Wire bonding

Last updated
Gold wire ball-bonded on a silicon die 07R01.jpg
Gold wire ball-bonded on a silicon die
Aluminium wires wedge-bonded to a BC160 transistor die BC160.jpg
Aluminium wires wedge-bonded to a BC160 transistor die
The interconnections in a power package are made using thick (250 to 400 mm), wedge-bonded, aluminium wires. Wirebonding2.svg
The interconnections in a power package are made using thick (250 to 400 μm), wedge-bonded, aluminium wires.
Inside a wire-bonded BGA package. This package has an Nvidia GeForce 256 GPU. NVIDIA@220nm@Fixed-pipline@NV10@GeForce 256@T5A3202220008 S1 Taiwan A DSC01376 (29588383793).jpg
Inside a wire-bonded BGA package. This package has an Nvidia GeForce 256 GPU.

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. [1]

Contents

Materials

Bondwires usually consist of one of the following materials:

Wire diameters start from under 10 μm and can be up to several hundred micrometres for high-powered applications.

The wire bonding industry is transitioning from gold to copper. [2] [3] [4] This change has been instigated by the rising cost of gold and the comparatively stable, and much lower, cost of copper. While possessing higher thermal and electrical conductivity than gold, copper had previously been seen as less reliable due to its hardness and susceptibility to corrosion. By 2015, it is expected that more than a third of all wire bonding machines in use will be set up for copper. [5]

Copper wire has become one of the preferred materials for wire bonding interconnects in many semiconductor and microelectronic applications. Copper is used for fine wire ball bonding in sizes from 10 micrometers (0.00039 in) up to 75 micrometers (0.003 in). [6] Copper wire has the ability of being used at smaller diameters providing the same performance as gold without the high material cost. Smaller diameters are possible due to copper's higher electrical conductivity. Copper wire bonds are at least as reliable if not more reliable than gold wire bonds. [7]

Copper wire up to 500 micrometers (0.02 in) [8] can be successfully wedge bonded. Large diameter copper wire can and does replace aluminium wire where high current carrying capacity is needed or where there are problems with complex geometry. Annealing and process steps used by manufacturers enhance the ability to use large diameter copper wire to wedge bond to silicon without damage occurring to the die.

Copper wire does pose some challenges in that it is harder than both gold and aluminium, so bonding parameters must be kept under tight control. The amount of power used during ultrasonic bonding must be higher [9] and copper has a higher fusing current so it has a higher current carrying capacity. [10] The formation of oxides is inherent with this material, so storage and shelf life are issues that must be considered. [7] Special packaging is required in order to protect copper wire and achieve a longer shelf life. Palladium coated copper wire is a common alternative which has shown significant resistance to corrosion, albeit at a higher hardness than pure copper and a greater price, though still less than gold. During the fabrication of wire bonds, copper wire, as well as its plated varieties, must be worked in the presence of forming gas [95% nitrogen and 5% hydrogen] or a similar anoxic gas in order to prevent corrosion. A method for coping with copper's relative hardness is the use of high purity [5N+] varieties. [5]

Long-term corrosion effects (Cu2Si) and other stability topics led to increased quality requirements when used in automotive applications [11]

Red-green-blue surface mount LED package with gold wire bonding details Very small 1.6x1.6x0.35 mm RGB Surface Mount LED EAST1616RGBA2.jpg
Red–green–blue surface mount LED package with gold wire bonding details

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 most common approach in thermosonic bonding is to ball-bond to the chip, then stitch-bond to the substrate. Very tight controls during processing enhance looping characteristics and eliminate sagging.

Junction size, bond strength and conductivity requirements typically determine the most suitable wire size for a specific wire bonding application. Typical manufacturers make gold wire in diameters from 8 micrometers (0.00031 in) and larger. Production tolerance on gold wire diameter is +/-3%.

