Page 89 - Wire Bonding in Microelectronics

68 Cha pte r T h ree

and are frequently associated with 25 µm or less diameter wire that is
plastic encapsulated. Cu wire can crack near the stitch heel and at the
ball neck during temperature cycling.∗ Therefore, there are still stud-
ies going on to implement 25 µm or smaller diameter wire. Addi-
tional industry comments indicate that other problems may be
equally important to metallurgical ones. Examples are that to opti-
mize for wire bonding, it is necessary to harden the bond pads, and
this presents manufacturing problems for companies that have their
chips bonded in foundries. Tools wear faster, and threading at finer
pitch (≥40 µm) is slower. Complex metallurgy pad areas (i.e., Cu/Lo-k)
are easily damaged. [Currently there is some production using fine
(~25 mm) Cu wire in plastic encapsulation for low-end amusement devices,
where only limited temp cycling is encountered.] Thus, even with all the
listed problems Cu wires cost, advantages are so great over Au that,
in time, the remaining problems should be solved.
A concise summary of Cu wire, advantages and problems, was
given in a technical bulletin published by Gaiser Tools. It is repro-
duced in App. 3B.

Insulated Bonding Wire
The concept of insulating bonding wire to prevent shorts between
adjacent wires has been around for many years [3-21, 3-23]. One such
coating system was applied to Al wire by heavily anodizing it. This
was effective for wedge bonding, but without the need for fine pitch,
it offered few advantages. Other coatings were tried later, but there
was little demand for such products in the course-pitch wire bonding
era, and they were not implemented. There have been many patents
for such technology. Recently, [3-24] developed a coating compatible
with Au ball bonding. This is currently available from at least one
wire supplier. As such, with very fine pitch (to 20 µm) advancing rap-
idly, it is possible that such will be used significantly in the future.

3.10 Conductor Burn Out (Fusing)

3.10.1 Bonding Wires
The current-carrying capacity of interconnection wires in semicon-
ductor devices is an important packaging design parameter, and
many papers have been written on the subject [3-25 to 3-35]. Wire
burn out (fusing) is a complex subject that is influenced by the metal-
lurgy and length of the wire, the ambient gas, or (if used) the plastic
encapsulant, as well as the duty cycle of the current, the type of bonds
(ball or wedge), and the degree of heat-sinking out through the bonds
to the chip or package. Several other factors influencing wire burn

∗Summarized from comments made by Cesar Chavez, private communication.