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Technology Advances, Shortages Seen For Wire Bonders

Old tech still accounts for 75% of all packages, and likely will continue to play important role as equipment improves.

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A surge in demand for IC packages is causing long lead times for wire bonders, which are used to assemble three-fourths of the world’s packages. The wire bonder market doubled last year, alongside advanced packaging’s rise.

Wirebonding is an older technology that typically flies under the radar. Still, packaging houses have multitudes of these key tools that help assemble many — but not all — package types. Over the years, customers have demanded faster and more capable wire bonders. In response, wire bonder vendors have developed faster systems, including those with AI-enabled defect detection and factory automation capabilities. Wirebonding is completely different than copper hybrid bonding, which is a far more advanced and costly technology used for die stacking, packaging, and other applications.

Nonetheless, in wirebonding, packaging customers face important new challenges. For some time, many packaging houses have been sold out of wirebonding capacity amid huge demand in the arena. So many packaging houses want more wire bonders to meet demand. There’s just one problem. Last August, delivery lead times for many wire bonder types spiked and were hovering around 10 months. So far in 2022, the situation is improving, but it’s still an issue. All of this impacts the delivery schedules of many package types.

“There was panic buying of wire bonders in 2021. OSATs and others were placing orders even for 2022, as they feared being too late in the ordering queue,” said Charles Shi, an analyst at Needham. “Lead times are probably shorter than six months now. The demand is still strong, but ordering is moving towards more moderate levels.”

Wirebonding always has been a dynamic but unsung part of the semiconductor ecosystem. Invented back in the 1950s, these bonders are used to create low-cost wired interconnects inside a package. Interconnects are used to connect one die to another, or to a substrate in a package. Over time, wire bonders evolved and became the workhorse assembly tool for many package types. TechSearch reports that 75% to 80% of today’s packages are based on wirebonding. Wire bonders are used for low-cost legacy packages, midrange packages, and memory die stacks.

Wirebonding is a key part of the assembly process. In one example of an assembly flow, a die is placed on a small rectangular frame with metal leads using a die-attach system. Then, a wire bonder takes these parts and automatically attaches tiny wires from the die to the metal leads at high speeds, forming electrical connections. Finally, using a different system, the structure is encapsulated, forming a package.

Fig. 1: Wirebond wires in packages. Source: K&S

Fig. 1: Wirebond wires in packages. Source: K&S

Wirebonding isn’t the only way to provide interconnects in packages, and it does have technical limitations. Beyond wirebonding, there are other schemes in which two or more dies are assembled using more advanced interconnects methods in an advanced package such as fan-outs, 2.5D or 3D-ICs.

Advanced packaging tends to grab most of the headlines and is making sizeable market gains. In comparison, wirebonded-based packaging is a mature technology with single-digit-growth rates. Even so, wire bonders are seeing meteoric growth. Worldwide sales reached $1.6 billion in 2021, up from $800 billion in 2020, according to VLSI Research. In 2022, the market is expected to remain flat with $1.6 billion in sales, according to the firm.

Besides the growth rates, other notable trends include:

  • The latest high-end wire bonders have machine learning and factory automation capabilities. Learning on these systems helps prevent defect excursions, while operating without human intervention.
  • Wire bonders are being used for more complex packages.
  • These systems are pushing the limits of technology in memory stacking.

What is wirebonding?
Bell Laboratories, once the world’s leading R&D organization, invented the first wire bonder in 1957. (Bell Labs also demonstrated the first transistor in 1947. In 2016, Nokia assumed control of Bell Labs.)

With the help of Bell Labs, Kulicke & Soffa (K&S) in 1959 developed the first commercial wire bonder, a system that paved the way toward chips with new and inexpensive packages.

The early wire bonders were manual systems, which were used to assemble simple dual-in-line packages (DIPs) and other commodity products. In DIPs, a die sits on a metal frame with leads. A bonder connects the die to the leads with tiny wires.

