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Wire bonding Failure Analysis: intermetallics, corrosion, and fatigue in varied metal systems

Consider a semiconductor chip that passes factory tests with flying colors. It is used in an electric car’s battery management system, working perfectly for a considerable period of time. However, occasional errors begin to manifest, leading to a total failure of the system. The problem? It is not with the intricate semiconductor design but rather due to a few wire bonds that have failed due to degradation.
 
Failures in wire bonding can be insidious in nature. Sometimes, the failure may not be detected during the initial testing phase, but the failure can occur catastrophically with massive recalls, especially in industries where the failure is related to safety, such as the automotive industry, space exploration, and medical equipment. Understanding the root causes of these failures is not just an academic exercise; it’s a crucial discipline for ensuring product longevity and reliability.
 
This paper progresses from the basics and introduces a systematic approach for understanding, analyzing, and preventing typical wire bonding failure modes and mechanisms.

Intermetallic Compounds (IMCs)

Intermetallic compounds refer to the crystalline material produced at the interface of two different metals during the bonding stage and later exposed to heat. Intermetallic compounds created by two different metals, such as Au-Al and Cu-Al, used in wire bonding, have a substantial effect on reliability. Gold and aluminum will increase the strength of the wire bonds formed with the substrate material. However, temperature and other parameters can create an intermetallic compound beyond the permissible range, giving rise to a decrease in performance, especially when the temperature is high.

3 Key Failure Mechanisms of IMC

Embrittlement and Fracture

The IMCs AuAl₂, Au₅Al₂, and Au₄Al exhibit natural hardness and brittleness characteristics. They do not possess ductility properties and cannot withstand stresses generated by the difference in the expansion coefficients of the materials, and so micro-cracks begin to form within the IMC interface area due to these brittle characteristics. [1]

Kirkendall Voiding

This is a subtler and potentially more hazardous situation. Because of a substantial difference in the diffusion rates of the two metals (for example, gold has a greater diffusion rate in aluminum than aluminum has), a buildup of vacancies occurs on the surface of the faster-diffusing metal. The vacancies develop into voids within the bond interface, as described below.
 
The increased resistance generated by a reduction in contact area can lead directly to an open circuit with little evidence of other functional degradation, because the path itself is so elevated in resistance due to those voids.[2]

The “Purple Plague” Revisited

The “purple plague” is a specific reference to the purple color of the compound AuAl₂. Although this is a strikingly distinctive color, the problem with the compound is actually its brittle characteristics and its relation to Kirkendall voids. It remains a classic case study in material incompatibility.

Factors Accelerating IMC Growth:

The growth of IMCs is a diffusion-controlled process and is mainly temperature and time-dependent according to the Arrhenius equation. The most important factors influencing the growth process are:
  • Temperature: This is the most important factor of all. Any temperature slightly above the design specification can create an exponential growth increase for IMCs.
  • Time: Long-term exposure to high temperature conditions, although these can be moderate.
  • Current Density: High currents can trigger electromigration that contributes significantly to the diffusion of atoms and the formation of voids.

Corrosion

Corrosion is the gradual destruction of metal because of the chemical reaction of the surroundings with the metal. Corrosion due to wire bonding may cause a lack of strength within the bond interface, increased electrical resistance, and open circuits. Some of the factors influencing corrosion include:
 
  • Environmental Conditions: Temperature, humidity, and the presence of corrosive gases (H₂S, SO₂, Cl₂).
  • Contaminants: Ionic residues of manufacturing processes such as Chlorides and Bromides.
  • Electrical Bias: Applied voltage can induce electrochemical migration and corrosion reactions.
  • Material Properties: The inherent corrosion-resistant qualities of the wire and pad materials being used.

Types of Corrosion in Wire Bonds

Understanding types of corrosion is imperative for semiconductor packaging engineers with regard to failure analysis and design for reliability (DFR).
This is a table showing the six most significant corrosion mechanisms in wire bonding with their respective causes and effects:
six key corrosion mechanisms relevant to wire bonding

Galvanic (Electrochemical) Corrosion:

  • Cause: This happens when two different metals, such as Aluminum and Gold, come into contact with each other in the presence of an electrolyte solution, such as moisture with ionic contamination. The metal with lower nobility will corrode.
  • Wire Bond Failure Mode: For a gold wire bonded onto an aluminum pad, the aluminum pad sacrificially corrodes, creating a non-conductive oxide layer. This leads to a rise in the resistance value with a consequent failure in the bond. This is a significant problem with aluminum pads and gold wires.

