Manufacturing Processes for a 4 Layer Multi-layer PCB

Section through PCB

Via hole

SMD Pad

The following presentationcovers the main processesduring the production ofa 4 layer or a multi-layer PCB.

The diagrams which followrepresent a section througha PCB, as indicatedin red.

Tracks under solder mask

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Layer 1 (Outer)

Layer 4 (Outer)

Layer 2 (Inner)Layer 3 (Inner)

COPPER FOIL

COPPER FOIL

PRE-PREG

PRE-PREG

INNER LAYER (CORE)

Copper Laminate

Typical Layer Construction - 4 Layer PCB

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

4 Layer PCB’s

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Inner Layer Processing

Layer 2 (Inner)

Layer 3 (Inner)

Core

Copper Laminate (Dielectric)

• For Multilayer PCB’s 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.

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Layer 2 (Inner)

Layer 3 (Inner)

Core

Laminating and Imaging of Internal Layers

UV sensitive film is laminated over top and bottomsurfaces of the CoreAreas of the Core where no copper is required are left exposed

Inner Layer Imaging

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

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Core

• 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 approximately0,025 mm (0.001”) thinner than the original design.

Inner Layer Etching

Etch Process - Remove Exposed Copper

Copper Removed

Layer 2 (Inner)

Layer 3 (Inner)

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Core

Remove Laminating Film

Layer 2 (Inner)

Layer 3 (Inner)

Film Strip

• The laminated film is removed prior to AOI inspection.

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Core

Completed Inner Layer Core

Layer 2 (Inner)

Layer 3 (Inner)

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

• Automatic Optical Inspection (AOI).

• Inner layers are scanned and compared to Gerber data for errors.

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Layer stackup

Layer 1 (Outer)

Layer 2 (Inner)

Layer 3 (Inner)

Layer 4 (Outer)

FOIL

FOIL

PRE-PREG

PRE-PREG

INNER LAYER

Layer 1 (Outer)

Layer 4 (Outer)

Layer 2 (Inner)

Layer 3 (Inner)

FOIL

FOIL

PRE-PREG

PRE-PREG

INNER LAYER

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.

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Layer 1 (Outer)

Layer 2 (Inner)

Layer 3 (Inner)

Layer 4 (Outer)

FOIL

PRE-PREG

PRE-PREG

INNER LAYER

Layer stackup

FOIL

Layer 1 (Outer)

Layer 4 (Outer)

Layer 2 (Inner)

Layer 3 (Inner)

FOIL

FOIL

PRE-PREG

PRE-PREG

INNER LAYER

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Layer stackup

Layer 1 (Outer)

Layer 2 (Inner)

Layer 3 (Inner)

Layer 4 (Outer)

FOIL

PRE-PREG

PRE-PREG

INNER LAYER

FOIL

Layer 1 (Outer)

Layer 4 (Outer)

Layer 2 (Inner)

Layer 3 (Inner)

FOIL

FOIL

PRE-PREG

PRE-PREG

INNER LAYER

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Layer stackup

FOIL

PRE-PREG

PRE-PREG

INNER LAYER

FOIL

Layer 1 (Outer)

Layer 2 (Inner)

Layer 3 (Inner)

Layer 4 (Outer)

Layer 1 (Outer)

Layer 4 (Outer)

Layer 2 (Inner)

Layer 3 (Inner)

FOIL

FOIL

PRE-PREG

PRE-PREG

INNER LAYER

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Layer stackup

FOIL

PRE-PREG

PRE-PREG

INNER LAYER

FOIL

Layer 1 (Outer)

Layer 2 (Inner)

Layer 3 (Inner)

Layer 4 (Outer)

Layer 1 (Outer)

Layer 4 (Outer)

Layer 2 (Inner)

Layer 3 (Inner)

FOIL

FOIL

PRE-PREG

PRE-PREG

INNER LAYER

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Layer stackup

FOIL

PRE-PREG

PRE-PREG

FOIL

Layer 1 (Outer)