Wirebonding initially was a labor-intensive task. Starting in the 1960s, many North American chipmakers began moving their assembly plants from the U.S. to low-cost sites in Asia. Human operators were required to move unbonded parts to the bonders, where they were loaded into the machine.

Early on, wire bonders morphed into two types, ball bonders and wedge bonders. Ball bonders, the largest market, are used for a plethora of package types, and utilize copper, gold or silver wires. Wedge bonders are used for power devices.

Generally, a ball bonder consists of a monitor, keyboard, handlers, conveyor systems, and a capillary unit. In a ball bonder application, let’s say you want to form a wired connection between a bond pad on a die with a separate lead post positioned off the chip. In operation, a spool of wire is loaded into the system. From the spool, a single wire is fed through a capillary unit with a tiny hollow tube. In the system, a spark is generated, which melts the wire end and forms a spherical ball at the tip.

Next, the bonder scans while depositing a wire strip from post to bond pad. Then, in the ball bonder, the ball on the pad is mechanically pressed to electrically secure the connection. The capillary tube retracts, completing the wire loop between die pad and lead.

Next, the high-speed process is repeated until every connection needed in the package is connected.

Simply put, wirebonding is a low-cost way to make interconnects in packages. But years ago, wire bonders were supposed to disappear due to increasing technical demands.

“People were talking about advanced packaging erasing wirebonding way back in the late-1970s and early-1980s,” said Dan Hutcheson, CEO of VLSI Research.

That never happened. In the 1980s, the advent of automated wire bonders helped simplify the process. “If you think about wirebonding, you have this leadframe, and the leads extend out. There’s a space between those leads in the pad where the die goes,” Hutcheson said. “If the die size changes, it doesn’t matter. All you have to do is reprogram the wire bonder.”

All told, wirebonding took off early on. “The biggest reason is because it’s less expensive and more flexible,” Hutcheson said. “There are two things that drove the growth. One is the total number of packages. It’s not just packages that needed to be bonded. It’s also the number of leads that need to be bonded for any given year.”

Wire bonders are used to make several package types. Each package has a different number of bond wires with various wire lengths, loops, and pitches. The pitch is bigger than the space between the wire center to the center or bond pod center to the center on a wafer.

“The main two initiatives we were driving with each new equipment development were to increase the throughput of the bonder and reduce its bond pad pitch capability,” said John Foley, director of product development at K&S. “In the late 2000s, customers were driving down to 40μm, and more recently moving towards 35μm pad pitches. Today, our equipment is capable of 30μm in-line bond pad pitches, although most applications don’t yet require this capability.”

Today, in wirebonded packages, the mainstream bond pad pitch ranges from 40μm to 45μm. “According to that requirement, the wire diameter stabilized around 0.7- to 0.8-mil diameter, depending on the bond-pad pitch. As we move toward 35μm pad pitches, 0.6-mil diameter wire will be required,” Foley said.

Nonetheless, there are several challenges with wirebonding. For one thing, the packages have become more complex. “In the past, we bonded older, more unwieldy packages such as plastic leaded chip carriers (PLCCs) and DIPs,” said Rosie Medina, vice president of sales and marketing at Promex, the parent company of QP Technologies. “What’s changed is the need for smaller bond pad openings, higher pin count/finer pitch, and staggered bond pads – all of which lend themselves to custom substrates and packages.”

While the packages have become more complex, the wire bonders themselves must stay ahead of the curve. “For a high-yield process, you require low loops. You require small and spherical balls that must be uniformly formed at high speeds (accelerations of >25g),” said Choon Lee, chief technology officer of JCET.

Reliability is key. Copper, the mainstream wire type in bonding, is inexpensive and has high conductivity. But copper can corrode, causing failure in wirebonded packages. This is due to halogens like chlorine, which are present in mold compounds.

Wire bonder vendors have met all of those challenges. The above failure mode can be prevented by using reasonably priced, halogen-free compounds.

Still, customers want more capable and faster equipment. In recent years, wirebonding throughput has improved roughly 2% per year.

