12 Industrial Laser Solutions SEPTEMBER/OCTOBER 2017 www.industrial-lasers.com
PROCESS ELIMINATES NEED FOR
EXTENSIVE SURFACE PREPARATION
BENJAMIN MEHLMANN and JOSEF SEDLMAIR
Power electronics constantly pres- ent packaging technologies with new challenges to handle ever- larger currents. At the semiconduc- tor level, the most important inter- connection technology by far is wire bonding with aluminum-heavy wires
in diameters up to 500µm, welded directly onto a bond pad of
the semiconductor chip by an ultrasonic friction welding process. These wire bonds have a fuse current of up to 35A for
a wire measuring 20mm in average length. For considerably
higher currents found in today’s power modules, several wires
are bonded in parallel on the chip.
Wire bonding features enjoy major advantages: the bonds are
highly reliable and have long lives; the process is very flexible
because distances and positions of the bonds can be freely programmed and adapted; and it is a very cost-efficient process.
However, two drawbacks to wire bonding are that the bonding surfaces must be very clean, and the bond pads on chips
or substrates must be held rigidly and stably. The thicker the
bonding wire, the more ultrasonic energy must be employed,
making it progressively more difficult to clamp the parts rig-
idly. The bond forces also must increase with thicker wires.
Taken together, these factors set a practical limit to about
500µm-diameter wire, as thicker wires pose a danger to sensitive chip surfaces.
Nonetheless, larger connector cross-sections have been
processed for a few years now using ultrasonic wire bonding.
These are usually rectangular ribbons measuring 2mm wide
and 0.2mm thick, replacing three 500µm-diameter wires. The
main area of application is automotive electronics, especially
A further application has developed in the field of e-mobility
for storage batteries. Depending on their design, they contain
battery cells wired together by a
variety of interconnection technol-
ogies. At present, the dominating
battery type is the 18650 lithium-ion
(Li-ion) consumer battery, which is
especially well suited for wire bond-
ing with heavy aluminum wire. Here,
the advantages of the continuous bulk joint regarding life-
time and stability combine with excellent ease of automati-
zation. This is the reason why wire bonding is by far the lead-
ing interconnection process for battery cells, for example, at
Even here, ultrasonic wire bonding is running into limitations
of manageable currents. 18650 Li-ion-type consumer battery
cells are operated with cells below about 20A, allowing a single wire to connect a cell to, for instance, a busbar. To connect
several cells directly with a bond wire, however, the maximum
current carrying capability is soon reached. Moreover, newer
cells at the same approximate size are designed for currents
up to 45A and would require several wires in parallel.
The alternative, using the aluminum ribbon mentioned earlier, is not really feasible: on the one hand, these ribbons are
still quite expensive, and on the other it is difficult to clamp
Laser bonding: A new
technology for high currents
FIGURE 1. The
a wire-bonder base
(left) and a standard
fiber laser with a
control unit (right).