Introduction

Wire bonding is a method used to electrically and mechanically join conductive wires to semiconductor chips and substrates to make interconnections. It is a key process used extensively in the assembly and packaging of integrated circuits (ICs). This article provides a comprehensive overview of wire bonding fundamentals, bonding techniques, materials, processes, inspections, applications, advantages and challenges.

Definition of Wire Bonding

Wire bonding attaches thin wires to electrically connect the contact pads on ICs to the leads or bonding areas on printed circuit boards or other assemblies. It forms a bridge to allow electrical signals to cross gaps between the IC and substrate.

The wire bonds also help conduct heat and provide shock protection to the chip.

Why Use Wire Bonding?

• Provides electrical connectivity for IC chips
• Allows routing signals in 3D space
• Bridges gaps between ICs and substrates
• Bonded wires are flexible
• Very precise bonding placement
• Can support high frequencies and data rates
• Cost effective for mass production

Wire bonding is a mature and reliable interconnect technology suitable for most IC assembly applications.

Types of Wire Bonds

The main types of wire bonds used are:

Ball Bonds

A ball bond is formed by melting the end of the wire into a sphere using a flame, spark discharge or electric arc and pressing it against a bond pad under force and ultrasonic energy. This creates the initial connection.

Wedge Bonds

A wedge bond is made by pressing a wire against the bonding area through a wedge-shaped bonding capillary, deforming it and pressing it against the surface under force and ultrasonic energy. The wire is then torn at the edge of the capillary tip.

Ribbon Bonds

Ribbon bonding joins a thin flat wire to the IC die or substrate in the same manner as round wedge bonding. It makes multiple bonds simultaneously in parallel.

Wire Bonding Techniques

The main wire bonding techniques include:

Thermosonic Bonding

Using a combination of heat and ultrasonic energy, along with force, to weld the wire to the bonding area. It is used for both ball and wedge bonding.

Thermocompression Bonding

Applying heat, force and welding pressure through the bond tool tip to deform the wire and bond it to the area. Mainly used for ball bonding.

Ultrasonic Bonding

Utilizing just ultrasonic energy and force to create a cold weld between the wire and bond area. Used for aluminum wire wedge bonding.

Wire Bonding Materials

Common wire materials include:

• Gold – Most prevalent, offers very reliable bonds
• Copper – Lower cost than gold, challenges with oxidation
• Aluminum – Only for ultrasonic wedge bonding
• Silver – Good electrical properties but bonds less reliably

Gold wire is generally preferred for critical electronics due to its excellent conductivity, bondability and corrosion resistance.

Wire Properties

Key wire properties for bonding include:

• Composition – Gold, copper, aluminum etc.
• Diameter – Typically 15 to 50 μm
• Temper – Annealed soft vs hard temper
• Coatings – Bare or coated with alloys
• Shapes – Round, ribbon, shaped customized wires

The composition, diameter and temper determine the best bonding approach.

Bonding Process Steps

Basic wire bonding process flow:

1. Prepare bond sites – Clean pads, apply heat
2. Position wire – Thread through capillary tip
3. Form first bond – Ball or wedge bond on chip die
4. Extend wire – Feed out to leadframe/substrate
5. Form second bond – Wedge bond wire to destination
6. Repeat bonding – For multiple wire connections
7. Cut excess wire – Sever final bond

This sequence is computer numerically controlled for automation.

Wire Bonding Equipment

• Bonding Tool – Precisely positions capillary tip
• Work Holder – Secures substrate using heating/cooling
• Ultrasonics – Provides ultrasonic vibration energy
• Vision – Aligns tools and materials optically
• Wire Feed – Feeds wire on demand during bonding
• Controls – Automates all parameters and positioning

Sophisticated wire bonders provide the accuracy, speed and flexibility needed.

Advantages of Wire Bonding

• Mature and well established technology
• High throughput and automation
• Handles high density I/O pitch <40 μm
• Low parasitic capacitance and inductance
• High purity wires available
• Adaptable to complex geometries
• Allows stacking dies and interposers
• Fine pitch capability down to 15 μm
• Cost effective for mass production

These benefits make wire bonding a widely utilized interconnect solution.

Wire Bonding Challenges

• Limited by length-to-diameter aspect ratio
• Wires block die access and view
• Long wires may sag during encapsulation
• Parasitic capacitance/inductance increase with length
• Not well suited for high current due to wire gauge
• Subject to corrosion and contamination over time
• Bonds may fail due to temperature cycling or moisture
• Repairs often require removing and replacing bonds

While very capable, wire bonds have inherent limitations.

