Page 60 - Wire Bonding in Microelectronics

Ultrasonic Bonding Systems and Technologies 39

ribbon wires. One advantage of ribbon over round wire is that its high-
frequency impedance is lower, depending on the width-to-thickness
ratio which reduces both the inductance and the skin-effect losses. As
such, it is often used in microwave devices and hybrids with w/t ratios
of 5 to 10, or even higher. The inductance, L, in nH, of a straight ribbon
wire is given by Eq. (2-1) [2-64].

L = 2 × 10 (1n[2 /(t + w)] + 0.5 + 0.2235 × [t + w]/ ) (2-1)
−4
where t is the ribbon thickness, w is its width, and is the length of
the wire, all in micrometers.
The skin effect will decrease the inductance (in the range of 2–6%)
at high frequencies but can increase the impedance considerably (see
Chap. 9). The high-frequency losses of ribbons with large w/t ratios
can be much lower than that of TAB leads (which are almost square)
as well as round wire.
There are two correctable problems encountered when bonding
ribbon wires. As the w/t ratio is increased (>5), the tool and substrate
must be maintained extremely parallel (within less than one degree),
or one side of the ribbon will be poorly welded. (To avoid this for
very wide ribbons, bonding is often done with a small tool and using
multiple welds across the width.) Also, there is seldom a very large
deformation of the ribbon during bonding, so there is little surface
cleaning of oxides and contaminants from the interface. Thus, to
obtain good welding, the bond pads should be plasma or UV-ozone
cleaned shortly before bonding (see Chap.7). The above reasons imply
that it is extremely important to use good bondable metal on the pads
and to carefully optimize the bonding-machine parameters. If ther-
mosonic bonding Au ribbon/wire, then using the highest practical
interface tem perature is helpful. Currently, there are several suppliers
of ribbon wires.
Recently, the use of large Al ribbon [e.g., 80 × 10 mils (2 × 0.25 mm)]
for high power devices has increased and autobonders have been
made to increase throughput. Also special textured bonding tool shapes
have been developed that help deform the wire and clean the surfaces
during bonding increasing the yield. Ribbon reduces the number of
individual interconnections, reducing the wire bonding time required
for high power chips and also facilitates spreading the current across
a large die surface/metallization (see Fig. 2-18). These large ribbons
need special techniques for pull-testing and otherwise evaluate their
strength. See Ref. [2-65].

2.7.2 Parallel Gap and Tweezer Welding
Parallel-gap electrode welding (PGW) (sometimes called split elec-
trode welding) is often used for resistive and harder metal wires, both
round and ribbon, as well as for special metallizations. As an example,