www.industrial-lasers.com SEPTEMBER/OCTOBER 2017 Industrial Laser Solutions 13
the cells well enough to sustain the higher
ultrasonic energy input.
Laser welding is an alternative. In several regards, it is almost a perfect com-
plement to wire bonding, as it has no
trouble with lower-quality surfaces and
it does not need substantial forces to
clamp the parts during the process.
Also, the connector elements can have
large cross-sections to conduct very
large currents. Finally, the requirements
on the bulk properties of the connectors
are lower, leading to lower material cost.
As with wire bonding, there are also a few disadvantages—
notably the more difficult automatization compared to wire
bonding. In wire bonding, the connector (wire or ribbon) is
reeled from a spool in a virtually endless manner; is handled
solely by the bonder; can be attached in any position and direction (also with tolerance compensation by pattern recognition
systems); and can be cut at any length as needed.
Laser welding is more complex—a pre-fabricated connecting element must be positioned on the contact points and held
there for welding. Therefore, the connector must be handled
as a separate part and needs to be precisely positioned before
the laser beam can start working. Furthermore, the connector must be pre-fabricated to the right dimensions, usually as
a punched part. Tolerances and height variations in the contact positions cause extra work during process automation.
Combining laser welding and wire bonding offers the best of
both worlds, and it has now been accomplished within a joint
research project supported by the German Federal Ministry
for Education and Research (BMBF) called RoBE (Robustness
for bonds in e-vehicles). Besides F&K Delvotec Bondtechnik
(Munich, Germany) and the Fraunhofer Institute for Laser
Technology (ILT; Aachen, Germany), the consortium included
Audi, Continental, Infineon, and the Fraunhofer Institute for
Reliability and Microintegration (IZM; Berlin, Germany).
Developed within the RoBE project is the LaserBonder sys-
tem, which consists of two main components: a standard wire
bonder base from F&K Delvotec and a near-infrared fiber laser
with 1k W power in a separate unit (FIGURE 1). The laser beam
is brought into the bond head through an optical fiber, and
ends up in a galvanometer scanner that
contains optics generating a very sharp
focus of about 35µm in diameter. The
optics move the focused laser beam on the bond ribbon, just
in front of the bond tool.
This bond tool is a modified type of bond wedge from ultrasonic bonding. Its main purpose is to hold down the bond ribbon properly on the contact pad. The rest of the bond head is
unchanged from the ultrasonic bonder, including the wire or
ribbon guide and the cutting unit, which indents the ribbon after
the second weld step prior to tearing it off (FIGURE 2).
Aluminum and copper connectors
Ribbon-shaped connectors are preferred over wires for laser
bonding. Thanks to their rectangular cross-section, they feature
a constant profile and, therefore, constant thickness of metal.
Aluminum is easily processed, as is copper. The latter is
preferred on battery cells because of its higher conductivity.
The bonder setup used is still largely restricted to the ribbon
dimensions also used in the ultrasonic bonder—approximately
2mm wide and up to 0.3mm thick. Larger ribbons don’t present a challenge for laser welding, but they do require a differ-
ent layout for feeding and cutting, both of which are under
In general, copper is considered much harder to process
by laser welding than aluminum because at 1µm wavelength, it has poor absorption. One alternative is a green
laser, which has much better absorption, but a much higher
At Fraunhofer ILT, a technology was used that takes advantage of a very small focus spot. The extremely high energy
density at this local spot permits deep-penetration welding
instead of heat conduction welding, generating very deep
and steep welding zones by internal multiple reflections. It
is also termed keyhole welding because of this behavior.
FIGURE 2. The LaserBonder’s bonding
head with its laser scanner.