Alloyed aluminium wires are generally preferred to pure aluminium wire except in high-current devices because of greater drawing ease to fine sizes and higher pull-test strengths in finished devices. Pure aluminium and 0.5% magnesium-aluminium are most commonly used in sizes larger than 100 micrometers (0.0039 in).

All-aluminium systems in semiconductor fabrication eliminate the "purple plague" (brittle gold-aluminium intermetallic compound) sometimes associated with pure gold bonding wire. Aluminium is particularly suitable for thermosonic bonding.

In order to assure that high quality bonds can be obtained at high production speeds, special controls are used in the manufacture of 1% silicon-aluminium wire. One of the most important characteristics of high grade bonding wire of this type is homogeneity of the alloy system. Homogeneity is given special attention during the manufacturing process. Microscopic checks of the alloy structure of finished lots of 1% silicon-aluminium wire are performed routinely. Processing also is carried out under conditions which yield the ultimate in surface cleanliness and smooth finish and permits entirely snag-free de-reeling.

Attachment techniques

Demonstration of ultrasonic wedge bonding of an aluminium wire between gold electrodes on a printed circuit board and gold electrodes on a sapphire substrate, reverse bonding order

The main classes of wire bonding:

Ball bonding usually is restricted to gold and copper wire and usually requires heat. For wedge bonding, only gold wire requires heat. Wedge bonding can use large diameter wires or wire ribbons for power electronics application. Ball bonding is limited to small diameter wires, suitable for interconnect application.

In either type of wire bonding, the wire is attached at both ends using a combination of downward pressure, ultrasonic energy, and in some cases heat, to make a weld. Heat is used to make the metal softer. The correct combination of temperature and ultrasonic energy is used in order to maximize the reliability and strength of a wire bond. If heat and ultrasonic energy is used, the process is called thermosonic bonding.

In wedge bonding, the wire must be drawn in a straight line according to the first bond. This slows down the process due to time needed for tool alignment. Ball bonding, however, creates its first bond in a ball shape with the wire sticking out at the top, having no directional preference. Thus, the wire can be drawn in any direction, making it a faster process.

Compliant bonding [12] transmits heat and pressure through a compliant or indentable aluminium tape and therefore is applicable in bonding gold wires and the beam leads that have been electroformed to the silicon integrated circuit (known as the beam leaded integrated circuit).

Manufacturing and reliability challenges

There are multiple challenges when it comes to wire bond manufacturing and reliability. These challenges tend of be a function of several parameters such as the material systems, bonding parameters, and use environment. Different wire bond-bond pad metal systems such as Aluminium-Aluminium (Al-Al), Gold-Aluminium (Au-Al), and Copper-Aluminium (Cu-Al) require different manufacturing parameters and behave differently under the same use environments.

Wire bond manufacturing

Much work has been done to characterize various metal systems, review critical manufacturing parameters, and identify typical reliability issues that occur in wire bonding. [13] [14] When it comes to material selection, the application and use environment will dictate the metal system. Often the electrical properties, mechanical properties, and cost are taken into account when making a decision. For example, a high current device for a space application might require a large diameter aluminium wire bond in a hermetically sealed ceramic package. If cost is a large constraint, then avoiding gold wire bonds may be a necessity. Some recent work has been done to look at copper wire bonds in automotive applications. [15] This is only a small sampling, as there is a vast body of work reviewing and testing what material systems work best in different applications.

From a manufacturing perspective, the bonding parameters play a critical role in bond formation and bond quality. Parameters such bond force, ultrasonic energy, temperature, and loop geometry, to name a few, can have a significant effect on bond quality. There are various wire bonding techniques (thermosonic bonding, ultrasonic bonding, thermocompression bonding) and types of wire bonds (ball bonding, wedge bonding) that affect susceptibility to manufacturing defects and reliability issues. Certain materials and wire diameters are more practical for fine pitch or complex layouts. The bond pad also plays an important role as the metallization and barrier layer(s) stackup will impact the bond formation.

