wirebonding process of
semiconductor packaging consists of forming a metallurgical bond between
the bond wire and the bonding substrate. The bonding substrate can be a
bond pad on the die or a metal pad on the lead frame, package, or
circuit substrate. The metal wire and the metal bonding substrate
of the wirebonding process. There are many different wirebonding
metallurgies used in the microelectronics industry, the common ones of
which are presented below. Each of these systems has
its own distinguishing characteristics, so proper choice of the
wirebonding metallurgy in relation to the application is important.
is the most widely used system in the semiconductor industry, being
employed primarily in thermosonic bonding of
packages for the formation of the bond between a
gold wire and an aluminum
bond pad on the die.
Au-Al thermosonic bonding
is usually done at an elevated temperature (at about 250 deg C) to
promote intermetallic formation between the
Au ball bond
Al bond pad.
During bonding, Au and Al interdiffuse into each other, forming five
different intermetallic compounds that exhibit different colors, i.e.,
white, tan, purple. The combination of these colors in various
proportions can produce many other observable colors. The interdiffusion
mechanism between gold and aluminum is quite active and thus inclined to
such as those formed in the aluminum bond pad when too much aluminum has
diffused into the gold ball. These voids can lead to bond
fractures that cause
ball bond lifting, which is a major reliability issue in
Au-Al wirebond systems.
The gold-silver or
Au-Ag system is
applications. It is used primarily in the formation
of a wedge or fish-tail bond between
silver-plated lead finger
the leadframe in
most leadframe-based plastic package technologies today.
In fact, it is
usually the 'second bond' of Au-Al thermosonic bonding, the 'first bond'
being the Au gold ball bond onto the Al bond pad itself.
The Au-Ag system has been
proven to be very reliable even at elevated
temperatures, since it does not form excessive intermetallic compounds
nor is it vulnerable to corrosion issues.
caused by contaminants (such as sulfur), however, may be an issue.
Fortunately, Au-Ag second bond formation is done at an elevated
temperature (about 250 deg C), which tends to
films, enhancing the bondability of the silver pad in the process.
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are used primarily in hermetic packaging, for bonding
aluminum bond pads
of the die. The Al-Al system is also not prone to intermetallic formation and corrosion,
making it a reliable wirebonding metallurgical system as well. Aluminum wire on aluminum
bond pad is usually done ultrasonically at room temperature, although a thermo-compression
bond can be created through high deformation.
The gold-gold or
wirebonding metallurgical system, since it is not
prone to interface corrosion, intermetallic formation, and other
mechanisms that degrade the bonding. In fact, even non-ideal formation
of gold-gold bond welds can further develop and increase in strength
with time and temperature. Au-Au bonding is usually performed at
elevated temperature by thermocompression or thermosonic means, although cold ultrasonic Au-Au wire bonding
can also be achieved. Au-Au bonds formed by thermocompression, however,
is sensitive to surface
Au-Cu bonding metallurgy
is usually employed in bonding gold wires to bare
copper lead frames. Gold wire-copper leadframe bonding produces three ductile intermetallic phases
(i.e., Cu3Au, AuCu, and Au3Cu),
which tend to form voids at high temperatures. These voids degrade the
bond and lowers its reliability. Cleanliness of the bonding
surface is therefore imperative in gold-copper systems to ensure
The aluminum-silver or
is commonly used in thick-film hybrid technologies, to bond an aluminum
wire onto a
alloy (with Pt or Pd) bonding pad on the leadframe. The Al-Ag phase
diagram is very complex, involving many different intermetallic phases.
Al-Ag bonds have a tendency to degrade as a result of excessive
between Al and Ag. They also tend to
in the presence of moisture. These drawbacks make Al-Ag systems less
popular than other metallurgies for wirebonding.
The presence of corrosive
contaminants, primarily chlorine (Cl), and moisture can result in
serious corrosion problems in an Al-Ag system. The potential risk of
in the field is usually addressed by using large-diameter aluminum wires
and thick silver alloy bonding sites. Needless to say, the bonding
surface must likewise undergo cleaning (usually with a good solvent wash
and DI water rinse) prior to bonding. For further protection against
corrosion, moisture-resistant silicone gel may be used to encapsulate
the hybrid device.
voiding can occur in an Al-Ag system, but may not be a major concern
most of the time because it typically occurs at temperatures higher than
the operating temperature range of IC's.
are used in the wirebonding of power
devices or devices used in high-temperature applications. It is also
used in bonding chips directly onto various special substrates. This
usually consists of
a large diameter
aluminum wire and
nickel bonding pads deposited through electroless means.
Al-Ni wirebonding systems are generally
reliable, but may present some bondability
problems because of the tendency of the nickel surface to
quickly. Thus, wirebonding should occur immediately
in an inert atmosphere
after the pads are
nickel-plated. Another alternative is to clean the pads or subject them
just prior to bonding. Some substrate manufacturers deposit a very thin
layer of gold over the nickel surface to prevent it from oxidizing.
and Cu-Au Systems
and copper-gold or
generally pertain to the bonding of
aluminum or gold bonding pads or substrates.
As discussed on
system is quite vulnerable to void formation at high temperatures, and
requires bonding surface cleanliness to ensure bonding reliability.
Cu-Al system, void formation due to intermetallic growth is not much of
an issue compared to a Cu-Au system. However, a Cu-Al system
produces more intermetallic compounds, one of which is the
phase. Excessive growth of this brittle
which is accelerated by temperature,
lowers the over-all shear strength of the bond. Cu-Al systems are also
in the presence of chlorine and moisture.
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