Wire bonding

Wire bonding is generally considered the most cost-effective and flexible interconnect technology and is used to assemble the vast majority of semiconductor packages.

[1] Bondwires usually consist of one of the following materials: Wire diameters start from under 10 μm and can be up to several hundred micrometres for high-powered applications.

While possessing higher thermal and electrical conductivity than gold, copper had previously been seen as less reliable due to its hardness and susceptibility to corrosion.

[6] Copper wire has the ability of being used at smaller diameters providing the same performance as gold without the high material cost.

Annealing and process steps used by manufacturers enhance the ability to use large diameter copper wire to wedge bond to silicon without damage occurring to the die.

Copper wire does pose some challenges in that it is harder than both gold and aluminium, so bonding parameters must be kept under tight control.

[7] Special packaging is required in order to protect copper wire and achieve a longer shelf life.

[5] Long-term corrosion effects (Cu2Si) and other stability topics led to increased quality requirements when used in automotive applications.

[11] Pure gold wire doped with controlled amounts of beryllium and other elements is normally used for ball bonding.

All-aluminium systems in semiconductor fabrication eliminate the "purple plague" (brittle gold-aluminium intermetallic compound) sometimes associated with pure gold bonding wire.

One of the most important characteristics of high grade bonding wire of this type is homogeneity of the alloy system.

Microscopic checks of the alloy structure of finished lots of 1% silicon-aluminium wire are performed routinely.

Processing also is carried out under conditions which yield the ultimate in surface cleanliness and smooth finish and permits entirely snag-free de-reeling.

The correct combination of temperature and ultrasonic energy is used in order to maximize the reliability and strength of a wire bond.

Compliant bonding[12] transmits heat and pressure through a compliant or indentable aluminium tape and therefore is applicable in bonding gold wires and the beam leads that have been electroformed to the silicon integrated circuit (known as the beam leaded integrated circuit).

Much work has been done to characterize various metal systems, review critical manufacturing parameters, and identify typical reliability issues that occur in wire bonding.

For example, a high current device for a space application might require a large diameter aluminium wire bond in a hermetically sealed ceramic package.

This leads to low cycle fatigue due to shear or tensile stresses in the wire bond.

Proper understanding of the use environment and metal systems are often the most important factors for increasing wire bond reliability.

While there are some wire bond pull and shear testing techniques such as MIL-STD-883, ASTM F459-13, and JESD22-B116,[17][18][19][20] these tend to be applicable for manufacturing quality rather than reliability.