Wire Bonding Loop Profiles


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 standards. 


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 'looping', 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 'wire loop'.  The wire loop is characterized by its shape, length, and height, all of which define what is known as the wire's 'loop profile'.


The loop 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 wires.


Not desirable 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.


Low-loop 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 reliable.


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.


One concern that comes with low loop profiles in ball bonds is the wire's heat-affected zone, 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.


See also:   Lead FinishPb-free Manufacturing




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