The purpose of this report is to review the reliability issues between the gold wire and copper wire in thermosonic ball bonding. By comparing the advantage and disadvantage of both gold wire and copper wire to see if the copper wire can be used to fully replace the gold wire in thermosonic ball bonding.
(Russell Tobias, May 2015)
In IC packages, thermosonic bonding is widely used to create electrical connections between the die pad and lead frame. These interconnect are made using a combination of lower heat and pressure with ultrasonic energy, as shown in the diagram.
Thermosonic bonding is a process which requires the help of thermal, mechanical and ultrasonic energy to create an electrical connection to connect the contact pad to the lead frame. The wire is pressed against the hot surface with a low force and ultrasonic force that vibrated for a period of time to achieve the bond as shown in the picture.
There are two types of wires used in thermos-sonic ball bonding:
Gold Wire Bonding:
Firstly, a ball or sphere is formed on the end of the contact pad with an EFO (Electronic Flame Off). The flame-off was done with an open hydrogen flame which would rotate towards the end of the wire and melt it, creating a ball at the end of the wire.
(Small Precision Tools)
Copper Wire Bonding:
The copper wire bonding process is very similar to gold wire bonding as instead of gold wire; it is replaced by copper wire. Basically, it uses the same wire bonder equipment just with an additional minor hardware and software.
(Small Precision Tools)
Advantage & Disadvantage
Advantage of copper:
· Lower cost
· Strong mechanical properties
· Excellent ball neck strength
· Better Heat distribution
· Increase power ratings with thinner wire diameters
· Lower resistivity
· Very little void formation in the Al-Cu system
· Can be bonded onto bare Cu substrates
Disadvantage of copper:
· Cu is harder than both Al and Au, the risk of damaging the underlying pad and dielectrics.
· Easy to be oxidized in the air, requires additional processing tools or coating to prevent oxidation.
(Z.W. Zhong, 2009)
Poor bondability for stitch/wedge bonds
· Additional bonding parameters for using forming inert gas
Copper VS Gold
As shown in the table as compared to Gold, Copper has a better reliability and superior product performance in terms of electrical and thermal conductivity. As Copper has better product reliability due to slower intermetallic growth that causes voids in between contacts and higher break load during wire pull testing.
(Preeti Chauhan, Z.W. Zhong and Michael Pecht, 2009)
Since Copper wires conduct heat faster and better than Au, they allow for a shorter heat-affected zone (HAZ). As when a very high current and temperatures are reached during electronic flame-off (EFO) firing, the heat generated during free air ball (FAB) formation will promote grain growth along the wire. It is well known that grain growth is undesirable for wire bond reliability. Thus we can conclude that copper wire is better as compared to Gold.
In conclusion, I personally think that using copper wire as the material for thermosonic ball bonding is a better option. Firstly, due to the rising cost of Gold, Copper is a cheaper choice.
Secondly, as Copper has better mechanical and electrical properties, and better interfacial reliability of Copper with Aluminium pads are the primary reasons for replacing Gold with Copper.
As mention previously copper wire have many advantages as compared to wire bonding using gold wire. Even though there are disadvantages, which brings many new challenges to wire bonding. I personally think that if companies can solve the problems and challenges faced by using copper wire bonding. Then they can fully benefit from Copper’s many advantages and at the same time save the cost.
(Z.W. Zhong, 2009)
Russell Tobias, May 2015, Joining Methods in Electronics Packaging: Sintered Silver and Eutectic Bonding, https://www.researchgate.net/figure/282330752_fig1_Figure-12-Thermosonic-gold-ball-wire-bonding-schematic
Z.W. Zhong, 2009, Wire bonding using copper wire, http://www.ntu.edu.sg/home/mzwzhong/Wire%20bonding%20using%20copper%20wire.pdf
Small Precision Tools, Copper Wire Bonding Process, http://www.smallprecisiontools.com/products-and-solutions/chip-bonding-tools/bonding-capillaries/technical-guide/basics-of-ball-bonding-process/copper-wire-bonding-process/?oid=1260=en
Preeti Chauhan, Z.W. Zhong and Michael Pecht, 2009, Copper Wire Bonding Concerns and Best Practices, http://www.ntu.edu.sg/home/mzwzhong/Copper%20Wire%20Bonding%20Concerns%20and%20Best%20Practices.pdf