Gold-aluminium intermetallics and bonding Au-Al IMCs

spore: profile
id: aluminium-gold
spore_version: 0.2
family: aluminium-alloys
name: Gold-aluminium intermetallics and bonding
abbreviation: Au-Al IMCs
role: intermetallic phases that form where gold wire or gold foil meets aluminium metallization in wire bonds and thermocompression joints
category: metals
type: aluminium
summary: |
  When gold meets aluminium in a bonded joint the two metals react and form intermetallic phases. The main ones are AuAl2, called purple plague, and Au2Al, with Au5Al2 and Au8Al3 also named in the sources. A patent reports that Au2Al forms at about 100 degC during heat treatment and converts to AuAl2 at about 230 degC when the aluminium layer is thicker than the gold. A 2025 Korean study of 15.3 µm gold wire on 1 µm and 5 µm aluminium pads, annealed at 175 degC, found voids after 250 hours and stable bonds up to 1,000 hours on the thinner pads. The reaction is a known reliability limit of gold-to-aluminium bonds.
best_for: Gold-to-aluminium bonds on chips that are kept cool and free of halide contamination, where the joint must carry current for a long time at moderate temperatures (KJMM 2025 and the background of US patent 5184206)
weakest_at: Long exposure above about 175 degC, which grows voids in the joint; the sources read here give no melting point, density, hardness or strength for the phases
identity:
  formula: AuAl2 (purple plague, about 78.5% Au and 21.5% Al by mass); Au2Al (about 93% Au and 7% Al by mass); Au5Al2 (about 95% Au and 5% Al by mass); Au8Al3 and Au4Al also named as phases in the bonded joint
  source: wiki-au-al
  where: Lead and Properties sections, composition by mass for AuAl2, Au2Al and Au5Al2; Au8Al3 and Au4Al named in the Xu et al. 2011 abstract (Intermetallics), which is a different source
tags:
  environments: []
  stresses:
    - extreme-heat
  applications:
    - electronics-and-chips
properties:
  - benchmark: gold_mass_percent
    label: Gold content of AuAl2 (purple plague), by mass
    value: 78.5
    unit: '%'
    scope: material
    conditions:
      basis: mass fraction of gold, as printed as about 78.5% Au and 21.5% Al
      phase: AuAl2
    method: Composition printed in an encyclopedia article; stated as about
    status: ground
    source: wiki-au-al
    where: Properties section, AuAl2 composition
  - benchmark: gold_mass_percent
    label: Gold content of Au2Al, by mass
    value: 93
    unit: '%'
    scope: material
    conditions:
      basis: mass fraction of gold, as printed as 93% Au and 7% Al
      phase: Au2Al
    method: Composition printed in an encyclopedia article
    status: ground
    source: wiki-au-al
    where: Properties section, Au2Al composition
  - benchmark: gold_mass_percent
    label: Gold content of Au5Al2, by mass
    value: 95
    unit: '%'
    scope: material
    conditions:
      basis: mass fraction of gold, as printed as typically 95% Au and 5% Al
      phase: Au5Al2
    method: Composition printed in an encyclopedia article; the source says the phase typically forms at this composition
    status: ground
    source: wiki-au-al
    where: Lead, Au5Al2 composition sentence
  - benchmark: formation_temperature
    label: Au2Al first forms in a gold-aluminium bond during heat treatment
    value: 100
    unit: degC
    scope: material
    conditions:
      basis: first interaction of gold and aluminium layers; the patent calls it heat treatment at about 100 degC
      phase: Au2Al
    method: Value stated in the background section of a US patent on thermocompression bonding of thin power chips; no measured data set
    status: ground
    note: The patent gives a single temperature, not a rate or time.
