Purpose – This report attempts to review on the reliability issues between gold wire and copper wire
Design/methodology/approach – reviewed some reports and articles available on the web
Findings – Copper wire have better electrical, thermal properties and many more advantages but there are also some disadvantages like wire open and short tail defects.
· Introduction – What is thermos-sonic ball bonding?
· Types of wires use in bonding
· Wire materials (Au) and (Cu) on Current carrying capacity and 3D electrical parasitic parameters
· Advantages and disadvantages of copper wire
· Reliability of copper wire over gold wire
What is Thermo-sonic ball bonding?
Thermo-sonic wire bonding is an electrical interconnection technique using a combination of pressure and ultrasonic energy to heat up and produce a solid-state weld. The thermos-sonic ball bonding Is the most common method of establishing a connection between integrated circuits and substrates. In thermos-sonic ball bonding we usually use thin gold wire to bond between the components. Once the bonding is done there will be connections, this connection is called wire bonds. (SPT , n.d.)
There are two types of wires use in thermos-sonic ball bonding
1) Gold ball wire bonding process
Thermo-sonic ball bonding using gold ball wire bonding process uses a fine gold wire made of soft, face-centered-cubic metal(FCC) usually ranging from 18 micro meter to 33 micro meter in diameter is fed down through the capillary. The small incursions of ultrasonic energy at the tip of the capillary are transmitted to the Au ball and down to the Al bond pad to form the ball bond. After that the capillary lifts up and form the looping profile. This cycle is repeated until the device has finish bonding. (SPT , n.d.)
2) Copper ball wire bonding process
Thermo-sonic ball bonding using copper ball wire bonding process is very similar to gold wire bonding process as it basically uses the same wire bonder equipment with minor hardware and software retrofits. Just that instead of using gold wire it is replaced with copper wire. Copper wire has offered serval mechanical and electrical advantages as well as it is relatively cheaper compared to gold wire.
Wire material cost savings Copper(Cu) vs Gold (Au)
Reference from Integrated Circuit Packaging Assembly and Test (ICPAT 009894)
Advantages and disadvantages of using Cu in Thermo-Sonic ball bonding process. Table 2
Lower Cost compared to Gold wire
Easy to be oxidized in air
Better thermal properties
Additional bonding parameter for using forming inert gas
Better electrical properties
Needs more ultrasonic energy and higher bonding force, which can damage the Si substrate, form die cratering and induce cracking and peeling of the bond pad
Excellent ball neck strength
Lower strength of anneal wire resulting in breakage
High pressure that is on pad structures, which can contain low-k-materials having very low stiffness.
It can be bonded on bare Cu substrates
A larger strain-hardening effect at higher strain rate
High-loop stability and better looping control
Wire open and short tail defects
Very little void formation in the Al-Cu system
Poor bond ability for stich and wedge bonds
Reliability of Copper wire or Gold wire which is better?
Wire bonding using gold (Au) wire is still the most preferred interconnection for microelectronics packaging. But recently the price of gold has gone up hence it would be very costly to use gold on every semiconductor die. Since gold wire prices are increasing we must find cheaper alternatives such as using Copper(Cu). If you refer from table 1 the resistance of gold wire is higher compared to the resistivity of the Copper wire when you have higher resistance you will require more current to turn on the circuit and even more current to reach the peak voltage of the device.
Not only that the resistance of gold wire is slightly higher the Copper wire, that not the only disadvantage of Gold wire they also has higher electrical parasitic parameters compared to the copper wire which has a lower electrical parasitic parameter.
Example of 3D Electrical Parasitic parameters.
If we use copper wire as the wire material for thermos-sonic ball bonding the industry can save the material cost because it is relatively cheaper than gold wire, some of the parameters that copper wire have is lower cost, better thermal and mechanical properties and higher stiffness than gold, copper can also be bonded to bare Cu substrates.
Some issues on copper wire is copper wires can be easily oxidized compared to gold wires, copper wire also requires additional forming gas to prevent corrosion. Copper wire also require more ultrasonic energy and higher bonding force, these can easily damage the Silicon substrate.
The semiconductor industry should use copper wires to replace gold wires because this research has shown that using copper wire is better than gold wire as it offers many more advantages compared to gold wires. However, there are also issues on copper wires like it is easily oxidized, it is harder than gold and aluminum. Hence, need to control the bonding parameters of copper wire. Therefore, Researchers must deal with these disadvantages when using copper wires and find alternative solutions to resolve it.
Small Precision tools (SPT , n.d.) Basics of Ball Bonding process Retrieved June 17, 2017 From Small Precision tools: http://www.smallprecisiontools.com/products-and-solutions/chip-bonding-tools/bonding-capillaries/technical-guide/basics-of-ball-bonding-process/?oid=559&lang=en
Small Precision tools (SPT , n.d.) Gold Ball Wire Bonding process Retrieved June 17, 2017 From Small Precision tools: http://www.smallprecisiontools.com/products-and-solutions/chip-bonding-tools/bonding-capillaries/technical-guide/basics-of-ball-bonding-process/gold-ball-wire-bonding-Process/?oid=562&lang=en
NTU Singapore (Z.W.Zhong, 2009) Wire bonding using copper wire Retrieved June 17, 2017 From NTU Singapore: http://www.ntu.edu.sg/home/mzwzhong/Wire%20bonding%20using%20copper%20wire.pdf