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Wirebonding Process
Wirebonding
, or wire bonding, is the process of
providing electrical connection between the silicon chip and the
external leads of the semiconductor device using very fine
bonding wires. The wire used in wirebonding is usually made either of gold (Au) or aluminum (Al),
although Cu wirebonding
is starting to gain a foothold in the semiconductor
manufacturing industry. There
are two common wirebonding processes: Au ball bonding and Al wedge bonding.
During gold ball wire bonding, a gold ball is first formed by melting the end of the wire (which is held by a bonding tool known as a capillary) through electronic flame-off (EFO). This free-air ball (Fig. 1a) has a diameter ranging from 1.5 to 2.5 times the wire diameter. Free air ball size consistency, controlled by the EFO and the tail length, is critical in good bonding. For a discussion on how the melting of the wire affects grain size distribution and wire strength, see: Grain Size Distribution in Gold Ball Bonds.
The free-air ball is then brought into contact with the bond pad. Adequate amounts of pressure, heat, and ultrasonic forces are then applied to the ball for a specific amount of time, forming the initial metallurgical weld between the ball and the bond pad as well as deforming the ball bond itself into its final shape (Fig. 2).
The wire is then run to the corresponding finger of the leadframe, forming a gradual arc or "loop" (Fig. 1b) between the bond pad and the leadfinger. Pressure and ultrasonic forces are applied to the wire to form the second bond (known as a wedge bond, stitch bond, or fishtail bond and shown in Fig. 3), this time with the leadfinger. The wirebonding machine or wirebonder (see Fig. 5) breaks the wire in preparation for the next wirebonding cycle by clamping the wire and raising the capillary.
During aluminum wedge wire bonding, a clamped aluminum wire is brought in contact with the aluminum bond pad. Ultrasonic energy is then applied to the wire for a specific duration while being held down by a specific amount of force, forming the first wedge bond (Fig. 4) between the wire and the bond pad. The wire is then run to the corresponding lead finger, against which it is again pressed. The second bond is again formed by applying ultrasonic energy to the wire. The wire is then broken off by clamping and movement of the wire.
Because it is non-directional, gold ball bonding is much faster than aluminum wedge bonding, which is why it is extensively used in plastic packaging. Unfortunately, gold ball bonding on Al bond pads can not be used in hermetic packages, primarily because the high sealing temperatures (400-450 deg C) used for these packages tremendously accelerate the formation of Au-Al intermetallics that can lead to early life failures. Gold ball bonding on gold bond pads, however, may be employed in hermetic packages.
Unlike Al-Al
ultrasonic wedge bonding, Au-Al thermosonic ball bonding requires heat to
facilitate the bonding process. The Al bond pad is
harder
than the Au ball bond, making good bonding between them through
purely ultrasonic
means impossible without causing
wire, bond pad, or silicon substrate damage. The application of thermal
energy to the Al bond pads
'softens'
them, promoting the inter-diffusion of Au and Al atoms that ultimately
form the Au-Al bond.
Front-End Assembly Links: Wafer Backgrind; Die Preparation; Die Attach; Wirebonding; Die Overcoat
See Also:
Bond
Strength Tests;
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