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THE ADVANTAGES AND LIMITATIONS OF WIRE BONDING IN EV APPLICATIONS

Posted August 17, 2022 by Charged EVs & filed under Sponsored Content, The Tech.

Sponsored by Kulicke & Soffa

By: Peter Klaerner, senior manager for Wedge Bond Systems Engineering,
Kulicke & Soffa



Wire and ribbon bonding have traditionally been the methods of choice for
creating reliable interconnects in the electronics industry. The progress in
electric vehicle production has created the need for better, more reliable
interconnects that can be scaled to high-volume manufacturing. It is no surprise
therefore that leading automotive manufacturers in the electric vehicle space
have turned to wire bonding for their interconnect needs. Such interconnects
must be able to handle extreme temperature ranges in addition to shock and
vibration conditions over increasingly longer lifetimes. There are several areas
of application for interconnects in electric cars. Some of the applications
include: battery cell-to-cell connections; battery cell-to-busbar connections;
battery management system (BMS) interconnects; and power module interconnects in
the electric car drive train.

A leading electric manufacturer claims that their previous battery generation
was designed to last up to 500,000 miles of service and their current generation
of batteries is targeted to last up to 1,000,000 miles of service. While only
limited real-world data is available due to the somewhat nascent proliferation
of electric vehicles, long-term data available so far has shown that those
claims may be credible. In addition to the battery chemistry that holds the
electric charge, the interconnect elements between battery cells and sections
are components that see stress over the product lifetime. Between the multiple
interconnect methods being used in battery interconnects (see Figure 1), wire
bonding is the method with the most real-world data available demonstrating its
reliability.

Figure 1: Common Battery Cell Interconnects By Cell Type

What are the advantages of wire bonding making it a preferred method of
interconnect formation in the electric car industry?

Advantages

Quasi room-temperature process

The wedge wire bonding processes do not require external thermal energy. In
contrast to other methods like resistance welding, soldering or laser welding,
the substrate to be bonded does not need to be heated nor does the wedge bond
process induce heating. During the wedge bonding process, very localized heating
occurs only for a very short time due to the micro-friction occurring between
the bond interfaces. This is a great advantage when forming interconnects in
heat sensitive battery cells or integrated circuits.

Flexibility

Wire interconnects can span considerable distances and height differences
between interconnect points. Product iterations leading to changes in both
distances or heights can be easily accommodated with a simple software
adjustment – no new costly hardware has to be manufactured. This flexibility
also enables different product configurations to be processed on the same bonder
equipment requiring only a product-specific process program to be loaded.

Ambient and product-related thermal stresses can be accommodated with stress
relief loops between the interconnect points. Wire interconnect processes have
been developed between 18-micron wire diameter on the low side, and up to
600-micron wire diameter on the high side. Suitable wire sizes can be chosen to
match resistance and conductivity requirements. For fine-tuning of exact
resistivity or impedance, as is sometimes required in electronics processes, the
loop shape can be dynamically adapted to changes in bond locations for desired
resistivity or impedance matching. Ribbon bonding processes – typically used to
enable higher current transfer due to their higher cross sections – have also
been developed to cover a large range of sizes.

This flexibility enables cost savings during the pre-production phase, and also
during the production phase.

In-process monitoring

During the wire bond process, a number of electrical and mechanical signals are
being captured in real-time. While the bond process is taking place, this allows
for an immediate evaluation of the bond conditions and a judgment of bond
quality. With modern data analysis tools, it is possible to classify and match
signatures of common failure modes such as contamination or misplaced bonds and
store questionable bonds for later post-bond review. Connecting and storing the
bond data with product serial codes it is possible to have full traceability
back to each individual bond.

Mechanical process assurance

Wire bonding offers the ability to perform a mechanical pull test in-situ. In
this non-destructive test, a pre-defined pull force is applied to test the bond
strength after each bond is completed. Pull testing detects infant mortality
failures of weak or non-stick bonds. Similar to the bond process data, pull test
results are immediately available and can be stored for full traceability.

Ability to re-work

Wire bonding offers the ability to remove weak or failed wires on an individual
wire level and attempt rework. The ability to rework offers a unique advantage
compared to other interconnect methods which only have a singular chance of
success.

Proven technology & reliability

Wire bonding as an electrical interconnect technology has been used since at
least the 1960s and has continuously been improved. As explained earlier in this
article, reliability in electronic components has been demonstrated, and more
recent evidence confirms that wire bonding is suitable also for the extended
shock and vibration demands placed on it for battery cell applicationsiii.

Established supply chain for both equipment and consumables

For large scale automotive production, a stable supply chain for both equipment
and also consumables (such as bonding wire) is available with a global reach.

