One of the
challenges in the assembly of modern semiconductor packages is the
formation of wire bond loops and shapes
that would satisfy all the complex interconnections required by the
device while maintaining the package dimensions within industry
Wirebonding, which is the packaging step that connects the die to the
package so it can be accessed by the outside world, consists of repeated
performance of a basic bonding cycle: a) formation of the first bond on
the die; b) pulling of the wire to the lead frame or bonding post of the
package where the second bond will be formed; c) formation of the second
bond; and d) cutting of the wire in preparation for the next cycle.
Step (b) above, which feeds and forms the wire that runs from the first
to the second bond, involves an action often referred to as
wherein the wire fed between the bonds takes the form of an arc.
The arc formed when the bonding tool traveled in a natural parabolic or
elliptical curve is the
The wire loop is characterized by its shape, length, and height, all of
which define what is known as the wire's
profile of a wire is a critical aspect of wirebonding because it affects
both the performance and reliability of the device. For instance,
the loop of a wire can not be too high, since excessively high loops can
result in exposed wires after molding. Even if the wires are not
exposed, high loops can also produce long and
sagging wires that are prone to being swept by the molding compound in
the direction of its flow during the encapsulation process, a phenomenon
known as 'wire sweeping.' Wire sweeping can result in
shorting between wires. Unnecessarily long wires also degrade the
electrical performance of the device because of cross-talk between the
too is a wire that is too low. Low-looped wires mean that they
have been pulled down too much by the bonding tool from the first bond
to the second bond. A low loop generally indicates that the wire
is too taut, such that enormous stress had been and is being exerted on
the neck or heel of the first bond. It is common to see neck or
heel cracks, or even breaks, in wires that were pulled down too low by
the looping action of the bonding tool.
wires, however, have become more important over recent years to
semiconductor packaging, mainly because it is a necessity for producing
thinner and more complex packages. The challenge to every packaging
engineer, therefore, is to be able to produce low-loop wires that are
Modern packages that need
low-loop wires include those that employ
die stacking, those that are very thin,
and those whose substrates have multi-tiered or multi-layered bonding
shelves. Fortunately, many modern wirebonding machines are now able to
form low-loop bonds that don't necessarily have to be a reliability
risk. The high programmability and precise mechanical actuations of
these modern wirebonders now provide semiconductor manufacturers with
the ability to 'shape' the profile of the wire.
As mentioned, naturally
curved loops that are too low can damage the neck of a ball bond or the
heel of a wedge bond. One way to prevent this and still get low
loops in ball bonds is to modify the loop shaping technique so that
reverse motion is exaggerated, creating a crimp in the wire just above
the neck of the ball bond. The crimp is already above the neck of
the bond, so the neck is no longer subjected to large mechanical stress.
The sharp change in wire direction provided by the crimp achieves the
required ultra-low profile. A downside of this technique is the
reduced wire pull strength of the wire. There are, of course, many other
wire looping techniques, each of which cater to one special package
requirement or another.
that comes with low loop profiles in ball bonds is the wire's
the section of the wire immediately above the ball bond that has been
made weaker by the melting of the wire during the formation of the
free-air ball. The looping of the wire must not start anywhere within
the heat-affected zone so as not to compromise the strength of the wire.
As such, only wires designed for low looping must be
used for low-loop applications.
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