|
|
|
|
|
Wirebond
Metallurgical Systems
The
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
constitute the
metallurgical system
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.
Gold-Aluminum System
Gold-aluminum or
Au-Al metallurgy
is the most widely used system in the semiconductor industry, being
employed primarily in thermosonic bonding of
plastic
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
and the
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
form
voids,
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.
Gold-Silver
System
The gold-silver or
Au-Ag system is
another metallurgy
commonly seen
in semiconductor
applications. It is used primarily in the formation
of a wedge or fish-tail bond between
the
gold wire
and the
silver-plated lead finger
of
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.
Bondability problems
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
remove silver-sulfide
films, enhancing the bondability of the silver pad in the process.
Aluminum-Aluminum System
Aluminum-aluminum or
Al-Al systems
are used primarily in hermetic packaging, for bonding
aluminum wires
onto the
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.
Gold-Gold System
The gold-gold or
Au-Au system
is an
extremely reliable
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
contamination.
Gold-Copper
System
Gold-copper or
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., Cu 3Au, 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
reliable bonding.
<Proceed to Page 2 - Al-Ag,
Al-Ni, and CuAl Metallurgical Systems>
See
also:
Wirebonding;
Bond Lifting;
Package Failures
HOME
Copyright
©
2005.
SiliconFarEast.com.
All Rights Reserved. |
|
|