Pitting Corrosion:

  • Cause: A localized attack which is triggered by a partial loss of the oxide film on a metal surface, which is commonly caused by chloride ions.
  • Wire Bond Failure Mode: Copper wire bonds will fail due to the potential for a single type of chloride contaminant to create a pit on the wire, creating a stress riser from which a crack can be generated, leading eventually to a wire fracture with temperature cycles.

Crevice Corrosion:

  • Cause: A type of localized attack in a shielded and stagnant region, such as underneath a particle of contamination or in the poorly adhered region between the wire and mold compound.
  • Wire Bond Failure Mode: The entrapment of moisture within these micro-crevices leads to an acidic and corrosive environment. This is because the produced acidity leads to corrosion within the wire bond interface; thus, failure may occur due to a weak wire heel area, which is invisible during inspection.

Intergranular Corrosion (IGC):

  • Cause: Preferential corrosion along grain boundaries in a metal, often due to reactivity introduced by the segregation of impurities.
  • Wire Bond Failure Mode: This is a possibility for aluminum bond pads if the metallization technique is not fully optimized. The bond pad itself may degrade due to the IGC, leading to bond lift-off and pad cratering.

Stress Corrosion Cracking (SCC):

  • Cause: The effect of tensile forces coupled with a corrosive environment causes brittle cracks in a material to progress.
  • Wire Bond Failure Mode: SCC can be a concern, although a remote one, in prestressed wire bonds, especially in packages with high residual stresses due to the encapsulation material and process. With a corrodent environment present, a wire can fail suddenly with a brittle failure mode, preceded only by a slight elongation.

Uniform Corrosion:

  • Cause: A general chemical attack, such as oxidation, evenly distributed over the entire surface area of a metal.
  • Wire Bond Failure Mode: This can be seen in aluminum wires in the form of a gradual growth of aluminum oxide, which leads to an increase in the electrical resistance of the wires. And in copper wires in the form of a layer of copper oxide, which can also degrade conductivity and bondability.

Material Types & Their Corrosion Characteristics in Wire Bonds

The corrosion of wire bonds is caused by environmental factors such as moisture, temperature, and the presence of contaminants such as sulfur and chlorine.
Among metals, the performance will be different when in a corrosive environment; therefore, the selection of a metal over another is an important factor to be taken into consideration.
 
  • Gold (Au): This can be coupled with other aluminum metals that may be unstable. This is because these metals will eventually corrode the bond interface whenever they come into contact with moisture.
  • Aluminum (Al): This is prone to galvanic corrosion in the presence of moisture and other metals. The oxide scales can reduce the aluminum metal bond strength. The oxide surface scales will impact the electrical performance during bonding.
  • Copper (Cu): Copper has oxide properties and can be prone to sulfides in extreme conditions with moisture and sulfur in the atmosphere. Even if copper oxide is formed, sulfides may be other causes of bond failure, especially with moisture. The copper oxide is a problem with regard to reliability in the bond interface itself.
  • Silver (Ag): It is prone to tarnishing because of sulfur compound formation, resulting in the silver sulfide layer, thus leading to an increase in the resistance value of the bond. Since tarnishing can be rapid if the sulfur compound is high, restrictions need to be established within the testing environments.

 

MaterialKey AdvantagesMain ChallengesReliability Factors
AluminumCost-effective, easy bondingIntermetallics with Au, moderate corrosionMild corrosion, fatigue
GoldDuctile and corrosion-resistantExpensive, intermetallics with AlExcellent fatigue, low corrosion
CopperHigh conductivity, low costProne to corrosion, bonding difficultyHigh corrosion risk, hardening
SilverGood conductivity, promising reliabilityLimited data, tarnishingModerate corrosion, less proven fatigue

Mechanical and Thermal Fatigue

Fatigue failures occur due to cyclic stresses, which are well below the ultimate tensile strength of the material but still succeed in initiating and propagating cracks within the material.
 

Heel Crack

This is a classic example of a fatigue failure. The heel region of a wire bond, which is where the wire meets the ball or wedge, is a natural stress concentrator.
  • Root Cause: Relative movement is induced by the difference in coefficients of thermal expansion among the silicon chip, the bond wire, and the substrate due to the process of thermal cycles. This periodic movement causes cyclic bending stresses to act on the wire heel, ultimately leading to the formation of micro-cracks and a typical failure mode at the wire heel region.

Neck Break (Ball Bonds):

As with heel cracks, a secondary location with a cyclic fatigue tendency is the area where the wire leaves the ball bond, often referred to as the ‘neck’.
 

