Wirebonding Is Right here To Keep

Few applied sciences in semiconductor manufacturing have stood the check of time as steadfastly as wirebonding. This course of, which entails electrically connecting semiconductor gadgets to their packages, has been a cornerstone of the electronics trade for the reason that starting of the electronics trade.

Like every little thing else within the semiconductor market, wirebonding applied sciences have modified over time to maintain tempo with shrinking architectures, chip complexity, and growing density of interconnects. As transistors have shrunk and chips have turn into extra advanced, the necessity for finer pitch wire bonds, extra dependable connections, and supplies that may stand up to the pains of harsh environments has pushed innovation. New supplies, akin to copper, copper alloys, and silver have began to exchange conventional gold wires as a result of each value and efficiency advantages.

“The main advances in wirebonding in latest many years have been within the help of cell telephones and high-density electronics,” stated Dick Otte, CEO of Promex Industries. “And the foremost advances embody going to copper wire, which is a bit more conductive, however a giant financial savings over the price of the gold. That’s been crucial change that I see in wirebonding in recent times.”

What’s wirebonding?
Wirebonding is a generally used technique for creating interconnections between an IC on a silicon die and its packaging, during which a skinny wire is linked from a bond pad on the system to a corresponding pad (i.e. lead) on the bundle. This connection establishes {the electrical} path from the inner circuits of the chip to the exterior pins that hook up with a printed circuit board (PCB).

The method begins by positioning a wire beneath the top of a slender bonding instrument. A exact pressure is exerted, pushing the wire in opposition to the electrode’s floor, resulting in an preliminary deformation on the level of contact. This power is derived from an ultrasonic system that produces mechanical oscillations round 60 kHz. These oscillations are channeled by the bonding instrument to the fusion zone for a period of about 100 milliseconds.

The friction generated by the ultrasonic vibrations produces localized heating on the interface, which when mixed with the stress utilized by the bonding instrument, facilitates atomic diffusion throughout the interface. Because the wire and substrate supplies’ atoms come into shut proximity as a result of mechanical deformation and frictional results, they start to kind intermetallic compounds. These compounds are sometimes extra secure than the guardian supplies and guarantee a robust bond between the wire and the substrate.

There are two major kinds of wire bonds, ball bonds and wedge bonds. Ball bonds are probably the most prevalent, constituting about 90% of all bonds, and are sometimes fashioned utilizing a thermo-sonic bonding method for gold and copper wiring. Wedge bonds, which are sometimes created utilizing ultrasonic or thermocompression strategies, are extra frequent with aluminum alloy and gold wires and are continuously utilized in particular purposes.

Fig. 1: A comparability of three kinds of wirebonding applied sciences. Supply: NASA

Wirebonding runs out of steam at superior nodes and in advanced chips, nevertheless. Because the quantity and density of bond pads enhance, there’s restricted house to make sure wires don’t contact or trigger interference. That is the place superior packaging applied sciences, akin to thermo-compression (TCP) bonding and flip-chip packaging strategies, come into play. These strategies make the most of solder or copper bumps to create direct connections between the chip and the substrate, eliminating the necessity for lengthy wire interconnects. In consequence, they provide a extra compact, environment friendly, and high-performance answer for contemporary semiconductor gadgets, notably these in high-frequency or high-performance purposes.

“What’s actually driving the transition from wirebonding to flip chip are more and more high-speed necessities for the electronics,” says Jeff Schaefer, senior course of engineer at Promex Industries. “Wires create inductance, and inductance causes electrical parameter points for our high-speed world.”

Others agree. “As you go towards the a whole bunch, if not 1000’s of interconnects, then you definately’re going to start out taking a tough have a look at whether or not it’s nonetheless proper to go together with wirebonding versus shifting to flip-chip,” says Frank Chen, director of purposes and product administration at Bruker. “However usually, most firms will nonetheless use wire bond over flip chip until they’ve a excessive variety of I/Os that push them into an array format.“

The evolution of wirebonding
Whereas there have lengthy been predictions in regards to the demise of wirebonding in favor of superior packaging, wirebonding nonetheless is used to assemble greater than 80% of semiconductor packages right this moment.[1] Its confirmed reliability, cost-effectiveness, and well-established manufacturing processes for chips that don’t demand ultra-high densities of interconnects means it’s going to probably be round for many years to come back, as properly.

In keeping with a brand new report from Technavio, the wire bonder tools market is predicted to proceed rising at a CAGR of three.3% between 2022 and 2027, with the dimensions of the market forecast to extend by  $219.24 million over that interval. This development is being pushed by the growing electronics content material in automotive purposes and speedy development of the OSAT trade, notably in Asia-Pacific nations.