    source: us5184206
    where: Description, Background Information
  - benchmark: formation_temperature
    label: Au2Al converts to AuAl2 when the aluminium layer is thicker than the gold
    value: 230
    unit: degC
    scope: material
    conditions:
      basis: heating with the aluminium stratum thicker than the gold stratum
      from: Au2Al
      to: AuAl2
    method: Value stated in the background section of the same patent
    status: ground
    source: us5184206
    where: Description, Background Information
  - benchmark: corrosion_onset_temperature
    label: Halide-accelerated purple plague formation above this temperature
    value: 180
    unit: degC
    bound: lower
    scope: material
    conditions:
      basis: corrosion induced by bromide, fluoride or chloride impurities at temperatures over 180 degC
      effect: accelerates purple plague formation and premature bond failure at the Au-Al interface
    method: Value stated in the background section of the same patent
    status: ground
    note: A patent background statement, not a test with a stated rate; iodide is not named in it.
    source: us5184206
    where: Description, Background Information
  - benchmark: failure_temperature_long_exposure
    label: Chip contact likely to fail after long exposure above this temperature
    value: 200
    unit: degC
    bound: lower
    scope: material
    conditions:
      basis: temperatures exceeding 200 degC for long periods; also cited together with halide presence
    method: Statement in the background section of the same patent; no test data
    status: ground
    note: Custom benchmark name because the canonical list has no failure temperature. The patent says the contact will probably fail; it gives no time.
    source: us5184206
    where: Description, Background Information
  - benchmark: annealing_temperature
    label: Annealing range used to follow the phase change in Au-Al wire bonds
    value: 175
    value_high: 250
    bound: range
    unit: degC
    scope: material
    conditions:
      basis: annealing range printed in the abstract; HRTEM observation of the interface
      material: gold wire bonds on aluminium
    method: Range printed in the abstract of a 2011 paper on Au-Al wire bond phase transformations
    status: ground
    source: anu-intermetallics-2011
    where: Abstract
  - benchmark: void_onset_time
    label: Voids at the bond interface, 15.3 µm gold wire on 1 µm aluminium pads
    value: 250
    unit: h
    scope: cell
    conditions:
      temperature: 175 degC annealing
      wire: 15.3 µm gold
      pad: 1 µm aluminium
      atmosphere: not stated in the abstract
    method: Annealing test followed by cross-section; voids reported at the lateral sections
    status: ground
    note: Custom benchmark name. The abstract says voids began to form at the lateral sections after 250 hours.
    source: kjmm-2025
    where: Abstract
  - benchmark: void_onset_time
    label: Voids beneath the bond, 2N gold wire on 5 µm aluminium pads
    value: 250
    unit: h
    scope: cell
    conditions:
      temperature: 175 degC annealing
      wire: 2N gold
      pad: 5 µm aluminium
      note: as early as 250 hours
    method: Annealing test followed by cross-section; the abstract says significant voids were observed beneath the bond interface as early as 250 hours
    status: ground
    note: The source says as early as 250 hours, so this is the earliest reported time, not a typical value.
    source: kjmm-2025
    where: Abstract, 5 µm pad result
  - benchmark: bond_stability_time
    label: Bond stayed stable on 1 µm aluminium pads up to the end of the test
    value: 1000
    unit: h
    bound: lower
    scope: cell
    conditions:
      temperature: 175 degC annealing
      pad: 1 µm aluminium
      wire: 15.3 µm gold
    method: Annealing test ending at 1,000 hours with bonding still stable
    status: ground
    note: Custom benchmark name. This is the end of the test, not a failure time; the same paper reports voids earlier on 1 µm pads.
    source: kjmm-2025
    where: Abstract
corrosion:
  - environment: Halide impurities (bromide, fluoride, chloride) at the gold-aluminium interface above about 180 degC
    headline: Halides attack the interface and accelerate purple plague, which causes early bond failure.
    effect: The patent background says corrosion induced by these impurities at temperatures over 180 degC accelerates purple plague formation and gives premature bond failure at the Au-Al interface. Excess gold in conventional gold wire bonding means the reaction can continue, so purple plague may become critical.
    rating: severe
    conditions:
      basis: patent background statement; no test rate given
      impurities: bromide, fluoride, chloride
    source: us5184206
    where: Description, Background Information
  - environment: Annealing at 175 degC with 1 µm and 5 µm aluminium pads
    headline: Kirkendall voids form at the interface within 250 hours at 175 degC.
