How is a PCB Made ?

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44 (0)1992 510000
Manufacturing Processes for a 4 Layer Multi-layer
PCB
Tracks under solder mask
The following presentation covers the main
processes during the production of a 4 layer or a
multi-layer PCB. The diagrams which
follow represent a section through a PCB, as
indicated in red.
Via hole
SMD Pad
Section through PCB
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44 (0)1992 510000
Typical Layer Construction - 4 Layer PCB
Copper Laminate
COPPER FOIL
Layer 1 (Outer)
PRE-PREG
Layer 2 (Inner)
INNER LAYER (CORE)
4 Layer PCBs
Layer 3 (Inner)
PRE-PREG

1. Multilayer PCBs have inner tracking layers which
   are individually processed layer pairs which
   require bonding and connecting to form the
   finished PCB.

COPPER FOIL
Layer 4 (Outer)
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Inner Layer Processing
Copper Laminate (Dielectric)
Layer 2 (Inner)

• For Multilayer PCBs Inner layer Core materials
  are usually processed as Layer pairs
• Core materials can be typically 0.2mm thick which
  come pre coated with a ½ oz copper foil.

Core
Layer 3 (Inner)
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Laminating and Imaging of Internal Layers
UV sensitive film is laminated over top and
bottom surfaces of the Core Areas of the Core
where no copper is required are left exposed
Inner Layer Imaging
Layer 2 (Inner)

• The addition of the inner layer track patterns to
  layers 2 and 3.

Core
Layer 3 (Inner)
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Etch Process - Remove Exposed Copper
Copper Removed
Inner Layer Etching
Layer 2 (Inner)

• The etch process produces an etch back or
  undercut of the tracks. This can be specified by
  the W / W1 parameters
• This means that tracks will end up approximately
• 0,025 mm (0.001) thinner than the original
  design.

Core
Layer 3 (Inner)
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Remove Laminating Film
Film Strip
Layer 2 (Inner)

• The laminated film is removed prior to AOI
  inspection.

Core
Layer 3 (Inner)
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Completed Inner Layer Core
All inner layer Core materials are processed as
Layer Pairs prior to Bonding At this stage the
Cores are inspected visually (AOI) and defective
Cores rejected Sometimes a surface treatment is
applied to the Cores to aid with the Bonding
process
AOI Inspection
Layer 2 (Inner)

• Automatic Optical Inspection (AOI).
• Inner layers are scanned and compared to Gerber
  data for errors.

Core
Layer 3 (Inner)
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Layer stackup
FOIL
Layer 1 (Outer)
PRE-PREG
Layer 2 (Inner)
INNER LAYER
Layer 3 (Inner)
PRE-PREG
FOIL
Layer 4 (Outer)
Layer Stackup

• The order in which the layers are to be bonded
  together.
• The outer layer copper foils, layers 1 and 4 are
  purchased in sheet form. For a 1 oz finished
  copper weight typically a ½ oz foil would be used
  during bonding process to form the outer layers.

Layer 1 (Outer)
FOIL
Layer 2 (Inner)
PRE-PREG
Layer 3 (Inner)
INNER LAYER
PRE-PREG
Layer 4 (Outer)
FOIL
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44 (0)1992 510000
Layer stackup
FOIL
Layer 1 (Outer)
PRE-PREG
Layer 2 (Inner)
INNER LAYER
Layer 3 (Inner)
PRE-PREG
FOIL
Layer 4 (Outer)
Layer 1 (Outer)
FOIL
Layer 2 (Inner)
PRE-PREG
Layer 3 (Inner)
INNER LAYER
PRE-PREG
Layer 4 (Outer)
FOIL
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Layer stackup
FOIL
Layer 1 (Outer)
PRE-PREG
Layer 2 (Inner)
INNER LAYER
Layer 3 (Inner)
PRE-PREG
FOIL
Layer 4 (Outer)
Layer 1 (Outer)
FOIL
Layer 2 (Inner)
PRE-PREG
Layer 3 (Inner)
INNER LAYER
PRE-PREG
Layer 4 (Outer)
FOIL
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Layer stackup
FOIL
Layer 1 (Outer)
PRE-PREG
Layer 2 (Inner)
INNER LAYER
Layer 3 (Inner)
PRE-PREG
FOIL
Layer 4 (Outer)
Layer 1 (Outer)
FOIL
Layer 2 (Inner)
PRE-PREG
Layer 3 (Inner)
INNER LAYER
PRE-PREG
Layer 4 (Outer)
FOIL
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Layer stackup
FOIL
Layer 1 (Outer)
PRE-PREG
Layer 2 (Inner)
INNER LAYER
Layer 3 (Inner)
PRE-PREG
FOIL
Layer 4 (Outer)
Layer 1 (Outer)
FOIL
Layer 2 (Inner)
PRE-PREG
Layer 3 (Inner)
INNER LAYER
PRE-PREG
Layer 4 (Outer)
FOIL
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Layer stackup
FOIL
Layer 1 (Outer)
PRE-PREG
Layer 2 (Inner)
Layer 3 (Inner)
PRE-PREG
FOIL
Layer 4 (Outer)
Layer 1 (Outer)
FOIL
Layer 2 (Inner)
PRE-PREG
Layer 3 (Inner)
INNER LAYER
PRE-PREG
Layer 4 (Outer)
FOIL
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Layer stackup
FOIL
Layer 1 (Outer)
PRE-PREG
Layer 2 (Inner)
Layer 3 (Inner)
PRE-PREG
FOIL
Layer 4 (Outer)
Layer 1 (Outer)
FOIL
Layer 2 (Inner)
PRE-PREG
Layer 3 (Inner)
INNER LAYER
PRE-PREG
Layer 4 (Outer)
FOIL
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Layer stackup
FOIL
Layer 1 (Outer)
PRE-PREG
Layer 2 (Inner)
Layer 3 (Inner)
PRE-PREG
FOIL
Layer 4 (Outer)
Layer 1 (Outer)
FOIL
Layer 2 (Inner)
PRE-PREG
Layer 3 (Inner)
INNER LAYER
PRE-PREG
Layer 4 (Outer)
FOIL
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Bonding Heat
FOIL
Layer 1 (Outer)
PRE-PREG
Layer 2 (Inner)
Layer 3 (Inner)
PRE-PREG
FOIL
Layer 4 (Outer)
Bonding

