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By: Peter Klaerner, senior supervisor for Wedge Bond Methods Engineering,
Kulicke & Soffa
Wire and ribbon bonding have historically been the strategies of selection for creating dependable interconnects within the electronics business. The progress in electrical automobile manufacturing has created the necessity for higher, extra dependable interconnects that may be scaled to high-volume manufacturing. It’s no shock due to this fact that main automotive producers within the electrical automobile house have turned to wire bonding for his or her interconnect wants. Such interconnects should have the ability to deal with excessive temperature ranges along with shock and vibration circumstances over more and more longer lifetimes. There are a number of areas of utility for interconnects in electrical automobiles. Among the purposes embody: battery cell-to-cell connections; battery cell-to-busbar connections; battery administration system (BMS) interconnects; and energy module interconnects within the electrical automobile drive practice.
A number one electrical producer claims that their earlier battery era was designed to last as long as 500,000 miles of service and their present era of batteries is focused to last as long as 1,000,000 miles of service. Whereas solely restricted real-world knowledge is on the market because of the considerably nascent proliferation of electrical autos, long-term knowledge accessible up to now has proven that these claims could also be credible. Along with the battery chemistry that holds the electrical cost, the interconnect components between battery cells and sections are elements that see stress over the product lifetime. Between the a number of interconnect strategies being utilized in battery interconnects (see Determine 1), wire bonding is the strategy with probably the most real-world knowledge accessible demonstrating its reliability.
What are the benefits of wire bonding making it a most popular technique of interconnect formation within the electrical automobile business?
Benefits
Quasi room-temperature course of
The wedge wire bonding processes don’t require exterior thermal power. In distinction to different strategies like resistance welding, soldering or laser welding, the substrate to be bonded doesn’t must be heated nor does the wedge bond course of induce heating. In the course of the wedge bonding course of, very localized heating happens just for a really brief time because of the micro-friction occurring between the bond interfaces. It is a nice benefit when forming interconnects in warmth delicate battery cells or built-in circuits.
Flexibility
Wire interconnects can span appreciable distances and peak variations between interconnect factors. Product iterations resulting in adjustments in each distances or heights may be simply accommodated with a easy software program adjustment – no new expensive {hardware} must be manufactured. This flexibility additionally allows totally different product configurations to be processed on the identical bonder gear requiring solely a product-specific course of program to be loaded.
Ambient and product-related thermal stresses may be accommodated with stress reduction loops between the interconnect factors. Wire interconnect processes have been developed between 18-micron wire diameter on the low facet, and as much as 600-micron wire diameter on the excessive facet. Appropriate wire sizes may be chosen to match resistance and conductivity necessities. For fine-tuning of tangible resistivity or impedance, as is typically required in electronics processes, the loop form may be dynamically tailored to adjustments in bond places for desired resistivity or impedance matching. Ribbon bonding processes – sometimes used to allow greater present switch attributable to their greater cross sections – have additionally been developed to cowl a wide range of sizes.
This flexibility allows price financial savings through the pre-production part, and likewise through the manufacturing part.
In-process monitoring
In the course of the wire bond course of, quite a few electrical and mechanical indicators are being captured in real-time. Whereas the bond course of is going down, this permits for a direct analysis of the bond circumstances and a judgment of bond high quality. With fashionable knowledge evaluation instruments, it’s attainable to categorise and match signatures of frequent failure modes similar to contamination or misplaced bonds and retailer questionable bonds for later post-bond assessment. Connecting and storing the bond knowledge with product serial codes it’s attainable to have full traceability again to every particular person bond.
Mechanical course of assurance
Wire bonding gives the power to carry out a mechanical pull take a look at in-situ. On this non-destructive take a look at, a pre-defined pull pressure is utilized to check the bond power after every bond is accomplished. Pull testing detects toddler mortality failures of weak or non-stick bonds. Just like the bond course of knowledge, pull take a look at outcomes are instantly accessible and may be saved for full traceability.
Skill to re-work
Wire bonding gives the power to take away weak or failed wires on a person wire degree and try rework. The flexibility to transform gives a novel benefit in comparison with different interconnect strategies which solely have a singular probability of success.
Confirmed know-how & reliability
Wire bonding as {an electrical} interconnect know-how has been used since no less than the Nineteen Sixties and has constantly been improved. As defined earlier on this article, reliability in digital elements has been demonstrated, and more moderen proof confirms that wire bonding is appropriate additionally for the prolonged shock and vibration calls for positioned on it for battery cell purposesiii.
Established provide chain for each gear and consumables
For giant scale automotive manufacturing, a secure provide chain for each gear and likewise consumables (similar to bonding wire) is on the market with a worldwide attain.
