Wire Bonding: Turning Gold into Copper
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Wire bonding is used to assemble the vast majority of semiconductor packages, electrically connecting I/O bond pads on the IC to the corresponding bond pads on the package. Bond wires usually consist of conductor metals gold (Au), aluminum (Al), or copper (Cu). Gold is the most widely used because of its resistance to surface corrosion. However, the price of gold has risen significantly: it is about $1,600 a troy ounce at present while five years ago it was priced around $665 an ounce. Aluminum is presently used on discrete/power devices because of its current carrying capacity. However, aluminum has two major compromises: the lower productivity of wedge bonding compared to gold ball bonding and its lack of flexibility for complex wire schemes.
With the constant need to reduce the cost of IC packaging, this upward trend in the price of gold is driving conversion from gold to copper as the preferred material for wire bonding in many semiconductor applications. Overall, it is estimated that usage of copper wire has achieved over 20 percent market share and is expected to exceed 50 percent within the next two years. Products spanning the range from low pin count devices with relatively large wire diameters to FPGA’s with nearly a thousand wires and with wire diameters as small as 20 µm are now commonly using copper wire.
Apart from the cost factor, copper is 40 percent more conductive than gold, resulting in lower operating resistance. As a result, the switch can lead to higher product performance in digital, high-current mixed signal and, in particular, power applications. Power device manufacturers prefer copper wire over gold wire because copper enables an increase in the maximum allowable current and provides enhanced heat dissipation, meaning it is better at pulling heat away from the die, leading to better performance at elevated temperatures.
These advantages are especially evident when copper wire is bonded on die pads plated with thick copper and nickel palladium finish as in analog and power products, where support for high currents is essential to provide a safe electrical environment for the device and the rest of the system. For these reasons heavy copper wire, (> 2.0 mils in diameter), is already widely used in the industry today in products such as power MOSFETs.
Similar to the Pb-free soldering transition, however, such a replacement is not just a simple drop-in material change. While copper avoids some of the drawbacks of gold, the fundamental physical and chemical characteristics of copper wire are different from gold wire. These factors make copper wire bonding more challenging especially when implementing wire replacement in high-volume manufacturing of packages like quad flat no lead (QFN).
For the fine, thin pitch wiring that connects an integrated circuit’s die pads with its package leads, chip makers traditionally have used gold because it is easy to work with in manufacturing. Gold wire is bonded using ultrasonic bonding between the IC bond pad and the matching package pad. Ultrasonic energy allows the materials to plastically deform at much lower stress compared to pure thermal or mechanical energy.
Among the material differences that has to be taken into account, gold is relatively soft and non-reactive while copper is harder and more brittle, it is harder than aluminum as well, so bonding parameters must be kept under tighter control. The stiffness of copper means that the bonding process must use higher force. On the other hand, copper’s hardness means that bonds stay more stable mechanically in stressed operating environments.
The basic steps of ball bonding include the formation of 1) the first bond (normally on the chip), 2) the wire loop, and 3) the second bond (normally on the substrate). During formation, a high-voltage spark is used to melt the copper wire to form a ball and requires a non-reactive environment to prevent copper oxidation.
Once formed, the copper ball at the end of a wire is attached to an aluminum pad using a combination of ultrasonic energy, pressure (about 150 Megapascals, or MPa) and heat (150◦ C to 240◦C depending on the device). This process is known as thermosonic bonding because of the use of heat and ultrasonic energy. If care is not taken these conditions can lead to the risk of damaging bond pads and underlying circuitry and, hence, reduced reliability. Among other things aluminum splash can occur when the higher force used for copper causes the top aluminum layer of the pad to splash out beyond the ball footprint. The copper in the wire bonds may also diffuse into the Si chip and impose reliability threats to the electronic device.
As indicated a couple of paragraphs earlier, another formidable challenge of using copper is its reactivity with oxygen in the surrounding air. Copper oxidizes rapidly, hence the need for a reducing gas atmosphere to provide an inert environment that prevents oxidation during bonding and enables stronger bonds.
Once bonded under a protective reducing gas atmosphere, copper wire has greater mechanical properties resulting in excellent stability. As such the migration to copper has begun in earnest in logic and will soon be followed by MEMS and application in devices that demand assembly and operational specifications commonly characteristic of high-volume digital products, including precise, secure bonding of wires to aluminum pads.
Fine-pitch wire bonding with copper and palladium (Pd) coated copper wire has been in high volume manufacturing for several years. Texas Instruments (TI) recently announced it has shipped nearly 6.5 billion units of copper wire bonding technology in its analog, embedded processing and wireless products.
TI has demonstrated that copper wire bonding is a high volume production process across a wide variety of its products with the company claiming equal or better manufacturability compared to gold. TI began shipping copper wire in its products in 2008. Today, all seven of TI’s assembly and test (A/T) sites are running copper wire bonding production across a range of products and package types, including QFN packages, ball grid array packages such as nFBGA and PBGA; package-on-package (PoP); QFPs; TQFPs; SOICs; and others.
The cost benefits associated with converting to copper wire also are being sought for more advanced packages such as 3D packaging and System in Package (SiP) solutions.