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.
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.
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 predefined 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. More recent evidence confirms that wire bonding is suitable also for the extended shock and vibration demands placed on it for battery cell applications.
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 work piece.
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.
Suitable metallurgy combinations
Wire bonding performs best with a number of suitable metallurgy combinations. Much research has been done to explore the bond ability 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 bond ability. 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.