Typical failure modes that result from poor bond quality and manufacturing defects include: fracture at the ball bond neck, heel cracking (wedge bonds), pad liftoff, pad peel, overcompression, and improper intermetallic formation. A combination of wire bond pull/shear testing, nondestructive testing, and destructive physical analysis (DPA) can be used to screen manufacturing and quality issues.

Wire bond reliability

While wirebond manufacturing tends to focus on bond quality, it often does not account for wearout mechanisms related to wire bond reliability. In this case, an understanding of the application and use environment can help prevent reliability issues. Common examples of environments that lead to wire bond failures include elevated temperature, humidity, and temperature cycling. [16]

Under elevated temperatures, excessive intermetallics (IMC) growth can create brittle points of fracture. Much work that has been done to characterize the intermetallic formation and aging for various metal systems. This not a problem in metal systems where the wire bond and bond pad are the same material such as Al-Al. This does become a concern in dissimilar metal systems. One of the most well known examples is the brittle intermetallics formed in gold-aluminium IMCs such as purple plague. Additionally, diffusion related issues, such as Kirkendall voiding and Horsting voiding, can also lead to wire bond failures.

Under elevated temperature and humidity environments, corrosion can be a concern. This is most common in Au-Al metal systems and is driven by galvanic corrosion. The presence of halides such as chlorine can accelerate this behavior. This Au-Al corrosion is often characterized with Peck's law for temperature and humidity. This is not as common in other metal systems.

Under temperature cycling, thermomechanical stress is generated in the wire bond as a result of coefficient of thermal expansion (CTE) mismatch between the epoxy molding compound (EMC), the leadframe, the die, the die adhesive, and the wire bond. This leads to low cycle fatigue due to shear or tensile stresses in the wire bond. Various fatigue models have been used to predict the fatigue life of wire bonds under such conditions.

Proper understanding of the use environment and metal systems are often the most important factors for increasing wire bond reliability.

Testing

While there are some wire bond pull and shear testing techniques such as MIL-STD-883, ASTM F459-13, and JESD22-B116, [17] [18] [19] [20] these tend to be applicable for manufacturing quality rather than reliability. They are often monotonic overstress techniques, where peak force and fracture location are the critical outputs. In this case the damage is plasticity dominated, and does not reflect some wearout mechanisms that might be seen under environmental conditions.

Wire pull testing applies an upward force under the wire, effectively pulling it away from the substrate or die. [21] The purpose of the test is as MIL-STD-883 2011.9 describes it: "To measure bond strengths, evaluate bond strength distributions, or determine compliance with specified bond strength requirements". A wire can be pulled to destruction, but there are also non-destructive variants whereby one tests whether the wire can withstand a certain force. Non-destructive test methods are typically used for 100% testing of safety critical, high quality and high cost products, avoiding damage to the acceptable wired bonds tested.

The term wire pull usually refers to the act of pulling a wire with a hook mounted on a pull sensor on a bond tester. However, to promote certain failure modes, wires can be cut and then pulled by tweezers, also mounted on a pull sensor on a bond tester. Usually wires up to 75 μm diameter (3 mil) are classified as thin wire. Beyond that size, we speak about thick wire testing.

See also

Related Research Articles

<span class="mw-page-title-main">Solder</span> Alloy used to join metal pieces

Solder is a fusible metal alloy used to create a permanent bond between metal workpieces. Solder is melted in order to wet the parts of the joint, where it adheres to and connects the pieces after cooling. Metals or alloys suitable for use as solder should have a lower melting point than the pieces to be joined. The solder should also be resistant to oxidative and corrosive effects that would degrade the joint over time. Solder used in making electrical connections also needs to have favorable electrical characteristics.

The Kirkendall effect is the motion of the interface between two metals that occurs due to the difference in diffusion rates of the metal atoms. The effect can be observed for example by placing insoluble markers at the interface between a pure metal and an alloy containing that metal, and heating to a temperature where atomic diffusion is reasonable for the given timescale; the boundary will move relative to the markers.