Gold (Au) bonding wire was the trade normal for many years, and for good motive. Its unparalleled chemical stability, corrosion resistance, and dependability made it the logical materials for semiconductor packaging. That started to vary as gold costs elevated 478% within the decade between 2001 and 2011. Producers started looking for viable alternate options that provided decrease prices with comparable or higher conductivity.

Copper was the apparent selection, with its cost-effectiveness and superior electrical and thermal conductivity. Whereas the worth of copper jumped almost 700% over the identical interval, It’s high worth of $4.58 per pound is a pittance in comparison with the $2,100 per ounce of gold. Plus, copper’s skill to conduct the identical present as gold with a smaller diameter wire, with out succumbing to overheating, makes it extremely enticing. Moreover, its slower response charge with aluminum enhances its reliability, particularly beneath extended high-temperature storage situations.

“It is advisable obtain a top quality and really dependable wire bond, and there’s so many components influencing that objective,” says Per Viklund, director of IC packaging and RF product traces at Siemens EDA. “The best wire materials must be chosen for each electrical and bonding efficiency, and the suitable steel bonding course of must be used. What you’re after, after all, is a bond that stays and by no means loosens by itself.”

A key driver within the adoption of copper for wirebonding has been the expansion of automotive electronics. New automobiles right this moment can have greater than a thousand ICs, and whereas gold remains to be utilized in some automotive purposes, copper has confirmed extra dependable over time in harsh environments and excessive temperatures as a result of its superior thermal conductivity and resilience.

“Once you work with automotive energy modules, you might have particular necessities for wirebonding, or very excessive present/very low inductance,” provides Viklund. “You are able to do some particular methods with these kinds of designs, however there are some bodily limitations stopping you from including an infinite variety of wire bonds. It’s simply not sensible.”

The automotive trade, with its growing reliance on superior electronics for every little thing from engine management items to driver help programs, calls for supplies that may persistently carry out beneath difficult situations with defects beneath one half per billion, which interprets to 1 defect per billion kilometers. Copper’s inherent properties, akin to its excessive melting level and wonderful electrical conductivity, make it notably suited to these necessities.

“Historically, gold was the was the wire of selection,” says Prasad Dhond, vice chairman of Amkor‘s Wirebond BGA Merchandise. “It’s simpler to work with, it’s a softer materials, nevertheless it has proven some points throughout automotive reliability testing. And, after all, the fee is a giant concern.”

The reliability points for gold at excessive temperatures over an prolonged time frame embody the event of Kirkendall voids within the joint between the wire and the aluminum pad on the die. These voids don’t seem with copper-aluminum bonds.

Nonetheless, copper isn’t with out its challenges. Whereas copper does provide benefits when it comes to value and thermal conductivity, the complexities related to its use in wirebonding necessitate superior tools and specialised strategies. Some alternate options, like palladium-coated copper wire, have proven vital resistance to corrosion, albeit at the next hardness and value than pure copper, however nonetheless lower than gold. Copper should even be labored in an oxygen-free forming gasoline surroundings (95% nitrogen and 5% hydrogen).

“You need to get a very good, robust bond between the supplies,” says Chris Davies, product line supervisor for semiconductor merchandise at Nordson. “However copper is thought for corrosion, so you’ll be able to’t use another metals round it otherwise you get galvanic corrosion. With gold and aluminum, you get purple plaque. Copper is the most affordable, gold is the costliest, and aluminum is someplace in between, however aluminum solely tends to bond with particular processes and is kind of difficult to bond due to the oxide layers on the floor. There are all the time tradeoffs.”

Automotive driving modifications
Till about 2010, the semiconductor trade primarily used gold wiring in wire-bond-based packages, however when gold costs spiked, the trade migrated from gold to copper wiring, enabling chipmakers to cut back their packaging prices. Automakers initially have been reluctant to modify to copper as a result of its limitations, in addition to the dearth of long-term testing of copper wirebonding for important purposes in harsh environments. That has modified as copper has demonstrated to supply larger reliability in excessive warmth and excessive vibration purposes.

“In automotive, copper wires are the most well-liked now,” says Dhond. “From a reliability perspective, prospects want it. During the last 10 years there was an enormous shift towards copper, and nearly all of our new product introductions for automotive use copper wire.”

One problem with copper is that the warmth required to realize a bond makes it unsuitable for chips with fragile bond pads or sensitivity to excessive temperatures. Its stiffness can also result in challenges in forming constant and dependable loops, particularly in fine-pitch purposes. Moreover, the oxidation of copper can result in decreased electrical efficiency and potential long-term reliability points.

For these purposes, a silver alloy can provide properties much like these of gold with a value much like palladium-coated copper. Silver boasts superior thermal conductivity and decreased electrical resistivity in comparison with copper, making it a perfect selection for energy electronics. Its elasticity and hardness strike a stability between gold and copper, simplifying the bonding course of, and its decrease melting level helps defend fragile bond pads. Silver wire bonds are additionally much less inclined to corrosion in comparison with copper.

