Wirebond
Metallurgical Systems - Page 2 of 2
<Back to Page 1 - Au-Al, Au-Ag,
Al-Al, Au-Au and Au-Cu Metallurgical Systems>
Al-Ag
System
The aluminum-silver or
Al-Ag system
is commonly used in thick-film hybrid technologies, to bond an aluminum
wire onto a
thick-film silver
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
interdiffusion
between Al and Ag. They also tend to
oxidize
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
corrosion
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.
Kirkendall
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.
Al-Ni
System
Aluminum-nickel or
Al-Ni systems
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
metallurgical system
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
oxidize
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
to
sandblasting
just prior to bonding. Some substrate manufacturers deposit a very thin
layer of gold over the nickel surface to prevent it from oxidizing.
Cu-Al
and Cu-Au Systems
Copper-aluminum or
Cu-Al systems
and copper-gold or
Cu-Au systems
generally pertain to the bonding of
copper wires
onto
aluminum or gold bonding pads or substrates.
As discussed on
Page 1,
the Cu-Au
system is quite vulnerable to void formation at high temperatures, and
requires bonding surface cleanliness to ensure bonding reliability.
In a
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
brittle CuAl
2
phase. Excessive growth of this brittle
intermetallic compound,
which is accelerated by temperature,
lowers the over-all shear strength of the bond. Cu-Al systems are also
vulnerable to
corrosion
in the presence of chlorine and moisture.
<Back to Page 1 - Au-Al, Au-Ag,
Al-Al, Au-Au and Au-Cu Metallurgical Systems>
See
also:
Wirebonding;
Bond Lifting;
Package Failures
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