    effect: The abstract reports voids at the lateral sections after 250 hours on 1 µm pads and significant voids beneath the bond interface as early as 250 hours on 5 µm pads with 2N wire. On 5 µm pads, AuAl2 grew into Au2Al in 2N wire, which the authors link to the voids.
    rating: moderate
    conditions:
      temperature: 175 degC
      atmosphere: not stated in the abstract
      wire: 15.3 µm gold, 2N and 4N grades
    source: kjmm-2025
    where: Abstract
risks:
  hazards:
    - text: 'Aluminium is a flammable solid and reacts with water to release flammable gas, according to the PubChem aggregate of ECHA notifications for the element (4079 company reports): H228 in 60.6% of hazard reports, H250 in 34.2% and H261 in 90.8%. The aggregate covers all forms, so check the supplier''s safety data sheet for the powder or foil you handle.'
      ghs:
        - H228
        - H250
        - H261
      source: pubchem-aluminium
      where: Safety and Hazards, GHS Classification
  failure_risks:
    - mode: Kirkendall voiding at the gold-aluminium interface
      trigger: Annealing at 175 degC for 250 hours or more
      consequence: Voids reduce the bonded area; the abstract reports voids on 1 µm and 5 µm pads
      severity: high
      source: kjmm-2025
      where: Abstract
    - mode: Halide-accelerated purple plague
      trigger: Bromide, fluoride or chloride impurities above 180 degC
      consequence: Premature bond failure at the Au-Al interface
      severity: critical
      source: us5184206
      where: Description, Background Information
    - mode: Chip contact failure after long hot exposure
      trigger: Temperatures above 200 degC for long periods
      consequence: The patent says the chip contact will probably fail
      severity: high
      source: us5184206
      where: Description, Background Information
  side_effects:
    - text: In the 2N wire on 5 µm pads, AuAl2 grew into Au2Al, and the abstract links this change to the voids that formed beneath the bond.
      affects: Bond interface of the joint
      source: kjmm-2025
      where: Abstract
  handling:
    - text: The aluminium aggregate PubChem record lists H261, so keep aluminium powder or foil away from water and store it dry.
      source: pubchem-aluminium
      where: GHS Hazard Statements
  mitigations:
    - text: Keep halide contamination out of the bonding area; the patent ties halide attack to temperatures over 180 degC.
      source: us5184206
      where: Description, Background Information
uses:
  - application: electronics-and-chips
    device: Gold wire bonds to aluminium pads on integrated circuits
    example: The 2025 study bonded 15.3 µm gold wire to 1 µm and 5 µm aluminium pads and annealed the joints at 175 degC to watch the interface grow.
    source: kjmm-2025
    where: Abstract
  - application: electronics-and-chips
    device: Thin electronic power chips joined by thermocompression bonding of gold foil to aluminium metallization
    example: The patent describes the Au2Al and AuAl2 sequence in these joints, with the aluminium layer thickness setting which phase ends the reaction.
    source: us5184206
    where: Title and Description, Background Information
supply:
  - subject: Gold
    abundance: Not read in this run
    producers:
      - country: China
        share_percent: 11.5
        year: 2025
      - country: Russia
        share_percent: 9.4
        year: 2025
      - country: Australia
        share_percent: 8.5
        year: 2025
      - country: Canada
        share_percent: 6.1
        year: 2025
      - country: United States
        share_percent: 4.8
        year: 2025
    critical_lists:
      - list: USGS 2025 List of Critical Minerals
        listed: false
      - list: EU critical raw materials list as printed on the Commission page
        listed: false
    recycling: 'USGS gold chapter: an estimated 90 tons of new and old scrap was recycled in 2025, about 60% of reported consumption.'
    substitutes:
      - 'USGS: base metals clad with gold alloys are widely used to economize on gold in electrical and electronic products; palladium, platinum and silver may substitute for gold.'
