Wore Bonding is a method of connecting metal leads to the pad, that is, a technique of connecting internal and external chips.

Structurally, the metal leads act as a bridge between the chip’s pad (primary bonding) and the carrier pad (secondary bonding). In the early days, lead frames were used as carrier substrates, but with the rapid development of technology, PCBS are now increasingly used as substrates. The wire bonding connecting two independent pads, the lead material, bonding conditions, bonding position (in addition to connecting the chip and the substrate, but also connected to two chips, or two substrates) are very different.

1.Wire Bonding: Thermo-Compression/Ultrasonic/Thermosonic
There are three ways to attach the metal lead to the pad:

①Thermo-compression method, the welding pad and capillary splitter (similar to the capillary-shaped tool to move metal leads) by heating and compression method;
②Ultrasonic method, without heating, ultrasonic wave is applied to the capillary splitter for connection.
③Thermosonic is a combined method that uses both heat and ultrasound.
The first is the hot pressing bonding method, which heats the temperature of the chip pad to about 200 ° C in advance, and then increases the temperature of the capillary splicer tip to make it into a ball, and puts pressure on the pad through the capillary splicer, so as to connect the metal lead to the pad.
The second Ultrasonic method is to apply ultrasonic waves to a Wedge (similar to a capillary Wedge, which is a tool for moving metal leads, but does not form a ball) to achieve the connection of metal leads to the pad. The advantage of this method is low process and material cost; However, because the ultrasonic method replaces the heating and pressurization process with easily operated ultrasonic waves, the bonded tensile strength (the ability to pull and pull the wire after connecting) is relatively weak.
2.Material of bonding metal leads: Gold (Au)/Aluminum (Al)/Copper (Cu)
The material of the metal lead is determined according to the comprehensive consideration of various welding parameters and the combination of the most appropriate method. Typical metal lead materials are gold (Au), aluminum (Al) and copper (Cu).
Gold Wire has good electrical conductivity, chemical stability and strong corrosion resistance. However, the biggest disadvantage of the early use of aluminum wire is easy to corrode. And the hardness of the gold wire is strong, so it can form a ball well in the first bond, and can form a semicircular lead Loop (the shape formed from the first bond to the second bond) just right in the second bond.
Aluminum Wire is larger in diameter than gold wire, and the pitch is larger. Therefore, even if a high-purity gold wire is used to form a lead ring, it will not break, but pure aluminum wire is easy to break, so it will be mixed with some silicon or magnesium and other alloys. Aluminum wire is mainly used in high-temperature packaging (such as Hermetic) or ultrasonic methods where gold wire cannot be used.
Copper Wire is cheap, but too hard. If the hardness is too high, it is not easy to form a ball, and there are many limitations when forming a lead ring. Moreover, pressure should be applied to the chip pad during the ball bonding process, and if the hardness is too high, the film at the bottom of the pad will crack. In addition, there can be a “Peeling” of the securely connected pad layer.

However, because the metal wiring of the chip is made of copper, there is an increasing tendency to use copper wire. In order to overcome the shortcomings of copper wire, it is usually mixed with a small amount of other materials to form an alloy.