bond. A normal back radius will range between 50 to 100% of the wire diameter.

Front radius. The front radius and the bond length chiefly define the strength of the second bond. The front radius ensures a gentle transition from the wire into the weldment of the second bond.

Wire hole diameter. The wire hole diameter plays an important role in applications that require high placement accuracy. A typical guide for selecting the hole diameter is two times the wire diameter. If the hole is too large, the wire will tend to move under the foot of the wedge. This will result in poor accuracy (and poor looping). If the wire hole diameter is too small, the wire can scrape along the sides of the hole as the wire is fed through the wedge. This will result in wire slivers that can cause shorts and reduce reliability. Wire slivers can be prevented by requesting wedges with polished holes to remove any metal burrs.

Vertical radius. Vertical radius is also referred to as vertical side relief (VSR). High-frequency devices can have large numbers of I/O pads placed as close to each other as possible, or very small individual bond pads. Wedge bonding can achieve finer pad pitch geometries than ball bonding with the same wire diameter because of the smaller amount of bond “squash” or wire deformation. Wedge bonding squash is typically 1.3 times the wire diameter as compared to two times the wire diameter for ball bonding. A thinner vertical radius permits wedge bonding to smaller bond pads without the need to decrease wire diameter.

Tool face geometry. Cross groove tools increase the ultrasonic coupling and are recommended for hard gold wire bonding. Soft materials like aluminum wire are not recommended for use with a cross groove tool because of the possibility of aluminum buildup in the groove. Flat or concave tools usually are used when bonding with aluminum wire.

The American Competitiveness Institute (aciusa.org) is a scientific research corporation dedicated to the advancement of electronics manufacturing processes and materials for the Department of Defense and industry. This column appears monthly.