Pure silver (Ag) is inherently tender, making it unsuitable for direct use in wirebonding purposes. To handle this limitation, silver is normally alloyed with palladium (Pd) and gold in various proportions, relying on the specs. Silver’s excessive reflectivity additionally makes it a most well-liked selection in LED packaging, the place maximizing mild output is a major concern.

Lastly, aluminum is an trade normal wirebonding materials that has been used nearly so long as gold and has distinctive properties that make it superb for a variety of purposes. Its compatibility with aluminum pads on dies permits for bonding at room temperature with low power ranges, which helps forestall injury to delicate gadgets.

Like pure silver, pure aluminum, whereas drawable, is comparatively tender. To reinforce its mechanical properties for particular purposes, it’s usually alloyed with parts akin to silicon, sometimes round 1%, or typically magnesium. These aluminum alloys end in wires with finer sizes and elevated power.

Aluminum’s excessive electrical and thermal conductivity make it a most well-liked selection in energy gadgets and sign transmission. Whereas it’s generally utilized in low-frequency purposes like radio and audio tools, energy provides, and thermostats, its versatility extends to a broad vary of purposes. Aluminum is bonded utilizing wedge bonding strategies, which limits it use with very fine-pitch purposes. Moreover, aluminum’s susceptibility to oxidation could make it unsuited for environments the place excessive purity and corrosion resistance are paramount.

“There are totally different challenges for various supplies, particularly whenever you’re fusing one materials to a different,” provides Viklund. “ The wire materials reliably bonds to no matter materials you might have on the highest floor of your chip. It’s all the time been about how can we fuse the wire to the chip, and be sure that it stays there. And it’s just about been aluminum or gold and copper and numerous alloys. It comes right down to electrical efficiency and bonding efficiency, after which on the finish, value. And you need to have the suitable stability between these or it received’t get adopted by your buyer.”

Fig 2: Some traits of 0.7 mil gold, aluminum, copper and silver wire. Supply: ASME

Reminiscence
Reminiscence purposes are one other large driver for modifications in wirebonding know-how. With the introduction of multi-layer architectures in NAND flash reminiscence, each the intricacy of wirebonding strategies and the associated testing challenges have escalated. Fashionable reminiscence chips, particularly in 3D NAND configurations, can comprise a whole bunch of layers, that are stacked in tiers.

“For some kinds of designs, like NAND reminiscence, you do a whole lot of chip stacking and wirebonding,” says Viklund. “And the problem is, after all, to do the wirebonding in a means the place the wires are protected, as a result of the following chip overlaps it.”

That’s the place issues actually start. “A number of layers is a killer,” provides Schaefer. “The most important problem on the subject of the variety of wires is the growing variety of layers. We wire bond one layer, examine, rework, wire bond the second layer, repeat, and so forth. And it’s nearly unattainable to transform layers upon getting layers on high.”

The extra layers, the harder. “There generally is a dozen or much more layers of wire bonds in right this moment’s reminiscence, so it might get very advanced,” says Chen. “The query is, can wirebonding proceed to scale? Is it possible to maintain up with the ever-increasing density? If wires are too shut to one another, finally with all of the mechanical and thermal biking that occurs, it might be that the wires begin shifting and finally brief. The priority is how will we get sufficient margin between all of the wires so we will deal with some tolerances like that?”

Conclusion
Wirebonding continues to reveal its adaptability and relevance within the face of evolving technological calls for. As semiconductor gadgets have miniaturized and grown in complexity, wirebonding has innovated, incorporating new supplies and refining strategies to fulfill the challenges of contemporary electronics. Whereas various interconnection strategies, akin to flip-chip applied sciences, have emerged to deal with particular wants, wirebonding stays the dominant technique of connecting chips to packages.

The trade’s shift from gold to copper, pushed by each financial and efficiency components, exemplifies the adaptability of wirebonding. Moreover, the growing complexity of reminiscence purposes, with multi-layer architectures, underscores the necessity for wirebonding options that may accommodate intricate designs with out compromising reliability. Because the semiconductor panorama continues to evolve, wirebonding’s legacy of resilience and innovation suggests that it’ll stay an integral a part of the trade’s future, adapting and thriving amidst change.

References:
Zhou, H.; Zhang, Y.; Cao, J.; Su, C.; Li, C.; Chang, A.; An, B. Analysis Progress on Bonding Wire for Microelectronic Packaging. Micromachines. 2023. ,14, 432. https://doi.org/10.3390/mi14020432

— Karen Heyman and Ed Sperling contributed to this report.

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