Fusing

As explained earlier, in a typical wire bond application the interconnect is
designed to endure harsh environmental conditions. There is however a use-case
in which there is a designed and desired failure point of the wire bond: when an
un-intended overcurrent condition occurs. U.S. Patent No. 7,671,565, assigned to
Tesla, Inc., described it as follows: “A system and method links batteries in
parallel to conductors using wire bonds that act as fuses in the event of an
overcurrent condition in a battery.” The wire bond can be specifically tuned to
the required fusing characteristics by selecting a suitable wire size and loop
length – both of which determine the resistivity of the interconnect.



Limitations

Of course, every process has some limitations. For wire bonding processes,
exemplary limitations include:

Serial process of interconnection

Wire bonding is serial in nature in that each bond has to be formed
sequentially. The only method to parallelize work involves multiple bond heads
working on the same product at the same time. This adds complexity to the
equipment and often limits flexibility. If a product is very repeatable in
dimensions a multi-bond head approach can be executed. Such equipment has been
used, for example, in solar cell interconnects where the bond locations are
identical for each workpiece.

Contamination

Contamination at either bonding interface can greatly affect the bond integrity.
This applies to organic materials such as hydrocarbons and non-organic materials
such as oxidation and dust. Mild contamination may be addressed with selective
bond parameters, while severe contamination typically requires a cleaning
process. Particularly in the prototyping stage of battery development packs, it
is not uncommon to deal with epoxy and other polymer residue caused for example
by 3D printing during the wire bonding stage. While this contamination is caused
by outside factors it can greatly influence the successful outcome of the wire
bond process.

Requirement for a stable substrate surface

During the wire bonding process, ultrasonic energy is applied under pressure.
The ultrasonic energy can cause harmonics that create resonance with the
substrate surface. Excessive vibration of the substrate surface can negatively
interact with the bond formation leading to weak or no-stick bonds. Therefore, a
product design guideline used during the development process may include
providing for a stable bond surface to ensure the highest success for the wire
bond process. Particularly, battery cells tend to have a weak spot regarding
vibration with the top cover of the battery cell having only support on its
sides but none in the center (Figure 2).

Figure 2: Sketch Of Cross-Section Showing Unsupported Center Of Battery Cell Top
Cover

Requirements for accurate placement location

While requirements for accurate placement location exist for all manufacturing
processes, there are some unique challenges in the realm of battery cell
interconnects. Such applications typically require that each cell be located at
a known location so the bonder can accurately place each bond. Usually, this
task is aided by using machine vision to locate reference marks on the product
while each battery cell is located relative to that reference mark. It can be
appreciated that as product dimensions grow there are stack-up tolerances during
the battery module assembly process that negatively affect cell location
accuracy. While a more precise assembly process can improve cell location
accuracy it comes with the drawback of significantly higher product cost which
affects each module. Another approach is to introduce more reference marks
throughout the product to lessen the effect of stack-up tolerances across a
large area by sub-dividing the area into small sections. Locating additional
reference marks, however, requires additional find time prior to the wire
bonding process, thereby increasing the overall cycle time. A brute force
approach would be to use machine vision to locate each individual cell prior to
bonding. This cell locating operation can be performed within the bonder but
consumes an inordinate amount of time just for locating each cell. Another
approach would be to locate each cell before the product enters the bonder with
the drawback of having to transfer and synchronize a large amount of data
between various systems. In summary, the solution to accurate cell placement
comes at a significant cost and technical complexity.

Suitable metallurgy combinations

Wire bonding performs best with a number of suitable metallurgy combinations.
Much research has been done to explore the bondability of various material
combinations and a number of well-performing combinations have been identified.
Typically mono-metallic bonds form the best connections such as
aluminum-to-aluminum or copper-to-copper. While dissimilar metal combinations
can be used the exact combination has to be evaluated as to bondability. It is
advantageous if battery module and cell designers take the wire bonding process
into consideration during the design cycle to achieve optimum production
robustness.  In cases where certain metallization is required that by itself
wouldn’t lend itself well to bonding,  coatings can be explored since wire
bonding also works well on certain coated surfaces. This article only gives a
very high level overview – Kulicke & Soffa will be happy to evaluate your
specific application as to bondability.

Spreading beyond automotive

The success of wire bonding demonstrated in automotive battery applications has
begun to spread to adjacent applications. Given the advantages wire bonding
offers it is no wonder that wire bonding is now being utilized in applications
such as energy storage, electric bikes, and motorcycles.

Kulicke & Soffa continues to provide innovative solution to create future
possibilities.

Learn more of K&S’ wedge bond solutions:
https://www.kns.com/Products/Equipment/Wedge-Bonder



Sponsored by Kulicke & Soffa






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