Layer 2 (Inner)

Layer 3 (Inner)

Layer 4 (Outer)

Layer 1 (Outer)

Layer 4 (Outer)

Layer 2 (Inner)

Layer 3 (Inner)

FOIL

FOIL

PRE-PREG

PRE-PREG

INNER LAYER

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Layer stackup

Layer 1 (Outer)

Layer 2 (Inner)

Layer 3 (Inner)

Layer 4 (Outer)

Layer 1 (Outer)

Layer 4 (Outer)

Layer 2 (Inner)

Layer 3 (Inner)

FOIL

PRE-PREG

PRE-PREG

FOIL

FOIL

FOIL

PRE-PREG

PRE-PREG

INNER LAYER

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Layer stackup

Layer 1 (Outer)

Layer 2 (Inner)

Layer 3 (Inner)

Layer 4 (Outer)

Layer 1 (Outer)

Layer 4 (Outer)

Layer 2 (Inner)

Layer 3 (Inner)

FOIL

FOIL

PRE-PREG

PRE-PREG

INNER LAYER

FOIL

PRE-PREG

PRE-PREG

FOIL

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Bonding – Heat

Layer 1 (Outer)

Layer 2 (Inner)

Layer 3 (Inner)

Layer 4 (Outer)

Layer 1 (Outer)

Layer 4 (Outer)

Layer 2 (Inner)

Layer 3 (Inner)

FOIL

FOIL

PRE-PREG

PRE-PREG

INNER LAYER

FOIL

PRE-PREG

PRE-PREG

FOIL

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.

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Bonding – Multilayer Press

Layer 1 (Outer)

Layer 2 (Inner)

Layer 3 (Inner)

Layer 4 (Outer)

FOIL

PRE-PREG

PRE-PREG

FOIL

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

Layer 1

Layer 4

Copper Laminate Drilled Hole

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.

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Electroless Copper ProcessAddition of Copper to all Exposed Surfaces

Layer 1

Layer 2

Layer 3

Layer 4

Copper Drilled Hole

Plating

• 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.

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Layer 1

Layer 2

Layer 3

Layer 4

Laminating and Imaging of External Layers

UV sensitive film is laminated over top and bottomsurfaces of PCBIt is then exposed and developed, leaving an exposed image of the PCB pattern

Copper

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

Imaging

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Layer 1

Layer 2

Layer 3

Layer 4

Electro-plating Process 1Additional Copper to all Exposed Surfaces

Laminated Film Plate Additional Copper

• 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.

Plating

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Layer 1

Layer 2

Layer 3

Layer 4

Electro-plating Process 2Add Tin over Exposed Copper Areas

Laminated Film Additional CopperTin Plating

Tin Plating

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

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Layer 1

Layer 4

Electro-plating Process 3Remove Laminated Film

Laminated Film Removed Tin Plating

Layer 2

Layer 3

Film Strip

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

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Etch Process - Remove Exposed Copper

Copper Removed Tin Plating

• 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 approximately0,025 mm (0.001”) thinner than the original design.

Etching Process

Layer 1

Layer 4

Layer 2

Layer 3

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Layer 1

Layer 2

Layer 3

Layer 4

Tin Strip - Remove Tin Plating

Tin Plating Removed

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

Tin Strip

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PCB is now complete except forsurface finishes and panel routing/punching

Layer 4

Layer 1

Via Hole SMD PadTracks

Tracks

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Solder Mask ApplicationImage, Develop and Cure

Layer 4

Layer 1

UV Image, Develop and Cure

• 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.

Solder Mask

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Surface Finish Process

Layer 4

Layer 1

Apply Solder to Exposed Copper Areas

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

Surface Finish

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Component Ident

R34

IC3

R34

IC3

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 PCB’s are 100% Electrically tested

All PCB’s are 100% Visually Inspected

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