Wire Bonding Inspections

Key wire bond quality inspections include:

Bond Placement Accuracy

Confirming correct positioning of each bond on its pad.

Bond Orientation

Checking orientation angle of wedge bonds against leadframe edges.

Bond Quality

Inspecting bond integrity – shape, attachment, lifted balls or wedges.

Wire Shape

Verifying proper loop height, placement and absence of kinks.

Wire Sweep

Checking wires are not excessively low from die to substrate.

Automated optical inspection improves quality and throughput.

Wire Pull Testing

bond strength can be measured by pull testing:

• Uses hook or wedge tool to lift bonded wire
• Gradually increases force until bond breaks or wire severs
• Measures maximum force at failure in grams
• Compare to minimum pull spec for acceptable strength

Manual or automated systems perform statistical sampling.

Process Controls

Key parameters requiring controls:

• Bond force – Amount of force applied
• Ultrasonic power – For thermasonic bonding
• Bond time – Duration of bonding
• Bond temperatures – Heating zones
• Wire tension – Constant tension during feeding

SPC, visual checks, and product audits ensure stability.

Wire Bonding Applications

Wire bonding sees extensive usage across:

• Consumer electronics – Mobile, IoT, wearables
• Computing – Microprocessors, memory, chipsets
• Automotive – Sensors, controls, infotainment
• Aerospace/military – Guidance systems, radars
• Medical – Implants, stimulators, imaging
• Communications – 5G, networking hardware

Any application with ICs benefits from wire bonding technology.

Emerging Technologies

Some newer technologies aim to augment wire bonding:

Ribbon Bonding

• Joins multiple wires simultaneously
• Higher throughput bonding
• Challenging ribbon handling

Stud Bumping

• Deposit gold bumps on bond pads
• Allows finer pitch than bond pads
• Issue of bump bond reliability

Remote Plasma Bonding

• Creates plasma for lower temperature bonding
• Reduces damage to sensitive devices
• Complex plasma generator required

These methods are still maturing toward volume adoption.

The Future of Wire Bonding

Despite competition from flip chip and other techniques, wire bonding will remain essential:

• Ongoing materials and equipment innovations
• Nano-scale wires enabling ultra-fine pitch
• Extending capabilities to new applications
• Allowing hybrid bonding with other interconnects
• Roadmap to at least 10 years for next-gen adoption

Wire bonding equipment and material suppliers continue advancing this vital technology.

Conclusion

In summary, wire bonding provides a proven, adaptable, and cost-effective interconnect method widely adopted across the microelectronics industry. Advancements in bonding tools, materials, and process automation continue expanding capabilities while reducing cost even further. Despite some inherent limitations, wire bonding will remain an essential manufacturing process enabling ongoing miniaturization and performance increases of integrated circuits. Even as alternative approaches emerge, wire bonding delivers unique benefits for numerous applications demanding flexibile, fine pitch, and high throughput chip connections.

FAQs

Q: What are the main differences between ball bonding and wedge bonding?

A: Ball bonding forms an initial ball whereas wedge bonding uses the side of the wire pressed into the bond pad. Also, ball bonding can only make the first bond whereas wedge bonding can make both first and second bonds.

Q: What wire size is typically used for wire bonding?

A: Most wire bonding uses wire diameters in the range of 15 to 50 microns. Specialized nano-scale wire bonding can go below 15 microns.

Q: How close together can wire bonds be placed?

A: The finest pitch wire bonding today can achieve a density down to around 15 microns between adjacent connections.

Q: What is the key benefit of ribbon bonding versus round wire?

A: Ribbon bonding can make multiple wire bonds simultaneously, boosting throughput substantially compared to round wire.

Q: What is the main factor limiting the span between wire bonds?

A: The length to diameter ratio of the thin wire limits total wire length before unwanted sagging or sweeping may occur. Typical spans are under 5 mm.

Types of Bonding Based on Shape

Common bonding shape that is used nowadays are ball, wedge bonding and flip chip bonding. In recent times, people prefer using one shape to another. Then, Why the designer prefer to one method than another ? For comparing all methods, we will explain more and compare all bonding shapes in this articles.

Ball Bonding

Ball holding is the cycle where pads are bond into a silicon die and substrate or leadframe utilizing wire which has fine diameter. The essential process of ball bonding method involve the first bonding shaping (commonly above the chip), the second bond shaping (commonly on the substrate) and the wire loop.

