Tensile strength (N)
0.5 0.4 0.3 0.2
Material: CuFe2P (0.2/0.4mm)
14 Industrial Laser Solutions SEPTEMBER/OCTOBER 2017 www.industrial-lasers.com
FIGURE 5. Wide welding seams at
low welding penetration depth are
shown with no oscillation (a), at 0.2mm
amplitude (b), and at 0.3mm amplitude (c).
FIGURE 4. Seam strength and oscillation
amplitude are compared.
The side effect of a small connection
area would normally be a narrow weld-
ing seam and, therefore, an undesirably
high transition resistance. However, this
is easily avoided by “wobbling” the laser
beam, superimposed on the linear motion
across the ribbon.
advantage is that
the heat emerging
from the laser spot
is not just dissipated
on both sides of the
welding seam, but
is used efficiently to
melt the area inside
the circular movement (FIGURE 3).
The result is a weld
seam in which welding depth and seam
width can be controlled independently,
and that delivers consistently shallow
welding depths into the lower contact pad.
High current capacity/
low weld penetration
FIGURES 4 and 5 demonstrate how the
mechanical strength of the weld seam and
its width can be controlled by the advance
speed and oscillation amplitude of the
laser beam. Under optimized conditions,
they can be more than doubled without
increasing the process time and, there-
fore, the energy input. This is also attractive
because the metallurgical properties of the
welding zone depend strongly on the thermal load to which it is exposed.
Laser bonding allows creating a con-
necting interface of practically any size
(and correspondingly high current-car-rying capacity) while keeping the weld
penetration depth low. This approach is
extremely attractive for battery cells. In
small 18650 Li-ion battery cells, the plus
pole is a hat-shaped, punched sheet
metal disk that is contacted in the cen-
ter by a copper ribbon without large
forces (FIGURE 6).
The second bond to the minus pole, the
battery can, is even more advantageous.
The container is made of thin sheet metal
measuring 250 to 300µm, which carries
considerable risk of excessive heat input
if it is penetrated by the welding step. Any
uncontrolled heat input would risk degrading the battery lifetime.
This is an aspect where laser bonding
is highly advantageous, as it can make
the connection on top of the rolled rim of
the cell, right next to the plus pole. Having
both connections on the top side of the cell
leaves the entire remainder of the cell free
of connecting leads, enabling simpler manufacturing because there is no need to flip
the battery assembly, and far more space
available for thermal management of the
cell to help increase cell lifetime.
Different approach for
Larger prismatic battery cells where much
larger currents are employed and where
there are dedicated connector blocks on
the battery cell require even larger ribbons.
First steps are under development for rib-
bons up to 10mm wide and 1mm thick
within the framework of FlexJoin, a joint
research project funded by the Federal
Ministry of Economy and Energy (BMWi).
An alternative to this is using pre-
punched connector elements as in con-
ventional laser welding. However, in the
LaserBonder, only a different bond tool
is needed and the bond ribbon is simply
left out. The main advantage of the equip-
ment is that it can push the connector ele-
ment down to the contact block, ensuring
a zero welding gap and making the clamp-
ing setup much simpler.
Because of the programmable laser
beam pattern, a welding interface of any
shape within a window about 10mm in
diameter can be formed, allowing large
operating currents. The downward push
force and required distance can be controlled and monitored, allowing additional
process and parts control. For some
application areas, it may also be attrac-
tive to have equipment that is capable
of both processes with extremely sim-
There are several further potential appli-
cations beginning to take shape. The
lower demands of laser bonding regard-
ing surface quality make it possible
to bond aluminum ribbons directly to
FIGURE 3. A schematic of the deep-penetration welding process.