The benefits and limitations of wire bonding in electrical car functions

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Sponsored by Kulicke & Soffa

By: Peter Klaerner, senior supervisor for Wedge Bond Programs Engineering, Kulicke & Soffa

Wire and ribbon bonding have historically been the strategies of alternative for creating dependable interconnects within the electronics business. The progress in electrical car 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 car 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 software for interconnects in electrical automobiles. A few of the functions embrace: 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 obtainable because of the considerably nascent proliferation of electrical autos, long-term knowledge obtainable to this point has proven that these claims could also be credible. Along with the battery chemistry that holds the electrical cost, the interconnect parts between battery cells and sections are parts 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 tactic with probably the most real-world knowledge obtainable demonstrating its reliability.

Determine 1: Widespread battery cell interconnects by cell sort

What are some great benefits of wire bonding making it a most well-liked methodology of interconnect formation within the electrical automobile business?

Benefits

Quasi room-temperature course of

The wedge wire bonding processes don’t require exterior thermal vitality. In distinction to different strategies like resistance welding, soldering or laser welding, the substrate to be bonded doesn’t want to 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 quick time due to the micro-friction occurring between the bond interfaces. This can be 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 might be simply accommodated with a easy software program adjustment – no new expensive has to be manufactured. This flexibility additionally permits completely different product configurations to be processed on the identical bonder gear requiring solely a product-specific course of program to be loaded.

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Ambient and product-related thermal stresses might be accommodated with stress aid loops between the interconnect factors. Wire interconnect processes have been developed between 18-micron wire diameter on the low aspect, and as much as 600-micron wire diameter on the excessive aspect. Appropriate wire sizes might be chosen to match resistance and conductivity necessities. For fine-tuning of actual resistivity or impedance, as is typically required in electronics processes, the loop form might be dynamically tailored to adjustments in bond areas 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 variety of sizes.

This flexibility permits 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, a lot of electrical and mechanical alerts are being captured in real-time. Whereas the bond course of is going down, this permits for a right away 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 widespread failure modes equivalent to contamination or misplaced bonds and retailer questionable bonds for later post-bond overview. 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 affords 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 power is utilized to check the bond energy after every bond is accomplished. Pull testing detects toddler mortality failures of weak or non-stick bonds. Related to the bond course of knowledge, pull take a look at outcomes are instantly obtainable and might be saved for full traceability.

Skill to re-work

Wire bonding affords the power to take away weak or failed wires on a person wire stage and try rework. The capability to transform affords a novel benefit in comparison with different interconnect strategies which solely have a singular likelihood of success.

Confirmed know-how & reliability

Wire bonding as interconnect know-how has been used since no less than the 1960s and has constantly been improved. As defined earlier on this article, reliability in digital parts 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 functionsiii.

Established provide chain for each gear and consumables

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For big scale automotive manufacturing, a steady provide chain for each gear and likewise consumables (equivalent to bonding wire) is obtainable with a worldwide attain.

Fusing

As defined earlier, in a typical wire bond software the interconnect is designed to endure harsh environmental circumstances. There’s 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 methodology 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 might be particularly tuned to the required fusing traits by choosing an acceptable wire measurement and loop size – each of which decide the resistivity of the interconnect.

Limitations

In fact, each course of has some limitations. For wire bonding processes, exemplary limitations embrace:

Serial means of interconnection

Wire bonding is serial in nature in that every bond must be shaped sequentially. The one methodology to parallelize work includes a number of bond heads engaged on the identical product on the similar time. This provides complexity to the gear and infrequently limits flexibility. If a product could be very repeatable in dimensions a multi-bond head method might be executed. Such gear has been used, for instance, in photo voltaic cell interconnects the place the bond areas are similar for every workpiece.

Contamination

Contamination at both bonding interface can drastically have an effect on the bond integrity. This is applicable to natural supplies equivalent to hydrocarbons and non-organic supplies equivalent to oxidation and dirt. Delicate contamination could also be addressed with selective bond parameters, whereas extreme contamination sometimes requires a cleansing course of. Significantly within the prototyping stage of battery growth packs, it’s not unusual to take care of epoxy and different polymer residue induced for instance by 3D printing through the wire bonding stage. Whereas this contamination is brought on by exterior components it will probably drastically affect the profitable consequence of the wire bond course of.

Requirement for a steady substrate floor

In the course of the wire bonding course of, ultrasonic vitality is utilized below strain. The ultrasonic vitality could 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 embrace offering for a steady bond floor to make sure the very best success for the wire bond course of. Significantly, battery cells are likely 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 heart (Determine 2).

Determine 2: Sketch of cross-section displaying unsupported heart of battery cell high cowl

Necessities for correct placement location

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Whereas necessities for correct placement location exist for all manufacturing processes, there are some distinctive challenges within the realm of battery cell interconnects. Such functions sometimes require that every cell be positioned at a recognized location so the bonder can precisely place every bond. Often, this job is aided through the use of machine imaginative and prescient to find reference marks on the product whereas every battery cell is positioned 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 downside of considerably greater product price which impacts every module. One other method is to introduce extra reference marks all through the product to minimize the impact of stack-up tolerances throughout a big space by sub-dividing the realm into small sections. Finding extra reference marks, nevertheless, requires extra discover time previous to the wire bonding course of, thereby rising the general cycle time. A brute power method can be to make use of machine imaginative and prescient to find every particular person cell previous to bonding. This cell finding operation can be carried out throughout the bonder however consumes an inordinate period of time simply for finding every cell. One other method 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 varied methods. In abstract, the answer to correct cell placement comes at a big price and technical complexity.

Appropriate metallurgy combos

Wire bonding performs finest with a lot of appropriate metallurgy combos. A lot analysis has been achieved to discover the bondability of varied materials combos and a lot of well-performing combos have been recognized. Sometimes mono-metallic bonds type the perfect connections equivalent to aluminum-to-aluminum or copper-to-copper. Whereas dissimilar metallic combos 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 attain optimum manufacturing robustness.  In circumstances the place sure metallization is required that by itself wouldn’t lend itself nicely to bonding,  coatings might be explored since wire bonding additionally works nicely on sure coated surfaces. This article solely provides a really excessive stage overview – Kulicke & Soffa shall be completely satisfied to guage your particular software as to bondability.

Spreading past automotive

The success of wire bonding demonstrated in automotive battery functions has begun to unfold to adjoining functions. Given the benefits wire bonding affords it’s no marvel that wire bonding is now being utilized in functions equivalent to vitality storage, electrical bikes, and bikes.

Kulicke & Soffa continues to offer revolutionary answer to create future prospects.

Be taught extra of Okay&S’ wedge bond options:https://www.kns.com/Merchandise/Tools/Wedge-Bonder

Sponsored by Kulicke & Soffa


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