DBC Process

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DBC Process Void Free Solder Reflow Two
key processes for Concentrated Photovoltaic
Solar Cell Now available at Tong Hsing
Presented by Chris Chang Date May 11th, 2009
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Concentrating photovoltaic

• Concentrating Photovoltaic (CPV) Systems employ
  sunlight concentrated onto photovoltaic surfaces
  for the purpose of electrical power production.
• Solar concentrators of all varieties may be used,
  and these are often mounted on a solar tracker in
  order to keep the focal point upon the cell as
  the sun moves across the sky.

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Efficiency of CPV

• Semiconductor properties allow solar cells to
  operate more efficiently in concentrated light,
  as long as the cell junction temperature is
  kept cool by suitable heat sinks. CPV
  operates most effectively in sunny weather since
  clouds and overcast conditions create diffused
  light, which essentially cannot be concentrated.
• Expected future efficiencies are nearly 50.

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Example of CPV

• Fresnel reflectors are not as efficient as
  parabolic mirrors in Nevada Solar One however,
  those from Ausra are much cheaper to build.

• A 6.2 kilowatt array, part of a solar power plant
  project in Spain.

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Module of CPV
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Sketch of CPV
Sun
Frensel Lens
To achieve void free solder die attach for high
power application, advanced reflow system is
required.
Receiver module
Solar cell triple junction PV die
Surface between die and ceramic
Ceramic and Heat sink
Using DBC process to metalize the Ceramic
substrate.
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Why DBC? (Direct Bonding Copper)

• Excellent Electrical Performance
• Thick copper (? 75um)
• Electrical conductivity value ( Cu? 5.81X107 s/m)
• High trace conductivity
• Perfect line definition, sharp edge and flat
  surface
• High quality of thermal performance
• Heat spreading well from copper
• Heat conductivity through both the ceramic and
  copper vias
• High circuitry density - 2 mil line width/
  spacing 100um via
• Robust Mechanical Performance
• Outstanding adhesion and aged adhesion
• Excellent solder leach resistance
• Ease to assemble
• High accuracy and consistency of pattern
  locations
• Perfect pattern definition

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Strength of DBC Substrates

• Super electric conductivity (0.3 m ohm s/sq)
• Good thermal conductivity ( Alumina, or Alumina
  nitride)
• High accuracy of pattern registration (/- 25um)
• Tight pattern dimensions (/-10 um)
• Economic process for through vias (plugged or
  plated)
• Pb free and RoHS compliant
• Ease of pattern recognition, sharp pattern
  resolution
• Excellent solder leach resistance (10 dips, 250
  C)
• Excellent solderability
• Excellent adhesion, aged adhesion
• Free of silver migration

Ceramic substrate
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DBC Process Flow

• Sputtering Metalize the substrate with Ti/Cu.
• Dry filming Laminate the sputtered substrate
  with UV dry film.
• Exposure Expose the dry film under UV.
• Development The unexposed area to be washed out.
• Cu plating Plated with Cu on selected area.
• Ni plating Plated with Ni.
• Au plating Plated with Au.
• Stripping Strip the dry film off.
• Etching Remove the non-plated area by etching.

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DBC Process Flow - CO2 Laser machining -
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DBC Process Flow - Sputtering -
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DBC Process Flow - Dry Film Laminate -
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DBC Process Flow - Exposure -
UV
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DBC Process Flow - Development -
Na2CO3
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DBC Process Flow - Plating Cu/Ni/Au -
H2SO4
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DBC Process Flow - Striping Dry Film -
Na OH
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DBC Process Flow - Etching (Sputter Cu Ti) -
SPS HF
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Void Free Solder Reflow

• Tradition convection reflow
• Tong Hsings advanced reflow system
• - Constant temperature profile control
• - Void free solder for high power devices

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What is the emphasis of solder void free in CPV?

• CPV systems use lenses or mirrors to focus
  a large area of sunlight into a small beam. The
  high temperature causes the small beam of
  sunlight and will damage the component of the
  solar cell, especially when solder with void.
• If solder voidlt5, better thermal conductivity
  can be achieved to improve efficiency.

Sunlight
Lens
Die
The void may expand to damage the component at
high temperature.
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Background

• To achieve void free solder die attach for high
  power application, advanced reflow system is
  required.
• The evaluation is to compare the void between
  traditional convection reflow over vs. Tong
  Hsings advance reflow system.

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Process Flow
Print solder paste
Die attach
Standard reflow
X-ray inspection
Clean
X-Ray inspection
Advanced reflow
Solder paste composition 96.5Sn/3.0Ag/0.5Cu
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Test Strip Design
Solder pad 4.7 mm X 3.0 mm
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Solder Reflow Void Free Exp.1

• Traditional convection furnace

Advanced reflow system
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Solder Reflow Void Free Exp.1
Traditional convection furnace
Advanced reflow system
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Solder Reflow Void Free Exp.2
Stencil thickness 0.1mm
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Solder Reflow Void Free Exp.2
Traditional convection furnace
Advanced reflow system
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Solder Reflow Void Free Exp.2
Traditional convection furnace
Advanced reflow system
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Conclusion

• The advanced reflow system can efficiently reduce
  the solder void under 5 in CPV to get better
  thermal conductivity to improve efficiency and
  avoid damage to the component.
• DBC process can supply a good heat spreading
  surrounding and excellent electrical performance
  in CPV.

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DBC Substrate
Applications and Features Electrical and
Mechanical Performance
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Tight Spacing High Precision Pad Location
High circuit density, low profile and flat
surface on one side to be required.
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Combinations of TechnologyThick film resistors
Plated DPC conductor pads
Fine pad pitch, high pad adhesion, solder leash
resistance and RoHS required.
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Chip Wire Bonding
Good bondability due to flat and good uniformity
on gold plated pad surface.
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Ceramic Carrier
Fine pitch for C4 flip chip assembly, Good
adhesion and solderability are required.
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Staking Substrate Assembly Memory Module
Excellent electrical performance for high speed
memory.
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RF Microwave
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Niche Applications
Various clock oscillators require ultra high
stability of frequency output.
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High REL Military ModuleHermetic package
Good conductivity and pattern definition Super
electrical performance.
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High REL Military
Good bondability for reliable chip and wire bond
performance.
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High Power RF Microwave
Fine trace definition robust conductor pads,
Excellent pad adhesion solder leach
resistance.