<span class="mw-page-title-main">Ball bonding</span>

Ball bonding is a type of wire bonding, and is the most common way to make the electrical interconnections between a bare silicon die and the lead frame of the package it is placed in during semiconductor device fabrication.

<span class="mw-page-title-main">Flip chip</span> Technique that flips a microchip upside down to connect it

Flip chip, also known as controlled collapse chip connection or its abbreviation, C4, is a method for interconnecting dies such as semiconductor devices, IC chips, integrated passive devices and microelectromechanical systems (MEMS), to external circuitry with solder bumps that have been deposited onto the chip pads. The technique was developed by General Electric's Light Military Electronics Department, Utica, New York. The solder bumps are deposited on the chip pads on the top side of the wafer during the final wafer processing step. In order to mount the chip to external circuitry, it is flipped over so that its top side faces down, and aligned so that its pads align with matching pads on the external circuit, and then the solder is reflowed to complete the interconnect. This is in contrast to wire bonding, in which the chip is mounted upright and fine wires are welded onto the chip pads and lead frame contacts to interconnect the chip pads to external circuitry.

<span class="mw-page-title-main">Integrated circuit packaging</span> Final stage of semiconductor device fabrication

Integrated circuit packaging is the final stage of semiconductor device fabrication, in which the die is encapsulated in a supporting case that prevents physical damage and corrosion. The case, known as a "package", supports the electrical contacts which connect the device to a circuit board.

<span class="mw-page-title-main">Ultrasonic welding</span> Welding process

Ultrasonic welding is an industrial process whereby high-frequency ultrasonic acoustic vibrations are locally applied to work pieces being held together under pressure to create a solid-state weld. It is commonly used for plastics and metals, and especially for joining dissimilar materials. In ultrasonic welding, there are no connective bolts, nails, soldering materials, or adhesives necessary to bind the materials together. When used to join metals, the temperature stays well below the melting point of the involved materials, preventing any unwanted properties which may arise from high temperature exposure of the metal.

<span class="mw-page-title-main">Intermetallic</span> Type of metallic alloy

An intermetallic is a type of metallic alloy that forms an ordered solid-state compound between two or more metallic elements. Intermetallics are generally hard and brittle, with good high-temperature mechanical properties. They can be classified as stoichiometric or nonstoichiometic intermetallic compounds.

<span class="mw-page-title-main">Gold plating</span> Coating an object with a thin layer of gold

Gold plating is a method of depositing a thin layer of gold onto the surface of another metal, most often copper or silver, by chemical or electrochemical plating. Plating refers to modern coating methods, such as the ones used in the electronics industry, whereas gilding is the decorative covering of an object with gold, which typically involve more traditional methods and much larger objects.

<span class="mw-page-title-main">Tape-automated bonding</span> Places a microchip on a flexible circuit board

Tape-automated bonding (TAB) is a process that places bare semiconductor chips (dies) like integrated circuits onto a flexible circuit board (FPC) by attaching them to fine conductors in a polyamide or polyimide film carrier. This FPC with the die(s) can be mounted on the system or module board or assembled inside a package. Typically the FPC includes from one to three conductive layers and all inputs and outputs of the semiconductor die are connected simultaneously during the TAB bonding. Tape automated bonding is one of the methods needed for achieving chip-on-flex (COF) assembly and it is one of the first roll-to-roll processing type methods in the electronics manufacturing.

<span class="mw-page-title-main">Aluminium alloy</span> Alloy in which aluminium is the predominant metal

An aluminium alloy (UK/IUPAC) or aluminum alloy is an alloy in which aluminium (Al) is the predominant metal. The typical alloying elements are copper, magnesium, manganese, silicon, tin, nickel and zinc. There are two principal classifications, namely casting alloys and wrought alloys, both of which are further subdivided into the categories heat-treatable and non-heat-treatable. About 85% of aluminium is used for wrought products, for example rolled plate, foils and extrusions. Cast aluminium alloys yield cost-effective products due to the low melting point, although they generally have lower tensile strengths than wrought alloys. The most important cast aluminium alloy system is Al–Si, where the high levels of silicon (4–13%) contribute to give good casting characteristics. Aluminium alloys are widely used in engineering structures and components where light weight or corrosion resistance is required.