    source: usgs-gold-mcs2026
    where: World Mine Production table, 2025e (shares calculated by Space Mushroom against the 3,300 t world total); Recycling; Substitutes
  - subject: Aluminium
    producers:
      - country: China
        share_percent: 60.8
        year: 2025
      - country: India
        share_percent: 5.7
        year: 2025
      - country: Russia
        share_percent: 5.3
        year: 2025
      - country: Canada
        share_percent: 4.5
        year: 2025
      - country: United Arab Emirates
        share_percent: 3.6
        year: 2025
    critical_lists:
      - list: USGS 2025 List of Critical Minerals (listed as Aluminum)
        listed: true
      - list: EU critical raw materials list as printed on the Commission page (bauxite entry; the page says aluminium and bauxite are merged)
        listed: true
    recycling: 'USGS aluminium chapter: aluminium recovered from old scrap was equivalent to about 28% of apparent consumption in the United States.'
    substitutes:
      - 'USGS: composites, magnesium, steel and titanium can substitute for aluminium in ground transport; copper can replace aluminium in electrical and heat-exchange applications.'
    source: usgs-aluminum-mcs2026
    where: World Smelter Production table, 2025e (shares calculated by Space Mushroom against the 74,000 thousand t world total); Recycling; Substitutes; US net import reliance 60%
documents:
  - title: Xu et al. 2011, Intermetallic phase transformations in Au-Al wire bonds (Intermetallics 19, 1808-1816)
    type: paper
    url: https://doi.org/10.1016/j.intermet.2011.07.003
    year: 2011
  - title: Korean Journal of Metals and Materials 63(12), 946-954, gold wire bonds on aluminium pads (2025)
    type: paper
    url: https://doi.org/10.3365/KJMM.2025.63.12.946
    year: 2025
  - title: USGS Mineral Commodity Summaries 2026, Gold
    type: report
    url: https://pubs.usgs.gov/periodicals/mcs2026/mcs2026-gold.pdf
    year: 2026
  - title: USGS Mineral Commodity Summaries 2026, Aluminum
    type: report
    url: https://pubs.usgs.gov/periodicals/mcs2026/mcs2026-aluminum.pdf
    year: 2026
  - title: 'PubChem: Aluminum (CID 5359268), GHS classification summary'
    type: database
    url: https://pubchem.ncbi.nlm.nih.gov/compound/Aluminum
    year: 2026
gaps:
  - No density, melting point, electrical resistivity, hardness or elastic modulus for AuAl2, Au2Al or Au5Al2 was read in a fetched non-Wikipedia page. The melting points of 575 degC (Au5Al2) and 1,060 degC (AuAl2) appear on Wikipedia only, and the brief limits Wikipedia to composition and history, so they are not used. Springer phase-diagram papers (s13404-014-0140-2 and s13404-014-0146-9) redirected to a login page.
  - Thin-film Al/Au diffusion work (Xu, Sritharan and Mhaisalkar, Thin Solid Films 2007, activation energy 33.602 kJ/mol) was seen only in a search summary, not in a fetched page, so no value is used. Azadmanjiri et al. 2016 on sputtered Al/Au films (Springer redirect) was not readable.
  - 'Sputtering: no Al-Au binary sputtering target datasheet was found; the search returned only patents on multi-component gold targets, which report composition by weight and are not Al-Au binaries. No sputtering use is included.'
  - 'Space: no NASA or ESA source on gold-aluminium bond reliability was fetched; NTRS search hits were not opened.'
  - 'Gold hazard: PubChem gold GHS record was not retrieved (CID 79 and 23947 returned other compounds). No gold hazard entry is included.'
  - 'Humidity and atmospheric corrosion of Au-Al bonds: no source read. The corrosion section covers high temperature and halides only. The Tampere University thesis on halogen corrosion (trepo.tuni.fi) was blocked by an access check.'
  - Au5Al2 and Au8Al3 are named separately in different sources; equivalence of the two names was not read in a fetched page, so both names are kept as printed.
  - 'Custom benchmark names used: gold_mass_percent, formation_temperature, corrosion_onset_temperature, failure_temperature_long_exposure, annealing_temperature, void_onset_time, bond_stability_time. None is in spores/property-canon.yaml.'
