IeMRC Flagship Project: Power Electronics

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Power Device Packaging Reliability and Wear-out
Phenomena Pearl Agyakwa09/07/08
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Contents

• Introduction/overview of reliability issues
• Wear-out examples wire bonds and solder
  interconnects
• Other wear-out phenomena

In a typical power module the substrate tiles are
soldered onto a copper baseplate.
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Why is packaging needed?

• Physical containment for component building
  blocks e.g. semiconductor dies, capacitors,
  inductors, resistors
• Protection from environment e.g. ingress of
  liquids, dust etc.
• Circuit interconnections (internal and external)
• Electromagnetic management EMC issues
• Thermal Management

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Anatomy of a typical package heatsink
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What happens during operation?

• Current flow in the conductors of the package
  produces magnetic fields that spread into the
  region within and around the package. Rapidly
  changing currents can cause interference with
  neighbouring circuits
• Heat generated within the semiconductor is
  conducted through the package elements to the
  heatsink
• As the temperature changes, the package materials
  expand at different rates leading to mechanical
  stress and strain
• Operating conditions/switching thermal cycling
• Thermo-mechanical stresses are generated at
  interfaces bringing about fatigue
• Heat generation provides activation energy for
  diffusion controlled phenomena to take place

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Power module reliability issues
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Power electronic packaging is multi-disciplinary

• To be effective a package must manage the
  thermal, mechanical and electromagnetic aspects
  of operation
• Multi-disciplinary
• Materials science
• Thermodynamics and heat transfer
• Electromagnetism
• Mechanics
• Design
• Prof Mark Johnson
• Mahera Musallam prognostics diagnostics for
  health management
• Robert Skuriat thermal management
• Paul Evans software integration for power
  module design and optimisation
• Jianfeng Li development of die-attach and other
  interconnect technologies
• Pearl Agyakwa wire bonding and interconnect
  technologies

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• Wire bond reliability

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The wire bonding process

• Heat is generated at the bond when ultrasonic
  energy is applied
• Residual stresses are generated at the bond
  interface on cooling to room temperature
• During thermal cycling, residual stresses and
  thermo-mechanical stresses act together to bring
  about bond degradation

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Wire bond reliability cont/d.

• Experimental approach accelerated life testing
  through passive thermal cycling
• Assessment of reliability through shear strength
  data and microstructural characterisation
• Scenarios
• Effect of thermal cycling ranges (different DT
  and mean T) and wire type
• Enhanced bonding concepts
• High temperature bonding
• Post-bond annealing
• Effect of overlayer metal thickness
• Ribbon bonding
• Develop physics of failure models wear-out
  phenomena of wire bonds still not understood

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Wire bond degradation effect of bonding at high
temperature

• High temp bonding rids interface of oxides get
  better metallurgical bond but more deformation
  during bonding

Interfacial oxides
Voids / oxide debris
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Wire bond degradation effect of post-bond
annealing

• Post-bond annealing serves to annihilate some of
  the dislocations which build up during bonding
  this slows down crack propagation rate.

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Annealing/temperature-time effects during cycling?
High DT

• Conventional models (e.g. Coffin Manson) predict
  reduced life with increased mean temperature and
  temperature range
• Experimental results show that higher DT does not
  always give reduced life
• Fastest degradation for lowest maximum
  temperature
• Evidence of temperature-time effects?

Low DT
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Correlation with of crack growth rate with shear
strength

• Crack length is greater for samples cycled -55 to
  125 (DT 180) than -60 to 170 (DT 230), in
  agreement with shear data.

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Annealing effects during cycling?

• Driving force for thermally activated processes
• Recovery
• Recrystallisation
• Grain growth (Ostwald ripening)
• Other vacancy diffusion etc

Ideal grain growth temperature dependent
Driving pressure for recrystallisation is given
by the dislocation density
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Crack propagation is not straightforward!

• A number of modes have been identified. Mode can
  change as primary crack advances
• At wire-metallisation interface (e.g. 5N wires)
• At metallisation-silicon interface (observed in
  some 4N wires)
• Through bulk wire (observed in high temperature
  bonded wires)

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• Die-attach solder reliability

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Reliability of Pb-free solder interconnects

• Issues affecting solder reliability include
• Condition of/compatibility with substrate or
  metallisation
• Optimisation of solder reflow process
• Microstructural evolution in service effects on
  bulk properties (mechanical, creep and fatigue
  resistance, etc.)

• Potential failure mechanisms include
• Build-up of plastic strain as a result of CTE
  mismatch
• Loss of creep strength due to precipitate
  coarsening
• Thermo-mechanical fatigue due to thermal cycling

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Solder reliability cont/d.

• Microstructural evolution during operation
  effects on bulk properties
• Complex -Competing microstructural processes
  occur simultaneously!
• Thermally activated movement of species across
  concentration gradients (coarsening/Ostwald
  ripening of IMCs)
• Electromigration (voiding)
• Plastic flow, movement of dislocations,
  accumulating plastic damage (depending on sm and
  Ds)
• Strain-assisted transport of species (e.g.
  vacancies -pipe diffusion at low Ts) creep
  cavities
• Strain hardening or softening (depending on
  material), nucleation/ annihilation of
  dislocations

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Die-attach solder reliability
Cycling and time-temperature effects on secondary
phase morphology and damage accumulation
200 cycles
Diffusion
Increasing precipitate coarsening
Reflow Temp
Ag3Sn
500 cycles
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Die attach solder reliability
Effect of secondary phases/IMCs on crack
initiation and propagation
Crack propagating preferentially along coarse
intermetallic particle

• Annealed samples show more progressed coarsening
  of the intermetallic phases and greater
  inter-particle spacing
• Crack propagation appears to be more affected by
  number of cycles

IMCs provide nucleation sites for crack formation
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Other examples of wear-out phenomena
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Substrate fatigue failure conchoidal fracture
delamination

• Conchoidal fracture
• Fracture through ceramic below surface
• Shell-like fracture lines, one per cycle
• Associated with DBC ceramic of low fracture
  toughness

• Delamination
• Fracture at interface
• No apparent cracking of ceramic
• Associated with DBC and AMB ceramic with high
  fracture toughness

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Substrate solder mountdown delamination during
cycling
SAM scans reveal area delamination as function
of number of cycles
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Acknowledgements
Prof Mark Johnson, Dr SC Hogg, Dr Martin
Corfield, Robert Skuriat, Wei Sun Loh, Joseph
Ikujeniyah, Rod Dykeman, Dr Mike Fay, Paul Evans,
Dr Jianfeng Li, Dr Mahera Musallam. Semelab Plc,
Dynex Semiconductors Ltd.

• Thank you for your attention