Fusing
As defined earlier, in a typical wire bond utility the interconnect is designed to endure harsh environmental circumstances. There may be nevertheless a use-case in which there’s a designed and desired failure level of the wire bond: when an un-intended overcurrent situation happens. U.S. Patent No. 7,671,565, assigned to Tesla, Inc., described it as follows: “A system and technique hyperlinks batteries in parallel to conductors utilizing wire bonds that act as fuses within the occasion of an overcurrent situation in a battery.” The wire bond may be particularly tuned to the required fusing traits by deciding on an acceptable wire dimension and loop size – each of which decide the resistivity of the interconnect.
Limitations
After all, each course of has some limitations. For wire bonding processes, exemplary limitations embody:
Serial means of interconnection
Wire bonding is serial in nature in that every bond must be shaped sequentially. The one technique to parallelize work entails a number of bond heads engaged on the identical product on the similar time. This provides complexity to the gear and sometimes limits flexibility. If a product could be very repeatable in dimensions a multi-bond head strategy may be executed. Such gear has been used, for instance, in photo voltaic cell interconnects the place the bond places are equivalent for every workpiece.
Contamination
Contamination at both bonding interface can significantly have an effect on the bond integrity. This is applicable to natural supplies similar to hydrocarbons and non-organic supplies similar to oxidation and mud. Delicate contamination could also be addressed with selective bond parameters, whereas extreme contamination sometimes requires a cleansing course of. Notably within the prototyping stage of battery growth packs, it’s not unusual to take care of epoxy and different polymer residue brought on for instance by 3D printing through the wire bonding stage. Whereas this contamination is attributable to outdoors elements it will possibly significantly affect the profitable end result of the wire bond course of.
Requirement for a secure substrate floor
In the course of the wire bonding course of, ultrasonic power is utilized below strain. The ultrasonic power may cause harmonics that create resonance with the substrate floor. Extreme vibration of the substrate floor can negatively work together with the bond formation resulting in weak or no-stick bonds. Due to this fact, a product design guideline used through the growth course of could embody offering for a secure bond floor to make sure the best success for the wire bond course of. Notably, battery cells are inclined to have a weak spot concerning vibration with the highest cowl of the battery cell having solely assist on its sides however none within the middle (Determine 2).
Necessities for correct placement location
Whereas necessities for correct placement location exist for all manufacturing processes, there are some distinctive challenges within the realm of battery cell interconnects. Such purposes sometimes require that every cell be situated at a recognized location so the bonder can precisely place every bond. Normally, this job is aided through the use of machine imaginative and prescient to find reference marks on the product whereas every battery cell is situated relative to that reference mark. It may be appreciated that as product dimensions develop there are stack-up tolerances through the battery module meeting course of that negatively have an effect on cell location accuracy. Whereas a extra exact meeting course of can enhance cell location accuracy it comes with the disadvantage of considerably greater product price which impacts every module. One other strategy is to introduce extra reference marks all through the product to reduce the impact of stack-up tolerances throughout a big space by sub-dividing the realm into small sections. Finding further reference marks, nevertheless, requires further discover time previous to the wire bonding course of, thereby rising the general cycle time. A brute pressure strategy can be to make use of machine imaginative and prescient to find every particular person cell previous to bonding. This cell finding operation may be carried out throughout the bonder however consumes an inordinate period of time only for finding every cell. One other strategy can be to find every cell earlier than the product enters the bonder with the disadvantage of getting to switch and synchronize a considerable amount of knowledge between numerous methods. In abstract, the answer to correct cell placement comes at a major price and technical complexity.
Appropriate metallurgy mixtures
Wire bonding performs greatest with quite a few appropriate metallurgy mixtures. A lot analysis has been finished to discover the bondability of assorted materials mixtures and quite a few well-performing mixtures have been recognized. Usually mono-metallic bonds type one of the best connections similar to aluminum-to-aluminum or copper-to-copper. Whereas dissimilar steel mixtures can be utilized the precise mixture must be evaluated as to bondability. It’s advantageous if battery module and cell designers take the wire bonding course of into consideration through the design cycle to realize optimum manufacturing robustness. In circumstances the place sure metallization is required that by itself wouldn’t lend itself properly to bonding, coatings may be explored since wire bonding additionally works properly on sure coated surfaces. This text solely offers a really excessive degree overview – Kulicke & Soffa shall be completely satisfied to judge your particular utility as to bondability.
Spreading past automotive
The success of wire bonding demonstrated in automotive battery purposes has begun to unfold to adjoining purposes. Given the benefits wire bonding gives it’s no marvel that wire bonding is now being utilized in purposes similar to power storage, electrical bikes, and bikes.
Kulicke & Soffa continues to offer revolutionary answer to create future prospects.
Be taught extra of Ok&S’ wedge bond options:
https://www.kns.com/Merchandise/Gear/Wedge-Bonder
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