  - 'Aluminium crustal abundance: not in the Table A1 rows read in PP 1802-A (aluminium, iron and phosphorus were not among the rows printed).'
  - No media files were added.
sources:
  wiki-au-al:
    title: Gold–aluminium intermetallic (Wikipedia)
    id: Wikipedia article, composition and history only
    url: https://en.wikipedia.org/wiki/Gold%E2%80%93aluminium_intermetallic
    kind: web
    license: CC BY-SA 4.0
  kjmm-2025:
    title: Gold wire bonds to aluminium pads, 15.3 µm wire, 1 µm and 5 µm pads, annealing at 175 degC (Korean Journal of Metals and Materials)
    id: doi:10.3365/KJMM.2025.63.12.946 (KJMM 63(12), pp. 946-954)
    url: https://doi.org/10.3365/KJMM.2025.63.12.946
    kind: paper
    year: 2025
    license: as published by KJMM
  anu-intermetallics-2011:
    title: Intermetallic phase transformations in Au-Al wire bonds (Intermetallics 19, 1808-1816)
    id: doi:10.1016/j.intermet.2011.07.003
    url: https://researchportalplus.anu.edu.au/en/publications/intermetallic-phase-transformations-in-au-al-wire-bonds/
    kind: paper
    year: 2011
    license: ANU research portal record
  us5184206:
    title: US Patent 5184206, direct thermocompression bonding for thin electronic power chips (background section on purple plague)
    id: US 5184206
    url: https://patents.google.com/patent/US5184206
    kind: web
    license: patent document
  usgs-gold-mcs2026:
    title: Mineral Commodity Summaries 2026, Gold
    id: USGS MCS 2026, February 2026
    url: https://pubs.usgs.gov/periodicals/mcs2026/mcs2026-gold.pdf
    kind: report
    year: 2026
    license: public domain (USGS)
  usgs-aluminum-mcs2026:
    title: Mineral Commodity Summaries 2026, Aluminum
    id: USGS MCS 2026, February 2026
    url: https://pubs.usgs.gov/periodicals/mcs2026/mcs2026-aluminum.pdf
    kind: report
    year: 2026
    license: public domain (USGS)
  usgs-critical-2025:
    title: About the 2025 List of Critical Minerals (USGS, 60 minerals)
    id: 2025 List of Critical Minerals
    url: https://www.usgs.gov/programs/mineral-resources-program/science/about-2025-list-critical-minerals
    kind: web
    year: 2025
    license: public domain (USGS)
  ec-crm:
    title: Critical raw materials (European Commission, Regulation (EU) 2024/1252)
    id: Regulation (EU) 2024/1252
    url: https://single-market-economy.ec.europa.eu/sectors/raw-materials/areas-specific-interest/critical-raw-materials_en
    kind: web
    year: 2024
    license: European Commission web page
  pubchem-aluminium:
    title: PubChem compound record, Aluminum (CID 5359268), GHS classification
    id: CID 5359268
    url: https://pubchem.ncbi.nlm.nih.gov/compound/Aluminum
    kind: database
    year: 2026
    license: public domain (NCBI)
license: CC-BY-4.0
updated: '2026-10-26'
review:
  status: draft
  method: assistant
  checked_values: 11
  unreachable: 3
  caveats: 4
  note: Written by an AI assistant on 2026-10-26 from pages read in this run. 11 properties; 3 sources could not be opened (two Springer phase-diagram papers and the Tampere halogen thesis); the thin-film activation energy was only in a search summary and is not used.
story:
  recipe:
    line: The bonded phases are intermetallics of gold and aluminium. AuAl2 is about 78.5% gold and 21.5% aluminium by mass; Au2Al is about 93% gold and 7% aluminium; Au5Al2 is about 95% gold and 5% aluminium. The phases grow from the two metal layers during heating, and a patent describes how the thickness of the aluminium layer sets which phase ends the reaction.
    parts:
      - name: Gold
        symbol: Au
        share_percent: 78.5
        basis: mass, AuAl2
        role: The gold wire or foil. The gold diffuses into the aluminium and forms the purple AuAl2 and the tan Au2Al phases.
