What is Wire Bonding?
Wire bonding is a method of making interconnections between an integrated circuit (IC) or other semiconductor device and its packaging during semiconductor device fabrication.
Wire bonding can also be used to connect an IC to other electronics or to connect from one printed circuit board (PCB) to another, although these are less common.
Wire bonding is generally considered the most cost-effective and flexible interconnect technology and is used to assemble the vast majority of semiconductor packages. Wire bonding can be used at frequencies above 100 GHz.
Materials Used in Wire Bonding
Pure gold wire doped with controlled amounts of beryllium and other elements is normally used for ball bonding. This process brings together the two materials that are to be bonded using heat, pressure and ultrasonic energy referred to as thermosonic bonding.
The selection of wire material is a critical aspect of the wire bonding process, with each material offering distinct advantages and drawbacks:
#1. Gold (Au).
Wire bonding uses gold wire, which is most frequently used due to its conductivity, high corrosion resistance, conductivity and ease of bonding.
#2. Aluminum (Al).
Wedge bonding mostly employs aluminum wire due to its good conductivity in comparison to that of gold and its comparatively lower price.
#3. Copper (Cu).
Copper wire is more efficient in terms of electric and thermal conductivity than aluminum and gold.
#4. Silver (Ag).
Silver wire can on very rarely used in wire bonding due to the high conductivity of the material.
Types of Wire Bonding
Wire bonding can be classified into three primary types, each with its specific applications
1. Thermo sonic Ball Bonding
Thermo sonic ball bonding is the widely used wire bonding method, particularly in the case of gold (Au) and copper (Cu) wire.
In this method, ultrasonic energy, heat, and mechanical pressure are used to form a strong bond. The process starts by heating the end of the wire until it becomes the shape of a ball.
This is followed by applying pressure at the bond pad of the semiconductor device and simultaneously applying ultrasonic vibrations and moderate heat. These make a proper metallurgical bond of the combination that is quite robust.
a) Ball to Ball Wire Bonding: –
In this process, the bonding wire is passed through a Capillary tool which is mounted at the transducer, and a small portion of the wire is used to melt through an EFO (Electronic flame off system).
A ball is formed on the end of the bonding wire and this ball is pressed to the bonding pad on the die with sufficient force then welded between two metal surfaces using a Capillary tool. The below image shows the process of Ball-to-Ball Wire Bonding on the lead frame.
2. Wedge Bonding
Common wedges employed in wire bonding utilize aluminum (Al) and gold wires. While in ball bonding the process begins at the formation of the ball, in wedge bonding, the process begins at the flat bond.
This technique encompasses ultrasonic energy along with mechanical pressure but with a little heat.
The conventional process for making efficient wire bonding involves a wedge-shaped tool, which presses the wire against the bond pad; the remaining wire bond is created through the use of ultrasonic vibrations.
a) Ultrasonic Wedge to Wedge Bonding:-
In the Ultrasonic Wedge to Wedge Bonding, the wire is fed through a hole in the back of a bonding wedge at an angle of 30 to 60 degrees from the horizontal bonding surface.
A wedge-shaped tool flattens the wire and generates a weld below this point during wedge bonding of the wire.
One bond is made to the die and another is to the substrate. The below images show the process of Wedge to Wedge Wire Bonding.
b) Ultrasonic Ball to Wedge Bonding:-
In this process, one edge of the capillary is used to form the all to wedge bond. The Ball Bond is made to the silicon chip and the subsequent wedge bond to the leadframe. The below image shows the process of Ball to Wedge Wire Bonding.
3. Thermo compression Bonding
The Thermo compression Bonding involves the use of heat and pressure without the introduction of ultrasonic energy.
The wire and the bond pad are heated to such high temperatures as to enable diffusion bonding. This method is slow and is used for better regulation of temperature.
Wire Bonding Process
The wire bonding process involves several critical steps, each designed to ensure the creation of reliable and effective electrical connections:
#1. Die Placement.
The process starts with proper positioning of the semiconductor die to the substrate of the package or the lead frame.
This step is important because any disparities at this stage may affect the other wire bonding steps to follow. The die is generally bonded using a die attach material, which could be an epoxy or a solder, based on the kind of package and usage.
#2. Wire Selection.
The selection of the type of bare wire and its diameter is critical.
These include the electrical conductivity of the wire as well as the thermal coefficients and mechanical strength which are essential for any wire that is used in the manufacture of these devices.
