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.
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
(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,
Distribution in Gold Ball Bonds.
free-air ball is then brought into contact with the bond pad. Adequate
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).
Photo of a free-air ball prior to ball bond formation
Wire loop formed to connect the die to the lead finger
The wire is then run to the
corresponding finger of the leadframe, forming a gradual arc or
between the bond pad and the
Pressure and ultrasonic forces are applied to the wire to form the
bond (known as a
bond and shown in Fig. 3), this time with the leadfinger. The wirebonding machine
(see Fig. 5) breaks the wire in preparation for
the next wirebonding cycle by
clamping the wire and raising the capillary.
a clamped aluminum wire is brought in contact with the aluminum bond pad.
is then applied to the wire for a specific duration while being held
down by a specific amount of force, forming the first
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
gold ball bonding is much faster than aluminum wedge bonding, which is
why it is extensively used in plastic packaging.
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
ultrasonic wedge bonding, Au-Al thermosonic ball bonding requires heat to
facilitate the bonding process. The Al bond pad is
than the Au ball bond, making good bonding between them through
means impossible without causing
wire, bond pad, or silicon substrate damage. The application of thermal
energy to the Al bond pads
them, promoting the inter-diffusion of Au and Al atoms that ultimately
form the Au-Al bond.
application also improves bonding by removing organic contaminants on
the bond pad surface.
Photo of a gold ball bond (1st bond) on the bond pad
Photo of a gold wedge/stitch bond (2nd bond) on the leadfinger
Fig 4. Photo
of an aluminum wedge bond (first bond) on the bond pad
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Bonding Failures; Bond
Manufacturing; Assembly Equipment
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