The throughput depends on several factors, such as package type and wire count. At the low-end, LEDs might have 2 to 3 wires. A quad-flat package (QFP), a common package type, ranges from 50 to 80 wires per device.

“We see packages with more than 2,000 wires at the high end. That would be for chips that are being used in utility smartphones, tablets and IoT devices,” K&S’ Foley said.

Wire bonder landscape, trends
Meanwhile, wirebonding has become a sizable business. Among 14 companies competing in the wire bonder market, K&S has >60% share, followed by ASM Pacific with >20%, according to VLSI Research.

It’s by no means a static market. For years, wirebonded packages mainly used gold wires, because the material is highly conductive and reliable.

That began to change around 2009, when gold prices jumped by 300% in the worldwide market. Potentially, the soaring price of gold spelled trouble for the cost of wirebonded packages.

Fortunately, the industry anticipated the problem. Even before the crisis, companies developed wire bonders that used less expensive copper wire.

With the migration from gold to copper wirebonding, vendors were able to reduce assembly costs by up to 30%. Gold wires still are used in some applications today, but copper became the mainstream technology almost overnight.

The next big inflection point occurred in the last year or two, when ASM Pacific, K&S, and a few other suppliers began to develop their respective Industry 4.0 (fourth industrial revolution) initiatives, also known as smart manufacturing. The goal is to improve manufacturing efficiency by using new technologies and better communication.

This isn’t new in the semiconductor industry. Over the years, chipmakers moved toward more automated fabs. Then, fab equipment makers incorporated more sensors into their systems. This, in turn, creates vast amounts of data that enables chipmakers to pinpoint problems earlier in the manufacturing flow.

Some fab equipment also incorporates machine learning. A subset of artificial intelligence (AI), machine learning uses advanced algorithms in systems to recognize patterns in inline data, learn, and make predictions from the information.

Machine learning is used in some but not all parts of a semiconductor fab. It’s used heavily in inspection equipment to help locate potentially fatal defects in chips.

Some of these technologies are moving into the packaging world. Indeed, several memory makers began moving toward so-called “lights-out” assembly facilities, where the goal is to eliminate operator intervention, thereby reducing costs.

Recently, several OSATs and chipmakers with packaging units moved in a similar direction. Generally, many packaging houses deploy various types of factory automation systems, including automated guided vehicles (AGVs), rail guided vehicles (RGVs), and overhead transport systems.

Wire bonder suppliers have developed the interfaces to communicate with those systems. “AVGs involve a robot that travels in the front of the wire bonder equipment, providing magazines of unbonded parts and later picks up magazines of fully bonded parts,” K&S’ Foley explained. “We also see rail-guided vehicles, where rows of wire bonders are back-to-back with enough space for a robot on a rail that automatically loads material from the rear of the machine.”

In overhead transfer mechanisms, meanwhile, a robotic carrier moves around the plant in an overhead conveyer belt. The carrier can pick up and drop off unbonded or bonded parts.

Besides the automation aspect, high-end wire bonders incorporate computer vision systems with machine learning for defect control. Using these algorithms, the bonder can detect whether a ball bond is too big or small. That’s one of many applications.

“The current focus is on enabling real-time process monitoring and fault detection. There is a wealth of data available from key subsystems on the bonder and we are adding additional sensors to enable more advanced detection. We are monitoring data in real time and using advanced machine learning algorithms,” Foley said.

Initially, automotive customers drove these developments. Car makers want zero defects in the process, including wirebonding. Now, all customers want these capabilities.

Market outlook
For some time, the industry has experienced unprecedented demand for semiconductor chips and packages, which is driving the need for many more assembly tools.

“We have seen broad-based growth in all sectors, with momentum that carried over to at least 2022,” said Tien Wu, chief operating officer at ASE, during a recent conference call. “In 2021, we have seen advanced packaging revenue grow 23% year-over-year. We do expect the growth rate in 2022 to be better than that number. The 2021 wirebond revenue grew 36%. We continue to see wirebond to be fully loaded. And we do expect wirebond revenue in 2022 will achieve double-digit growth.”