#3. Tool Setup.
The bonding tool generally referred to as a capillary for the ball bonding technique. Regarding the geometry, the material of the tool, and the wire material tools for wire bonding play an important role in their selection.
#4. Bond Formation.
The process of thermo sonic ball bonding commences with the formation of the free-air ball (FAB) obtained by balling the end of the wire by melting it. The FAB is finally soldered to the die pad with the help of ultrasonic energy, heat, and pressure.
#5. Loop Formation.
The bonding tool translates to the position of the second bond and in the process, it is displaced in the form of a wire loop. The geometry of the loop as well as its cross section is adjusted to assure the mechanical stability as well as the desired performance of the loop electrically.
#6. Second Bond Formation.
The second bond is carried out at the location of the package substrate or lead frame in which the die is to be attached. In ball bonding, this implies creating a stitch bond by simply placing or touching the wire against the bond site with the help of the bonding tool and then applying ultrasonic energy.
#7. Inspection and Testing.
Following the wire bonding process, the bonds go through a sort of test to either check the quality or the reliability of the bonds. Some of the general kinds of inspection include, visual inspection, a pull test that shows the adhesive tensile strength , and adhesive shear strength.
Wire Bonding Techniques
Wire bonding is the process of creating electrical interconnections between semiconductors (or other integrated circuits) and silicon chips using bonding wires, which are fine wires made of materials such as gold and aluminium.
The two most common processes are gold ball bonding and aluminium wedge bonding.
#1. Gold wire bonding.
Gold wire bonding is achieved through thermosonic bonding. This involves melting the end of the wire to form a gold ball, which is known as a free-air ball.
The diameter of the free-air ball measures 1.5 to 2.5 times the diameter of the wire. After being formed, the free-air ball is placed into contact with the bond pad and pressure, heat and ultrasonic forces are applied to it for a certain amount of time.
This forms a metallurgical weld between the ball and the bond pad while also allowing deformation of the ball bond into its ending shape.
In the next step, the wire is run to a finger of the lead frame, which forms an arc between the bond pad and the lead finger.
A second bond, called a wedge bond, is then formed through applying pressure and ultrasonic forces to the wire using the lead finger.
A machine called the wirebonder then clamps the wire and raises the capillary in order to break the wire to prepare for the next wirebonding.
#2. Aluminium wedge wire bonding.
Aluminium wedge wire bonding involves bringing a clamped aluminium wire in contact with the aluminium bond pad.
The wire is then held down while ultrasonic energy is applied to it for a certain amount of time, which forms the first wedge bond between the wire and bond pad, before being pressed against the corresponding lead finger.
Further application of ultrasonic energy to the wire forms the second bond, before the wire is broken off by being clamped and moved.
Compared to aluminium wedge bonding, gold ball bonding is significantly faster due to being non-directional, resulting in its widespread use in plastic packaging.
However, gold ball bonding on aluminium bond pads cannot be used in hermetic packages due to the high sealing temperatures used in such packages, as these temperatures greatly accelerate the formation of gold-metal intermetallics which can result in early life failures.
Conversely, gold ball bonding on gold bond pads can be utilised in hermetic packages.
Applications of Wire Bonding
Wire bonding is used across a wide range of modern applications, offering flexibility, reliability, and cost-efficiency. Some of the key areas include:
#1. 3D Integrated Circuits (ICs).
In 3D ICs, where multiple semiconductor dies are stacked vertically, wire bonding is vital for connecting these layers. As devices become more compact, the demand for high-density processing power has grown, making wire bonding indispensable in managing fine pitch and high pin counts.
This technology is critical for high-performance computing, advanced mobile devices, and high-density digital electronics.
#2. Power Electronics and Wide-Bandgap Semiconductors.
Wire bonding is essential for packaging wide-bandgap semiconductors such as silicon carbide (SiC) and gallium nitride (GaN), which are used in high-power applications like electric vehicles and renewable energy systems.
These semiconductors operate at high voltages and temperatures, and heavy-gauge copper wire bonding is often used to handle the higher current loads and ensure efficient power management.
#3. Optoelectronics and Image Sensors.
As the resolution of image sensors increases, the number of connections required rises dramatically, making fine wire bonding essential. These high-performance, high-density designs are crucial for advanced consumer electronics, medical diagnostics, and security systems.
#4. Chip-on-Board (COB) LEDs.
COB technology is widely used in LED designs, providing higher lumen density and improved thermal management.