Reliability of a semiconductor device is the ability of the device to perform its intended function during the life of the device in the field.

A semiconductor package is a metal, plastic, glass, or ceramic casing containing one or more discrete semiconductor devices or integrated circuits. Individual components are fabricated on semiconductor wafers before being diced into die, tested, and packaged. The package provides a means for connecting it to the external environment, such as printed circuit board, via leads such as lands, balls, or pins; and protection against threats such as mechanical impact, chemical contamination, and light exposure. Additionally, it helps dissipate heat produced by the device, with or without the aid of a heat spreader. There are thousands of package types in use. Some are defined by international, national, or industry standards, while others are particular to an individual manufacturer.

Thermosonic bonding is widely used to wire bond silicon integrated circuits into computers. Alexander Coucoulas was named "Father of Thermosonic Bonding" by George Harman, the world's foremost authority on wire bonding, where he referenced Coucoulas's leading edge publications in his book, Wire Bonding In Microelectronics. Owing to the well proven reliability of thermosonic bonds, it is extensively used to connect the central processing units (CPUs), which are encapsulated silicon integrated circuits that serve as the "brains" of today's computers.

Thermocompression bonding describes a wafer bonding technique and is also referred to as diffusion bonding, pressure joining, thermocompression welding or solid-state welding. Two metals, e.g. gold-gold (Au), are brought into atomic contact applying force and heat simultaneously. The diffusion requires atomic contact between the surfaces due to the atomic motion. The atoms migrate from one crystal lattice to the other one based on crystal lattice vibration. This atomic interaction sticks the interface together. The diffusion process is described by the following three processes:

<span class="mw-page-title-main">Failure of electronic components</span> Ways electronic components fail and prevention measures

Electronic components have a wide range of failure modes. These can be classified in various ways, such as by time or cause. Failures can be caused by excess temperature, excess current or voltage, ionizing radiation, mechanical shock, stress or impact, and many other causes. In semiconductor devices, problems in the device package may cause failures due to contamination, mechanical stress of the device, or open or short circuits.

<span class="mw-page-title-main">Copper conductor</span> Electrical wire or other conductor made of copper

Copper has been used in electrical wiring since the invention of the electromagnet and the telegraph in the 1820s. The invention of the telephone in 1876 created further demand for copper wire as an electrical conductor.

<span class="mw-page-title-main">Alexander Coucoulas</span> American inventor, engineer and author

Alexander Coucoulas is an American inventor, research engineer, and author. He was named "father of thermosonic bonding" by George Harman, the world's foremost authority on wire bonding, where he referenced Coucoulas's leading edge publications in his book, Wire Bonding In Microelectronics. A thermosonic bond is formed using a set of parameters which include ultrasonic, thermal and mechanical (force) energies.

<span class="mw-page-title-main">Compliant bonding</span>

Compliant bonding is used to connect gold wires to electrical components such as integrated circuit "chips". It was invented by Alexander Coucoulas in the 1960s. The bond is formed well below the melting point of the mating gold surfaces and is therefore referred to as a solid-state type bond. The compliant bond is formed by transmitting heat and pressure to the bond region through a relatively thick indentable or compliant medium, generally an aluminum tape.

Wedge bonding is a kind of wire bonding which relies on the application of ultrasonic power and force to form bonds. It is a popular method and is commonly used in the semiconductor industry. Wedge bonding is directional, so the bonding head rotates to accommodate the different angles for bonding. Due to this rotation, wedge bonding is slower than ball bonding. The advantage of wedge bonding is a finer pitch is possible. Wedge bonding also accommodates the use of metal ribbon instead of wire for bonding.

A bond tester is a scientific instrument used to measure the mechanical strength of bonds, evaluate bond strength distributions or determine compliance with specified bond strength requirements of the applicable acquisition document.