On the start of the wire process, the bonding equipment move down until it reach first bond area. The first bond is accomplished by making bond between a pad and a spherical ball utilizing ultrasonic energy and heat treatment. The first bond is likewise alluded to ball bond. Looping movements are planned in program to meet the requirement of package for  loop shape and height.

The second bond includes stitch bond which bonds the tail bond and the opposite end. The tail bond is required to create a tail of wire for the following ball shaping process. After the bonding equipment moves up to release the wire tail, after that the tail is off and the bonding equipment ascends to the ball shaping height. The ball shaping process is accomplished by ionization air gap in a cycle called EFO or electronic fire off. The ball resulted on this process is called a FAB or free air ball.

Keeping ball size is important for this cycle as it sets the general size bond and is relying on dependable bonding process to guarantee a conformable length of wire tail before each ball shaping. In case each bond is not shaped appropriately, there will be huge bump sizing variations.

Mechanical force fine application is important in the bonding cycle as it also set bump’s height and form. At last, shearing step with precise determination to split up the the ball and wire is important to make planar and conformable height bond.

Wedge Bonding

The bottom side of the capillary is used to squeezed a wire stub toward the bond pad, using ultrasonic energy to shape the bond among bond pad and wire, In wedge bonding method. After that, the capillary move toward the second bond area and the cycle is processed with repeating those process. When the second bond is finished, the wire is clipped and snapped over the second bond.

The important process in the wire bonding cycle  comprise accomplishing dependable bond (including first, second, and tail bond), keeping up wanted loop, and placing the bonds precisely. Throughput is a necessary point too, as it influences the device production expense. Accomplishing wanted first bond and second bonds generally needs bonding parameters optimization. DOE or design of experiment should be used to exercise parameters optimization, that comprise effect ultrasonic energy levels, mechanical force, bonding power. An appropriate free air ball dimension regularly is resolved prior to starting the DOE first-bond. Looping directions are chosen by the application necessities. There are two common loop types and those are reverse and forward. Forward looping type firstly puts a ball bond over the die, and after that puts a stitch bond on lead frame. In the other hand, the first step in a reverse bonding type, nonetheless, is putting a bump die. Right after the bump is shaped, a ball bond is put over the substrate, then shaping the stich bond over the. Low-profile looping prerequisites have pushed the developing reverse ball bonding utilization, that is a less quick method than forward bonding method.

Application of Fine-pitch. fine-pitch wire bonding competence has been exhibited in lab at 35 µm pitch. Generally 15 µm wire is utilized with 35-µm pitch ball bonding and a bonded ball with diameter around 27 µm. Fine-pitch usage needs a higher wire bonder aptitude, such as better control of ultrasonic energy level, the bonding force, and also fine wires looping ability, that is more disposed to loop influence  and weaker. A wire bonder which fulfills the fine pitch needs ought to likewise having  precise movement and submicron accuracy on vision system.

Applications of Stacked Die. Stacked pass on implementations are one of the quickest developing patterns in the semiconductor business. The urge for lighter, more intelligent, and smaller gadgets encourage this 3-D packaging research and development. Stacked die usage present variance of wire bonding difficulties, including multi level and low loop wire bonding loop free space needed, loop resistance from wire clear during molding process , and bonding to hang unsupported die verge.

Most wire bonding implementation utilize the ordinary forward bonding method, since it is quicker and more adequate for finer pitch compare to reverse bonding method. Even though, forward ball bonding method has a restriction of loop height because of the neck region over the ball. Exaggerated bending over the ball can cause crack on the neck area, which brings about dependability issues. Reverse bonding can accomplish loop height smaller than 75 µm.

Flip-Chip Bonding

Flip chip bonding is a important innovation for cutting edge microelectronic circuits packaging. It permits connection of bare chip to a substrate for packaging in a face-down arrangement, with electrical associations among the substrate and chip through conductive bumps. Flip chip assembly has numerous benefits. A primer benefit is enhancing electrical performance. flip chip little bumps connection give short electrical ways, that yield incredible electrical properties which have low resistance ,capacitance, and inductance. This brings about extraordinarily improved performance in high frequency working when compared with other bonding techniques, for example, chip wire bond over substrate.

