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‘Understanding the Importance of Precision’
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Wirebonding in microelectronics
Chris Otter, TWI Ltd
The term “wire bonding” is generally accepted to mean the interconnection (via wire) of components and conducting tracks. The most frequently used method of joining the wires is ultrasonic welding.
In the late 1950s, interconnections were made using thermocompression bonding, where heat and force produce a solid phase diffusion bond. The addition of ultrasonic vibration (eg 60kHz) increased the reliability of the process and the technique is now used almost exclusively in microelectronics production.
The process uses a combination of vibration and force to effectively scrub the interface between wire and substrate, causing a very localised temperature rise, promoting the diffusion of molecules across the boundary. In the case of gold wire, substrate heating (100 to 150°C) is usually added to further encourage the migration of materials with reduced material deformation, this variation of the process being known as thermosonic bonding.
There are two basic types of wire bonds, ball/wedge and wedge/wedge, these descriptions relate to the geometry of the wire at the point where it is welded to the chip or substrate.
Ball/Wedge bonding
The machine used to make ball/wedge bonds incorporates a “flame-off” arm. At the end of each full bonding cycle an arc is struck between the arm and the end of the wire, producing a ball. The ball is deformed as it is welded to the IC, the second bond (known as the wedge, capillary-wedge, termination bond or stitch bond) is then made to the substrate. The figure below shows a schematic representation of the geometry of a ball/wedge bond.
Wedge/Wedge bonding
As the wire, in wedge/wedge bonding, is fed beneath a flat bonding face of the wedge tool, the second bond must be placed in-line with the first to ensure correct location beneath the tool. This effectively slows down the rate of bonding, as the operator (in the case of a manual machine) must orientate the substrate to maintain the bonding direction. The figure below shows a schematic representation of the geometry of a wedge/wedge bond.
A good wedge bond can be determined (in most cases) by the degree of deformation of the wire. The width of the flattened area should be approximately 1.7 x the diameter of the wire. eg for 25m the width should be 42m.
Selection of the right wire bonding process will be based on factors such as materials, product working environment (eg elevated temperature) and required production speed. Due to the restriction in the geometry of wedge/wedge bonding ie the two welds must be in-line with each other, wedge/wedge bonding is a slower process (typically 5 wires/second) than ball/wedge (typically 10 wires/second).
Wire bonds are found in any device which incorporates integrated circuits (ICs); eg automotive engine management systems, computers, missile guidance systems and medical instruments, plus the machines used to manufacture them. The continued increase in the number of IC driven or monitored devices/products has meant that ultrasonic wire bonding is probably the most widely used welding process in the world, with more than 4 x 1012 wires bonded per year. Most are used in the 40 to 50 billion ICs produced annually.
Wire bond interconnections can be made using gold, aluminium and copper wires, each material having different bonding characteristics and therefore requiring alteration in bonding parameters and equipment.
Wire bonding can be split into different technique variations based on the geometry of the interconnection type:
Gold and aluminium wires of up to 50µm in diameter are considered as “fine” wire, 25µm diameter is typically used for logic devices, but wires as small as 7µm have been successfully bonded.
Wires above 50µm diameter are considered as “heavy” wire and require different bonding equipment capable of applying higher bonding load than those used for fine wire. Heavy wire bonds are used where higher current levels are required and are almost exclusively aluminium.
Rectangular section gold ribbon (eg 30 x 12µm) can be used in place of wire. The characteristics of ribbon make it suited to high frequency circuits such as those used in RF telecommunications systems. The lateral spread of ribbon during the bonding process is less than that of round section wire, giving it potential for very fine pitch applications. Standard fine wire bonding equipment can be used, but specially designed bonding tools are required.
When all process variables are under control, ultrasonic and thermosonic wire bonding can be a highly reliable manufacturing process. There are a number of basic requirements necessary for successful and consistent wire bonding:
• Ultrasonics. The success of the process relies upon the efficient transfer of the ultrasonic vibration form the transducer to the bonding tool. Therefore the tool must be set using the correct height gauge and securely held in the transducer arm using the correct grub screw tightened to the correct torque.
• Clamping. The substrate must be securely clamped to the work stage. Any movement will dampen the interfacial scrubbing action
• Material condition. Variations in material quality are the prime cause of failure. Inspection, storage and handling of components and bond wire must be carefully controlled to avoid contamination.
Further information
Chris Otter is a Senior Project Leader in the Microtechnology Group at TWI Ltd.
TWI is one of Europe’s largest independent research and technology organisations. It has been actively involved in electronic packaging for over 40 years. The development of wire bonding technology has continued through this period with work being conducted on a wide range of materials (eg Au, Al, Cu, Pt, insulated wires) and sizes (eg 6µm – 500µm diameter).
If you require further information on this article or electronics packaging, please contact:
Chris Otter chris.otter@twi.co.uk +44 (0)1223 899000
Norman Stockham norman.stockham@twi.co.uk +44 (0)1223 899000
TWI Ltd, Granta Park, Great Abington, Cambridge CB21 6AL UK
www.twi.co.uk/electronicsandsensors
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