      - name: Aluminium
        symbol: Al
        share_percent: 21.5
        basis: mass, AuAl2
        role: The aluminium pad or metal layer. Its thickness decides whether the reaction stops at Au2Al or runs to AuAl2 at about 230 degC.
  risk_groups:
    - title: For people
      level: 0
      verdict: Aluminium powder is flammable
      lead: The intermetallic joint is a solid and is not a hazard in normal use. The metal feedstock is the concern, mainly as fine powder.
      points:
        - label: The aluminium feedstock
          line: The PubChem record for aluminium aggregates ECHA notifications and lists H228 (flammable solid), H250 (catches fire spontaneously if exposed to air) and H261 (releases flammable gas on contact with water), depending on the form.
        - label: The gold
          line: No gold hazard statement was retrieved in this run; check the supplier safety data sheet for the gold you handle.
    - title: If a joint overheats
      lead: These results come from laboratory tests and a patent background, not field failures.
      points:
        - label: Long annealing
          line: At 175 degC, a 2025 study saw voids in the joint after 250 hours on 1 µm aluminium pads and as early as 250 hours on 5 µm pads with 2N gold wire.
        - label: Halides
          line: The patent says halide impurities (bromide, fluoride, chloride) above 180 degC speed up purple plague and cause early bond failure.
        - label: Long hot exposure
          line: The patent says the chip contact will probably fail after long periods above 200 degC.
  why: Gold and aluminium react at a bonded joint and form several intermetallic phases. The reaction matters for electronics because gold wire bonds are made on aluminium pads on chips. The phases grow with time and temperature, and voids form where the metals diffuse unevenly, which can weaken the bond.
  uses:
    - where: Integrated circuits
      line: Gold wire bonds connect chips to packages by joining gold wire to aluminium pads. A 2025 study bonded 15.3 µm gold wire to aluminium pads of 1 µm and 5 µm and annealed the joints to track the interface.
    - where: Power chips
      line: A US patent on thermocompression bonding of thin electronic power chips describes gold foil joined to aluminium metallization, with the Au2Al and AuAl2 sequence set by the aluminium layer thickness.
  can_do:
    - label: Bond life at 175 degC
      benchmark: void_onset_time
      value: 250 h
      line: Voids appeared after 250 hours at 175 degC on the thinner pads in one 2025 study. On the same test, bonding stayed stable up to 1,000 hours on 1 µm pads.
    - label: Temperature threshold
      benchmark: corrosion_onset_temperature
      value: 180 degC
      line: A patent background says halide impurities accelerate purple plague above about 180 degC, which is the figure to keep bonds below when halides are possible.
  compare: The sources here are about the joint, not about the bulk metal. The patent names 100 degC for the first Au2Al formation and 230 degC for conversion to AuAl2 when the aluminium is thicker. The 2025 test saw voids at 175 degC within 250 hours. No source read in this run gives density, hardness or strength for the phases, so the table compares the reaction temperatures and times that were read.
  compare_rows:
    - label: First Au2Al formation
      benchmark: formation_temperature
    - label: Halide corrosion onset
      benchmark: corrosion_onset_temperature
    - label: Voids in a 175 degC anneal
      benchmark: void_onset_time
  handling:
    - Keep halide contamination out of the bonding area, since halides speed up the reaction above about 180 degC.
    - Keep aluminium powder dry and away from water; the aluminium record lists H261.
    - Limit long exposure above 175 degC if the joint must stay sound for years.
  good_for:
    - Gold wire bonds on aluminium pads that run cool and stay free of halides, where the joint only has to last a set time.
    - Thin power chips joined by thermocompression, where the aluminium layer thickness is controlled.
  watch_out:
    - Voids form within 250 hours at 175 degC in the 2025 study, so long hot storage is the main risk.
    - Halides at temperatures above about 180 degC cause early bond failure.
    - Sputtered Al-Au film data and phase melting points were not readable in this run, so this profile does not cover sputtering.
  verdict: The gold-aluminium joint is a known reliability limit. It works for moderate temperatures and clean bonds, and the time-at-temperature and halide data are the figures to design around.