Blog  /  Wire Bonding: An Affordable and Flexible Connection Technology for Semiconductor Packages

Wire Bonding: An Affordable and Flexible Connection Technology for Semiconductor Packages

Semiconductors and integrated circuits need electrical interconnections with their packaging during device fabrication. This interconnection also applies to connecting two printed circuit boards or ICs to other electronics.

Wire bonding is the most affordable and flexible connection technology for all these interconnections. Additionally, it can work at frequencies above 100 GHz. Therefore, it makes sense to use it in your project if you require such interconnections. We have looked at the technology in detail below, so take a look!

 

What is Wire Bonding?

 

Wire bonding is a solid phase welding process that joins a wire and pad surface (two metallic materials). The electrical interconnection technique uses a thin wire plus ultrasonic energy, pressure, and (or without) heat. It combines these to create intimate contact between the two materials.

 

A wire-bonded operational amplifier

A wire-bonded operational amplifier

Source: Wikimedia Commons

 

Types of Wire Bonding

 

  • Thermocompression Bonding: This process uses force, heat, and time to weld the two materials via inter-diffusion. The wire (heated in some cases) gets pressed against a hot surface (150°C or more). It occurs at high pressure for a limited period. But the process uses no friction and only works on a gold wire and a gold bond surface.
  • Ultrasonic Bonding: This requires pressure, ultrasonics, and time to weld the two materials. The process involves pressing the wire against the surface at ambient temperature. Additionally, it should be at a low force with vibration for a limited period to create the bond. You can use this method with aluminum, gold, silver, palladium, copper, or platinum (wire or ribbons). It has two forms: wedge and peg bonding.
  • Thermosonic Bonding: This bonding process uses force (pressure), heat, ultrasonics, and time to weld two materials. Usually heated in some cases, the wire gets pressed on the hot surface (150°C or less). It should occur at low pressure and undergo vibration for a limited period to create the bond. But like thermocompression bonding, the process only works with gold wire and a gold bonding surface. However, it has three forms: gold ball bonding, wedge bonding, and stud bumping (bump bonding).

 

Wire Bonding

 

Wire Bonding Forms

 

The following are the two primary wire bonding forms:

 

Ball Bonding

 

In ball bonding, the wire passes through a hollow capillary. After that, an EFO (Electronic Flame-Off) system melts the small portion extending outside. The surface tension of the molten wire forms a ball before the wire solidifies. Next, it gets pressed to the pad using sufficient force to cause plastic deformation. The result is an intimate bond with the bond pad surface.

 

Wedge Bonding

 

With wedge bonding, the capillary's bottom side squeezes the wire tab to the bonding pad at a 30-60° angle. After that, ultrasonic energy shapes the bond between the pad and wire. The capillary then moves to the second bond area and repeats the above process. After linking the two sections, the wire gets clipped and snapped.

 

A wedge bond

A wedge bond

Source: Wikimedia Commons

 

Wire Bonding Materials

 

The most commonly used materials for bonding wires are:

 

Gold Wire

 

Gold wire is the preferred material in thermosonic and thermocompression gold wire bonding. But the material must be clean and have a smooth surface finish to ensure sufficient bonding and no clogging along the capillary chamfer diameter.

However, ultra-pure gold is very soft. Therefore, small quantities of beryllium (5-10 ppm) or copper (30-100 ppm) get added to make the material workable. The former is stronger than the latter by 10-20%. Thus, it is ideal for rmosonic bonding (automated) because the quick capillary movements put stress on the wire.

 

A wire-bonded Motorola GM350 transceiver. Note the gold wires

A wire-bonded Motorola GM350 transceiver. Note the gold wires

Source: Wikimedia Commons

 

Aluminum Wire

 

Like gold, pure aluminum is also too soft to make a reliable wire. Therefore, the material gets alloyed using 1% silicon or magnesium for strengthening purposes. Aluminum with 1% magnesium is superior to the silicon alloy wire because it exhibits fatigue and strength degradation resistance after exposure to high temperatures. But Aluminum-silicon is stronger at room temperature and forms a more robust joint with the aluminum bond pad.

 

A wire-bonded bipolar PNP transistor. Note the aluminum wires

A wire-bonded bipolar PNP transistor. Note the aluminum wires

Source: Wikimedia Commons 

 

Copper Wire

 

Copper wire to bond pad metallization has become popular due to its affordability and sweep resistance during plastic encapsulation. However, copper is tougher than gold and aluminum, which means it can push the bond pad metallization aside. Also, the material oxidizes quickly, so you must carry out copper ball bonding in inert atmospheres.

 

A wire-bonded IC using copper wires

A wire-bonded IC using copper wires

Source: Wikimedia Commons

 

Wire Bonding Guideline

 

Developing wire bonds requires the following guidelines.

 

Process Optimization

 

Process optimization is essential when developing and improving production. The production process is usually a continuous loop that includes four steps, namely: design & development, characterization, control, and optimization. Process characterization and control stages are in a continuous loop in the middle. Meanwhile, optimization loops around the entire production process to improve each step.

The design and development stage usually includes the achievable goals and information required to set up the wire bonds mechanism. After that, characterization covers categorizing wire bonding failure data, such as wire breaking and bond off-center. Thirdly, control focuses on stabilization performance by regulating bonding parameters, bond-tool installation, wire pulling steps, etc.

Lastly, optimization focuses on improving all these steps to make the bonding process more efficient and reliable.

 

Wire Bonding Design

 

The design of a wire bonding mechanism should prevent failures associated with vibration fatigue, noise, wire flexure fatigue, interdiffusion, etc. Some inputs for this design process include the following:

  • Chip thickness and material
  • Bond pad thickness, material, pitch, etc.
  • Transistor conduction channel resistance
  • Output capacitive load (designed-for)
  • Max allowable interconnect resistance/length (wire diameter)
  • And more

 

Bond Evaluation

 

Bond evaluation occurs after the bonding process is complete. It can be a visual evaluation or a mechanical test of the bond patterns. The former involves evaluation using scanning electron microscopes, optical microscopes, and other similar instruments. On the other hand, mechanical testing involves using methods listed in the MIL-STD-883: test method standard. They include the following:

  • Delay measurements
  • Ball bond shear test
  • Random vibration
  • Internal visual
  • Stabilization bake
  • And more

 

Failure Mechanisms of Wire Bonds

 

A manual or commercial wire bonder can create imperfect wire bonds with a weak internal structure. Some typical failures include the following:

  • Cratering: Usually caused by over bonding. The damage occurs under the bond pad but can leave a hole in the substrate. Also, it can leave a divot fixated on the wire in severe cases.
  • Wirebond fracture/lifting: Lifting is when ball bonds detach from the silicon tabs or chips. Also, it refers to the ball bond not sticking to the bond pad.
  • Inconsistent tails: These failures can cause shorting with other traces, bonds, or bondable surfaces nearby.

 

The tail of a wedge bond

The tail of a wedge bond

Source: Wikimedia Commons

 

  • Improper positioning: This issue can lead to bad welds from the bond head or shorts.
  • Poor welding: Creates weak gold, aluminum, or copper ball bonding.

Pad cleanliness can help eliminate some of these faults, while failure shooting using a fishbone diameter can help isolate any arising issues when wire bonding.

 

Summary

 

As you can see, wire bonding is critical in making electrical interconnections for ICs or other semiconductor devices. The process can have failures, but you can minimize these by following the guidelines and using the appropriate material. If you have any questions, contact us for more details.