References

  1. ↑ V. Valenta et al., "Design and experimental evaluation of compensated bondwire interconnects above 100 GHz", International Journal of Microwave and Wireless Technologies, 2015.
  2. ↑ "K&S - ACS Pro". www.kns.com.
  3. ↑ Mokhoff, Nicolas (March 26, 2012). "Red Micro Wire encapsulates wire bonding in glass". EE Times . San Francisco: UBM plc. ISSN   0192-1541. OCLC   56085045. Archived from the original on March 20, 2014. Retrieved March 20, 2014.
  4. ↑ "Product Change Notification - CYER-27BVXY633". microchip.com. August 29, 2013. Archived from the original on March 20, 2014. Retrieved March 20, 2014.
  5. 1 2 Chauhan, Preeti; Choubey, Anupam; Zhong, ZhaoWei; Pecht, Michael (2014). Copper Wire Bonding (PDF). New York: Springer. ISBN   978-1-4614-5760-2. OCLC   864498662.
  6. ↑ "Heraeus Bonding Wires for Semiconductor Technology catalogue" (PDF). Heraeus.
  7. 1 2 Khoury, S.L.; Burkhard, D.J.; Galloway, D.P.; Scharr, T.A. (1990). "A comparison of copper and gold wire bonding on integrated circuit devices". pp. 768–776. doi:10.1109/ECTC.1990.122277. S2CID   111130335 . Retrieved 2024-02-06.
  8. ↑ Brökelmann, M.; Siepe, D.; Hunstig, M.; McKeown, M.; Oftebro, K. (October 26, 2015), Copper wire bonding ready for industrial mass production (PDF), retrieved January 30, 2016
  9. ↑ Breach, C. D. (2010). "What is the future of bonding wire? Will copper entirely replace gold?" (PDF). Gold Bulletin. 43 (3): 150–168. doi:10.1007/BF03214983. S2CID   137609854 . Retrieved 2024-02-06.
  10. ↑ "The Great Debate: Copper vs. Gold Ball Bonding | Semiconductor Digest".
  11. ↑ "AEC Q006" (PDF). www.aecouncil.com.
  12. ↑ A.Coucoulas, "Compliant Bonding" Proceedings 1970 IEEE 20th Electronic Components Conference, pp. 380-89, 1970. http://commons.wikimedia.org/wiki/File:CompliantBondingPublic_1-10.pdf https://www.researchgate.net/publication/225284187_Compliant_Bonding_Alexander_Coucoulas_1970_Proceeding_Electronic_Components_Conference_Awarded_Best_Paper
  13. ↑ Harman, George G. (2010). Wire Bonding in Microelectronics (3rd ed.). New York: McGraw-Hill. ISBN   978-0-07-164265-1. OCLC   609421363.
  14. ↑ S.K. Prasad, Advanced Wirebond Interconnection Technology. New York: Springer, 2004.
  15. ↑ Ensuring suitability of Cu wire bonded ICs for automotive applications
  16. ↑ Hillman, C., "Predicting and avoiding die attach, wire bond, and solder joint failures." International Symposium on 3D Power Electronics Integration and Manufacturing (3D-PEIM), 2016.
  17. ↑ MIL-STD-883: Test Method Standard for Microcircuits, Method 2011.7 Bond Strength (Destructive Bond Pull Test)
  18. ↑ MIL-STD-883: Test Method Standard for Microcircuits, Method 2023.5 Nondestructive Bond Pull
  19. ↑ "ASTM F459-13: Standard Test Methods for Measuring Pull Strength of Microelectronic Wire Bonds (Withdrawn 2023)". ASTM International. Retrieved 2024-04-22.
  20. ↑ JESD22-B116: Wire Bond Shear Test Method
  21. ↑ How to test bonds: How to Wire Pull? April 2016.

Resources

This page is based on this Wikipedia article
Text is available under the CC BY-SA 4.0 license; additional terms may apply.
Images, videos and audio are available under their respective licenses.