That’s the good news. The bad news is that the lead times for K&S’ wire bonders are six to seven months for most customers. In response, K&S is expanding its manufacturing capacity. Other wire bonder vendors are seeing similar demand.

Fig. 2: K&S wirebonding machine. Source: K&S

Fig. 2: K&S wirebonding machine. Source: K&S

“Ongoing global demand for 5G, connected devices, automotive and memory is continuing from 2021,” K&S’ Foley said. “2022 packaged semiconductor growth is expected to sequentially reduce from 2021, although this is still nearly 2X the historic industry average.”

In the early days, wirebonding was used to assemble simple packages. Over time, the wirebonded packages have become more complex. In the 2000s, for example, QFNs appeared.

QFNs fall into the leadframe family of packages. A leadframe is an alloy frame with extended leads. In QFN, a die is attached to a frame. Then, using a wire bonder, tiny wires connect the die to each lead. Finally, the package is encapsulated.

QFNs are widely used today, but they are more complex. “We are seeing multi-tier QFNs with three to four tiers to accommodate more I/Os. We are seeing bigger QFNs (<12mm),” JCET’s Lee said.

Wire bonders are used for other complex package types. “Today, we are doing more ball-grid arrays (BGAs), chip-on-board (COB), and multi-chip modules (MCMs),” Promex’ Medina said. “Although they aren’t pushing the boundaries as much since they have lower pin counts, sensors are another important wirebonding application. Many sensors require access to the surface of the die in the end application, thus using wirebonding for the interconnects is ideal.”

Memory is another big driver for wirebonding. In 2016, Apple introduced the iPhone 7. The phone stacked 16 NAND flash dies, enabling 128GB of storage. Each die was stacked in a pyramid-like formation and connected using tiny wirebonded wires.

Today, memory vendors are stacking 8 or 16 NAND flash dies in a package. In R&D, the industry is developing 24 die stacked packages.

This trend presents some challenges. “The memory die will be quite large and incredibly thin. So handling these fragile dies require the use of specialized needle-less pickup tools to minimize stress and reduce the risk of cracking during these processes,” said Knowlton Olmstead, a senior engineer at Amkor, in a video. “These thin dies will also have an overhang in the die stack. That requires selection of proper die-attach film and mold compound materials to minimize warpage and stress in the package, preventing failure in the final assembled package.”

That’s not the only issue. “Increasing the number of stack die in the package, while maintaining a low package height introduces challenges in a number of areas. There’s continual improvements being made in reducing the substrate ‘Z’ height to allow for higher stacking,” Olmstead said. “The wirebonding that cascades down the die also needs to be done in a controlled manner, so it minimizes stress on that whole overhanging area. Also, we want to maintain a low wirebond loop height, which lets us have a very low die-to-mold-cap clearance height.”

Conclusion
Clearly, wire bonders are an important part of the semiconductor ecosystem. Despite the growth and attention paid to advanced packaging, wirebonding will continue to be used for many package types.

Procuring enough wire bonders is one of the big challenges right now. But at some point there will be a glut of wirebonding capacity, which is the cyclical nature of an older but critical technology.

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Shortages, Challenges Engulf Packaging Supply Chain
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2 comments

Alex Voronel says:

Hello Mark, thank you for the great summary on the current status of wire bonding technology. Although the lion’s share of wire bonding is in ball bonding (Au and Cu), what about the wedge bonding part of it? It is a “little brother” of the ball bonding, however it has gained a good part of the overall market in last decade, especially for the assembly of power electronics devices (Power Modules and Discrete Packages). I am interested in your view and opinion on this segment of the wire bonding technology. Thanks.

Mark LaPedus says:

Hi Alex, Thanks for the comments. At some point, I may look at wedge bonding. It’s not as big as ball bonding. But it’s important too.

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