Another key benefit of flip chip assembly method is compactness of package which decrease weight and size contrasted with traditional wire bond method. The electrical interconnection between substrate surface and pads over chip can be spread out as a zone array, as opposed to around the chip that is a particular design for wire bond arrangement. This two dimensional structure can reduce chip footprint over substrate and reduce chip space. The small physical area and low profile of flip chip construction permit small electronic package size to be fabricated. Nowadays, you can find flip chip components in number hand held gadgets, PDAs, electronic coordinators, electronic watches, cameras, and any other products.

How to Choose Proper Bonding Shape for Certain Aim?

When we reach application step, we often deal with question which method that appropriate with our need: wedge bond, flip-chip bond , and ball bond? For which reason would an engineer pick a ball bonder and not a wedge bonder or the other way around? This question come to most the engineers, as a rule, electrical attributes of the package are influenced by the wire bonding technique. Even, there are situations where particular packages have physical restriction like temperature restriction (no heat or low heat usage), avoid gold material and choose aluminum, prefer using ribbon type to wire type and fine pitch usage. This is the case where the appropriate selection of wire bond method becomes play an important role.

Commonly, ball bonding implementation are related to thermosonic and thermocompression bonding techniques. Thermocompression uses temperature from around 150^oC and mechanical pressure to make intermetallic bonding. And thermosonic adds energy from ultrasonic from the past step. With the two techniques, in any case, sparkle from an EFO or electronic fire off  under the capillary create a free air ball prior to bonding shaping. This free air ball at that point deform when the capillary directly contact with the bond pad surface and applies ultrasonics and mechanical force for several time to change ball shape. Hence the interdiffusion between the bond pad and the wire metallization happens, that creates the intermetallic bond.

Until this day, over  90% of all wire bonds method in electrical packaging use gold ball bonding technique. It is caused by quick process to make ball bonding rather than wedge bonding technique. Ball bonding needs just three movement axis (X Y Z), in the other hand wedge bonding needs four movement axis (X Y Z θ).Just gold or Au wire might be utilized in the ball bonding technique in contrast aluminum (Al) and gold wires are utilized ordinarily in wedge bonding technique. This happen because aluminum wire will oxidize throughout the EFO or electronic fire off process to shape the ball. High-volume Cu or copper wire ball bonding technique is still on research phase. To prevent copper wire being oxidized throughout the ball shaping, the EFO process is implemented into the inert gas. Table below show brief comparison between wedge bonding and ball bonding.

In spite of the fact that wedge bonding technique need more time than ball holding application, wedge bonding has the other numerous benefits, for instance,  fine pitch, short and low loops, and profound access. That is the reason wedge bonding is being utilized widely in optoelectronics and microwave implementation.

Typically, ball bonding method is quicker for around 5 until more than 12 wires each second. Sorts of wire material utilized for this method such as coated palladium, copper, and gold wires. Common application and package for this technique are QFP, BGA, SOP, wafer level bumping, and hybrid MCM. The ball bonding technique is proper for fine pitch implementation on 40 micrometer or less.

The ball lacking  on the primary bond gives wedge holding a benefit for a lot better pitch utilizations of 40 micrometer or less than it. Wire from aluminum is the most popular wire utilized for this cycle, trailed by gold wire. Run of the mill bundles and applications incorporate high power, optoelectronic bundling, RF microwave,  BGA, QFP, SOP, MCM half and halves and temperature-touchy implementation. Wedge bonding speeds normally ranging from 3 until 6 wires each second.

Bonding process step by step

1. Cleaning is the important process required before doing wire bond process. The metallization should be exempt from inorganic and organic pollutant. For instance, residual oil on the bonding surface area will decrease the dependability of the connection. There are two well known cleaning techniques, the one is  bright or Ultraviolet ozone cleaning  and another is plasma cleaning and. Ultraviolet ozone cleaning produces a lot of radiation (having wavelength in 2537A and 1848A) to eliminate organic contaminants. Plasma cleaning is powerful for eliminating epoxy bleed out, that is created from outgassing.