Cratering:

This is a fail mode with significant importance because the bond pad and silicon will be adversely affected. This is due to the excessive ultrasonic energy or bond force, which can cause the bond process to remove some of the silicon material from the substrate. The failure mode can be an immediate failure and can also be a latent failure.

Failure Analysis in Wire Bonding

For a systematic failure analysis, a structured approach is required. The below flowchart defines the procedure:
wire bonding failure analysis flowchart

Non-Destructive Analysis (NDA):

  • Electrical Test: It helps in determining the type of failure (open, high-resistance, shortcut) and in isolating the defective net/bond.
  • X-Ray Imaging: Internal defects like wire sweep, wire breaks, shorts, and ball/wedge irregularities can be detected without opening the package.
  • C-Mode Scanning Acoustic Microscopy (C-SAM): This uses ultrasonic inspection to look for interfacial delamination, evidence of internal cracks, and voids within the package, especially at interfaces involving mold compound and the chip or leadframe.

Destructive Physical Analysis (DPA):

  • Decapsulation: A process of removal of mold compound using chemicals/plasma in order to view the die and wire bonds.
  • Optical Microscopy: First visual inspection aimed at locating the failure area and determining its characteristics, such as discoloration, lifted bond, and wire breaks.
  • Scanning Electron Microscopy (SEM): It gives detailed and high-magnification images of the failure location, which is an important criterion for the study of failure by ductile versus brittle materials as well as microscopic void paths. [3]
  • Energy Dispersive X-ray Spectroscopy (EDS): Simultaneously used with SEM for elemental composition analysis in a chosen area. It is a key tool for verifying the existence of intermetallic components (Au-Al ratio), corrosion products (oxygen with aluminum, sulfur with silver), and ionic contaminants (chlorine and potassium).

Mitigation Techniques for Corrosion in Wire Bonding

There are a few methods available for improving wire bond reliability:

Material Selection and Matching:

  • Use other than Au-Al wire bonding in high-reliability, high-temperature applications. This may include aluminum-to-aluminum bonding and use of palladium-coated copper wire instead of bare copper wire because of its better protection against aluminum oxidation and corrosion.
  • Use mold compounds with low ionic contamination and low moisture absorption rates.

Process Optimization:

  • Create a strong process window regarding the bonding forces, energy, duration, and temperature so that a strong bond can be formed without any harm to the wafer and with minimal initial intermetallic compound growth.
  • Secure tight control of the manufacturing environment against ionic contamination and moisture exposure.

Protective Barriers:

Design for Reliability (DfR):

  • Optimize wire loop shapes to reduce heel bond stress.
  • It must satisfy the design rules of the bond pad so that a strong base for bonding is ensured.

Conclusion

The failure modes in wire bonding, namely intermetallic growth, corrosion, and fatigue failures, often occur simultaneously with each other. A failure with a small growth of the intermetallic compound may progress with corrosion and ultimately result in a fatigue failure.
 
Today, in the aggressive semiconductor industry, the need for a transition from a reactive failure analysis mode to a proactive ‘reliability by design’ approach is paramount.
 
Understanding failure mechanisms helps in design-level decision-making related to materials and wire bonding design well in advance of deployment. For primitive understanding related to wire bonding types, materials, and related processes, see the foundational article:

Wire Bonding: The Ultimate Guide to Process, Materials, and Design Choices

FAQs about Wire Bonding Failures

The reactivity of copper to oxygen and sulfur compounds is easily corroded. Copper bonds are particularly prone to corrosion, especially when they come in contact with environmental factors like humidity and dirt. Galvanic corrosion can occur if there is any addition of dissimilar metals.

Gold and copper are both commonly used wire bond materials for high-power and high-frequency applications based on their good conductivity. Silver also has limited usage as a high-frequency or high-power bond material due to its tendency to tarnish. However, materials would ultimately need to be sourced based on cost, conductivity, and stability in environmental conditions.

Wire bonding failures present a risk, particularly in the case of corrosion and fatigue in battery management systems, which can promote short circuits or overheating in electric vehicles, contributing to range anxiety and safety issues that have been aggravated by the transition to renewable transportation on a global basis. 

The heating and cooling of the materials causes the materials to expand and contract, creating mechanical stresses at the bond interface. There may be fatigue cracking, heel breaks, or bond lift-off due to thermal cycling, compromising the mechanical integrity of the bond.

References:
[1] Harman, G. (2010). Wire Bonding in Microelectronics (3rd ed.). McGraw-Hill Professional.
[2] Su, B., Qu, J., & Pecht, M. (2019). Copper Wire Bonding. Springer.
[3] Lai, Z., & Mei, Y. (2018). Advanced Wirebond Interconnection Technology. Springer.

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