• Once the layers are built up heat is applied over
  a period of time, this is to allow the Pre-Preg
  material time to melt and flow evenly between the
  layers, this is in effect the material that
  sticks the layers together.

Layer 1 (Outer)
FOIL
Layer 2 (Inner)
PRE-PREG
Layer 3 (Inner)
INNER LAYER
PRE-PREG
Layer 4 (Outer)
FOIL
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Bonding Multilayer Press
FOIL
Layer 1 (Outer)
PRE-PREG
Layer 2 (Inner)
Layer 3 (Inner)
PRE-PREG
FOIL
Layer 4 (Outer)
Bonding

• Layers are pressed together over a set period of
  time typically a bonding cycle will last for
  approx. 4 hours.
• Bonding capacity would depend on the number of
  openings and size of press.

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Drilling of a Panel
Copper Laminate
Drilled Hole
Layer 1
Drilling

• The holes are drilled for both thru-hole
  components and via connections. Once plated this
  will form the connection from 1 side of the PCB
  to the other, at the same time connecting the
  inner layers where required.

Layer 4
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Electroless Copper Process Addition of Copper to
all Exposed Surfaces
Copper
Drilled Hole
Layer 1
Plating
Layer 2

• Plating is the same process for 2 or 4 Layer.
• The aim is plate down the barrel of the hole to
  make the connection from 1 side of the PCB to the
  other.

Layer 3
Layer 4
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Laminating and Imaging of External Layers
UV sensitive film is laminated over top and
bottom surfaces of PCB It is then exposed and
developed, leaving an exposed image of the PCB
pattern
Layer 1
Imaging
Layer 2

• The outer layer track patterns are photo-imaged
  onto the panels using UV sensitive film.

Layer 3
Layer 4
Copper
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Electro-plating Process 1 Additional Copper to
all Exposed Surfaces
Laminated Film
Plate Additional Copper
Layer 1
Plating
Layer 2

• Electro-plating increases the copper thickness on
  the outer layers and through the barrel of the
  hole. The copper foils for a 4 layer or the
  starting copper weight on 2 layer boards is most
  commonly ½ oz the plating then increases the
  finished copper weight to the desired 1oz each
  side.

Layer 3
Layer 4
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Electro-plating Process 2 Add Tin over Exposed
Copper Areas
Tin Plating
Laminated Film
Additional Copper
Layer 1
Tin Plating
Layer 2

• A tin plate is added to all exposed copper areas,
  this act as a mask during the etching process.

Layer 3
Layer 4
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Electro-plating Process 3 Remove Laminated Film
Tin Plating
Laminated Film Removed
Layer 1
Film Strip
Layer 2

• The laminated film is removed to expose copper
  areas ready for the etching process.

Layer 3
Layer 4
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Etch Process - Remove Exposed Copper
Copper Removed
Tin Plating
Layer 1
Etching Process
Layer 2

• The etching process removes unwanted areas of
  copper.
• The etch process produces an etch back or
  undercut of the tracks.
• This means that tracks will end up approximately
• 0,025 mm (0.001) thinner than the original
  design.

Layer 3
Layer 4
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Tin Strip - Remove Tin Plating
Tin Plating Removed
Layer 1
Tin Strip
Layer 2

• The Removal of Tin will very slightly reduce the
  copper thickness (T) on the outer layers

Layer 3
Layer 4
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PCB is now complete except for surface finishes
and panel routing/punching
Via Hole
SMD Pad
Tracks
Layer 1
Layer 4
Tracks
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Solder Mask Application Image, Develop and Cure
UV Image, Develop and Cure
Layer 1
Solder Mask

• The image is semi cured and then UV image
  exposed.
• Any areas that are not exposed to the UV light
  are then dissolved in the developer, exposing the
  areas where the final surface finish is required.

Layer 4
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Surface Finish Process
Apply Solder to Exposed Copper Areas
Layer 1
Surface Finish

• Surface Finish (Tin / Lead, Gold, Silver, OSP,
  Tin) is usually only added to pads. The solder
  mask prevents coating of any other areas.

Layer 4
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Component Ident
SCL2 9624
Ident / Silk Screen

• An optional component Ident or Silk Screen is
  printed onto the PCB.

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Routing or Punching
Routing or Punching

• The circuit profile is then punched or routed.

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Testing and Inspection All PCBs are 100
Electrically tested All PCBs are 100 Visually
Inspected