• Setting the appropriate temperature for ultrasonic, thermocompression, and thermosonic methods are important to assure wire bonding become conformable. Thermo sonic bonding process should be done on temperature ranging from 100^oC to 150^oC. Ultrasonic bonding might be done on surrounding temperature or around 25^oC or. In the other hand, Thermocompression bonding should be set around 300^oC and 500^oC.
• Setting the appropriate mechanical force for the ultrasonic, thermocompression, and thermosonic methods and gives the appropriate amount of pressure required to make solid wire bonds. Thermosonic holding needs between 0.5 and 2.5g force for each wirebond. And similar to thermosonic, ultrasonic bonding need 0.5- 2.5g mechanical force for each wire bond. Lastly, 15-25g mechanical force needed by thermocompression bonding for each wire bond.
• Setting the appropriate mechanical force is important for the ultrasonic and thermosonic bonding techniques. This is needed to guarantee bond quality, increment the force setting without over-stressing or applying on the wire. You shall know over-stressing is occurring when the mechanical pull testing tool shows a low break.
• Ensure the unit is appropriately cinched inside the work holder, because it is important that ensure no movement might happens. You can check this by prodding the item  using tweezers. In case movement is happening, the unit should be safe while high speed bonding process.
• Ensure the capillary is in good condition and works fine. Several factors, for example, bonding pad pitch, bond size, harness type, metallization, and wire diameter can affect bonding characteristics. The appropriate instrument selection is important for creating conformable wire bonding.

Wire bond Design Tips

Elude chip-to-chip interconnection – Unless performance need it, wire bonding straightforwardly between Integrated Circuit ought to be eluded. Making stitch bond will spread mechanical energy into pad surface, that may prompt cracking under, or in, the pad metallization. Crack refer to potential dependability issue; thus, intermediate bonding pads ought to be designed on the substrate.

Try not to cross the wires – Bond wires ought not crossing over between wire, bond pads, or other die. In condition that mechanical stress from external source is applied, the wire bond unsupported loop could hang and meet / touch a wire straightforwardly under it, prompting a short circuit that may damage whole system.

Keep in mind: bond pads is important – Bond pad ought to be arranged to make the the most concise possible wire bond. The wire bond length specifies connection capacitance, inductance, and total impedance of the. Long wire bonds might be bad to the package performance. Utilizing aluminum or gold wedge bonding with flash gold with is a special case. Even, it will be good to check the implementation notes for the integrated circuit package, since some integrated circuit producers do not use wedge bonding because of the mechanical force utilized to die on the production process. At least 0.005 mm is needed between a via and the verge of a bond pad. Created bond that is located near with substrate discontinuity may prompt to harming material caused by spreading mechanical energy to the substrate via bonding. If  multilayer substrates wire bonding, used pad ought to be at least 10 mm from the conductor edge to enable wire bonding tolerance, registration, and printing.

How to Select Wire for Bonding

The wire diameter selection upon maximum current limit, cost, and the wire bond pitch. Gold wire with 1-mil diameter is a typically used by designer, and has 1.17 mω electrical resistance per mil, and a maximum current limit around 0.7 A, relying upon heatsinking, wire length, and so forth Common inductance value for 1-mil wire bond is around 25 pico H per mil, however it fluctuates relying on height of bond wire. Aluminum wire is utilized exclusively in wedge-bonding implementation, as the high affinity of Aluminum to oxidize needs ball bonding with Aluminum wires to be applied in an inert environment.

Placement of Bonding

In the design process of IC package, it is critical to determine bond arrangement relative with different parts / components — a determination that will make smaller package dimension and increase layout densities. If wire bonds place the high components / parts, holding device need free space (X) and resilience for both die arrangement and bond precision should be thought of. On a stitch bond, the real bond surface is shift from centerline of capillary. Subsequently, clearance should incorporate extra tolerance, equivalent to a half of tip diameter of capillary to guarantee appropriate clearance. Therefore, Y ought to be 0.005 in. > X for tip capillary with 0.01-in. dimension. Another configuration is to utilize wedge bonding, where the bonding equipment has a vertical face. Tragically, wedge holding is more slow—and consequently more expensive—than ball-line holding.

Spacing and Pad Sizing

Table beneath gives rule of thumb to design substrate pad and determining size of die pad. The dimension relate to wedge and ball bonding and to ceramic substrate and PCB. Exemptions are noted as relevant.

Evaluating the Wirebond

Wire-bonded parts acceptability and wire bond strength might be assessed utilizing either a DPT / Destructive Pull Test or NDPT / Non-Destructive Pull Test. The most used standard are MIL-STD-883, specifically on Method 2011.7  about Bond Strength Method 2023.5. These standard portray sizes of sample for each test type and acceptance standards for various bonding and wire types. A few assessment test that is recorded in this standard such as:

• Internal visual
• Nondestructive bond pull test

• Destructive bond pull test

• Mechanical shock
• Ball bonding shear test

• Stabilization bake
• Constant acceleration

• Moisture resistance
• Random vibration