Wire bonding enables compact LED arrays with efficient heat dissipation, leading to brighter, longer-lasting lighting solutions in automotive, industrial, and consumer applications.
#5. Consumer Electronics.
Wire bonding in consumer electronics includes connecting the semiconductor dies to their packages in gadgets like Smartphone and Portable tablets, laptops, and other portable gadgets. Hence, this technique is suitable for use in larger production in the consumer electronics subsector due to its cost and efficiency.
#6. Automotive Electronics.
Wire bonding in automotive is used in MCUs, sensors, power modules, and many more significant parts of automobile. Therefore, applications of wire bonding electronics in automobiles are significant because of their complementary characteristics like high tolerance to environmental conditions for example temperature, humidity, etc
#7. Aerospace and defence.
Military and aviation-related products require very reliable electronic parts that can survive even the harshest conditions. Wire bonding is widely applied in radar apparatus, communication equipment, and missile control systems.
#8. Medical Devices.
Medical-related applications include pacemakers, hearing aids, electronically controlled body implants, and diagnostic equipment where wire bonding is vital for the development of good electrical connections.
Due to these parameters of wire bonding, their reliability and accuracy are very high, they are ideal for use in life-sensitive circuits where the failure rate is virtually nonexistent.
Cost Considerations in Wire Bonding
While wire bonding offers significant advantages in terms of performance and space efficiency, cost remains an important factor, especially for large-scale production.
The cost of wire bonding is influenced by several variables, including the type of material used, the complexity of the application, and the production volume.
Material Costs:
The cost of wire bonding materials varies widely. Gold wire bonding is the most expensive option, with a price around $349 per gram for 0.8 mil wire.
However, copper and aluminum offer much more cost-effective alternatives, especially in applications where high conductivity and durability are still essential.
For example, the same diameter of aluminum or copper bonding wire can be a fraction of the cost compared to gold, making them ideal choices for high-volume production.
Manufacturing Costs:
Wire bonding machines vary in cost depending on their level of automation.
Manual or semi-automated machines can cost tens of thousands of dollars and are suitable for smaller-scale or prototype production, whereas fully automated machines can cost hundreds of thousands of dollars and are essential for large-scale production.
For low-volume or non-recurring production runs, it is often more cost-effective to outsource the wire bonding process to an external manufacturer.
These service providers can offer more affordable solutions without the need for companies to invest in expensive wire bonding equipment.
Production Volume and Tooling Costs:
Wire bonding becomes more cost-efficient with larger production volumes. Although the initial tooling costs for wire bonding setups are fixed, the cost per unit decreases as production scales.
In high-volume production—such as hundreds of thousands to millions of units per year—COB designs can be more cost-effective than using standard packaged chips.
This is because COB eliminates the need for die packaging, reducing assembly costs and allowing for more compact designs with fewer components.
Example Cost Breakdown:
For a basic COB design with 1770 um x 1258 um die and 21 wire bonds, costs can vary significantly depending on the level of automation and production volume. Here’s an example breakdown for a small batch of 100 units:
- Wire bonding service and tooling charge: $500 (fixed);
- Wire bonding process (aluminum wedge bonds): $360;
- Bare die cost: $115 per unit;
- PCB with ENEPIG surface finish (50x50mm): $590;
- Packaging and shipping: $50.
Total costs for a production run of 100 units: $1,615. For higher production volumes, these costs drop significantly, making COB designs a more affordable choice for large-scale manufacturing.
The differences with traditional wire soldering
Wire bonding technology differs from traditional welding in several ways:
- Size: wire bonding employs extremely thin wires with diameters ranging from a few dozens to a few hundred microns, while traditional soldering wire typically has much larger diameters, around 0.5 – 1 mm.
- Materials: wire bonding uses gold, aluminum or copper wires, whereas traditional soldering wire is typically made from tin-lead alloys.
- Process: wire bonding requires specialized machinery applying controlled heat, pressure and ultrasonics, whereas traditional soldering is done manually or with soldering machines that melt the wire.
- Applications: wire bonding connects chip terminals during manufacturing, while traditional soldering usually joins components already mounted on PCBs.
- Reliability: wire bonding soldering must withstand the stresses of final production processes, demanding high precision and quality; traditional soldering usually doesn’t require such critical standards.
- Costs: wire bonding incurs higher costs due to raw materials and equipment, unlike traditional soldering, which is significantly cheaper.