How is a PCB Made ? What determines impedance ? .

How is a PCB Made ?What determines impedance ?

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Manufacturing Processes


Manufacturing Processes for a Multi-layer PCB

Section through PCB

Via hole

SMD Pad

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

The diagrams which followrepresent a section througha 6 layer PCB, as indicatedin red.

Tracks under solder mask

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Typical Layer Construction - 6 Layer PCB

Layer 1 (Outer)

Layer 6 (Outer)

Layer 2 (Inner)

Layer 3 (Inner)

Layer 4 (Inner)

Layer 5 (Inner)

FOIL

FOIL

PRE-PREG

PRE-PREG

PRE-PREG

INNER LAYER

INNER LAYER

Copper Laminate

• Layer build / stackup is one of the most important aspects of controlled impedance

• Many combinations of material thickness can be used.

• PCB Fabricators manufacturing techniques vary

Impedance Considerations

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Drilling of Bonded Panel

Layer 1

Layer 6

Layer 2

Layer 3

Layer 4

Layer 5

Copper Laminate Drilled Hole


• Press temperature and pressure have an effect on the flatness and hence impedance. This should be checked prior to drilling

• Drilling itself does not effect impedance

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Electroless Copper ProcessAddition of Copper to all Exposed Surfaces

Layer 1

Layer 6

Layer 2

Layer 3

Layer 4

Layer 5

Copper Drilled Hole

• Electroless copper effects copper thickness on outer layers (T)

• Sometimes other solutions are used containing carbon etc


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Layer 1

Layer 6

Layer 2

Layer 3

Layer 4

Layer 5

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

• Does not effect impedance


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Layer 1

Layer 6

Layer 2

Layer 3

Layer 4

Layer 5

Electro-plating Process 1Additional Copper to all Exposed Surfaces

Laminated Film Plate Additional Copper

• Electro-plating increases the copper thickness on outer layers (T)

• This will always be variations in the amount of copper added.

• This finished copper thickness should be used in structure calculations


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Layer 1

Layer 6

Layer 2

Layer 3

Layer 4

Layer 5

Electro-plating Process 2Add Tin over Exposed Copper Areas

Laminated Film Additional CopperTin Plating



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Layer 1

Layer 6

Layer 2

Layer 3

Layer 4

Layer 5

Electro-plating Process 3Remove Laminated Film

Laminated Film Removed Tin Plating



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Etch Process - Remove Exposed Copper

Copper Removed Tin Plating

• 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.


Layer 1

Layer 6

Layer 2

Layer 3

Layer 4

Layer 5

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Layer 1

Layer 6

Layer 2

Layer 3

Layer 4

Layer 5

Tin Strip - Remove Tin Plating

Tin Plating Removed

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


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PCB is now complete except forsurface finishes and panel routing

Layer 6

Layer 1

Via Hole SMD PadTracks

Tracks

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Solder Mask Application- Curtain Coated Method

Layer 6

Layer 1

Apply Liquid Photo-imageable Resist, then Dry

• Some PCB Fabricators chose to check the impedance before the solder mask is added

• Structures can be checked in Normal and Coated mode

• Thickness of solder mask should be specified using H1


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Solder Mask ApplicationImage, Develop and Cure

Layer 6

Layer 1

UV Image, Develop and Cure



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Surface Finish Process

Layer 6

Layer 1

Apply Solder to Exposed Copper Areas

• Surface Finish (Tin / Lead / Gold / Silver) is usually only added to pads

• If board has no solder mask the thickness of finish should be added to T.


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Component Notation

R34

IC3

R34

IC3

SCL2 9624



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Routing (includes second stage drilling)


• Controlled Impedance coupons are routed from the panel

• Good controls are necessary to ensure that coupons can be matched to manufacturing panels

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Process finished PCB and coupon fortesting



• Controls are necessary to ensure that coupons can be matched to manufacturing panels this should be performed on trial panels prior to production ramp up.

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Why as a designer do you need todiscuss your design with your PCB fabricator?

Impedance ConsiderationsPCB manufacture is a process, it uses materials which are not “Ideal”

FR4 for example is a glass resin mix madeof two substances with differing electrical properties.

PCB Manufacturers need to make small adjustments to designs to maximise yields

• Glass Er 6• Resin Er 3 (FR4)• Resin Er < 3 (High

performance laminates)

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Why as a designer do you need todiscuss your design with your PCB fabricator?

Impedance ConsiderationsProcess varies from one fabricator toanother.

Press pressures temperatures may vary

Pre preg and Core may vary from oneSupplier to another.

• Supplier variations

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Process finished PCB and coupon fortesting



• Controls are necessary to ensure that coupons can be matched to manufacturing panels this should be performed on trial panels prior to production ramp up.

Design & test tools for controlled impedance & Signal integrity

PCB Faultfinding systems & software www.polarinstruments.com

©2003 Polar Instruments

Your contacts at Polar:

USA: Richard Smith (800) 328 0817 UK / Europe: Neil Chamberlain +44 23 9226 9113Asia / Pacific: Amit Bhardwaj +65 6873 7470

http://www.polarinstruments.com/

For more information please visit:


©2003 Polar Instruments

Your contacts at Polar:

USA: Richard Smith (800) 328 0817 UK / Europe: Neil Chamberlain +44 23 9226 9113Asia / Pacific: Amit Bhardwaj +65 6873 7470

http://www.polarinstruments.com/

How is a PCB Made ? What determines impedance ? .

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