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EAT – E-learning

Module 3. Printed wiring boards 3.1 Types and materials of printed wiring boards

Department of Electronics TechnologyBudapest University of Technology and Economics

BME-ETT Elect2eat Team: Zsolt Illyefalvi-Vitéz, PhDwww.ett.bme.hu Olivér Krammer

János Pinkola, dr

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The Printed Wiring Board (PWB)

A printed wiring board (or PWB) is used to mechanically support and electrically connect electronic components using conductive pathways, or traces, etched from copper sheets laminated onto a non-conductive substrate.

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Types and materials of printed wiring boards

Cu layer: 17, 35, 70, (105) µmSubstrate (v): 0,2….3,2 mm

Cu wire

Fiberglass layer

epoxy

Functions:• Electric connection between component leads• Fixing components mechanically

Printed wiring:• Wiring made on resin based insulating board

(foil, surface)• Conductive layer is mainly copper

Substrate (base material):• Copper foil covered, reinforced resin

v

Insulating substrate

Cu layer

A Printed Wiring Board (PWB) is a substrate, which is made up of an insulating board with copper foil tracks on its outer surfaces.

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PWB categories according to the layer structure

• one sided

• double sided

• multilayer

• metal substrate

• metal core

• 3D, MID (Molded Interconnect

Device)

• multiwire

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PWB categories according to types and materials

For circuit cards and modules the rigid printed wiring boards are used, while flexible and rigid-flex printed wirings are popular for the interconnection of cards placed in different position in equipment or of different modules and units.

Rigid printed wiring board Flexible printed wiring board

Rigid - Reinforcing material (paper, fiberglass, polyaramide, metal…)

- Resin (phenol, epoxy, polyimide, PTFE…)

Flexible (polyester, polyimide, PTFE)

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Characteristics of PWB substrates

FR flame retardant

CEM Composite Epoxy Material:Cu

fiberglass

epoxy

paper

Resin phenol epoxy epoxy epoxy

Core material paper paper fibergl./paper fiberglass

Standard (NEMA) FR2 FR3 CEM1 FR4

Flexural strength (N/mm 2) 135 150 330 500

Water absorption (mg) 28 25 10 7

Solderbath resistance (sec) 15-20 25-30 30-40 >120

Adhesion of Cu foil (N/mm) 2,0 2,2 1,7 2,0

Sheet resistance (ohm) 10 11 2x10 11 10 12 >10 12

Cutting properties +++ +++ ++ +

Flammability V0 V0 V0 V0

Price percentages 55 65 80 100

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PWB categories according to pattern resolution

Normal Fine Very fine

Through-holemounted:

SMT, COB, MCM-L:

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• electrical connection between certain conducting layers• more reliable soldered joints when using through-hole mounted components

Aim of the through-hole metallization

Through-hole with

metallized wall

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delamination

Typical through-hole failures

barrel crack

corner crackpad rotationmissing plating

pad lifting

nodule

pull away

break between layer and plating

plating voidmisalignment of PCB during drilling

imprecisefitting

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3. Printed wiring boards3.2 Mechanical technologies in PWB-production

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Mechanical technologies in PWB-production1. Hole formulation

– punching: Ø > 1,0 mm, FR2, FR3– drilling: Ø > 0,1 mm, FR4...– laser-, photo-, plasmavia: Ø 0,05…0,2 mm

2. Brushing3. Milling (routing)4. (Pressing)

1. Hole formulation

a) PunchingThe shape is formed by pressing the material against a die with a huge force

The shear forces generated between the material and die separate the material into the desired shape

Removal of material by the relative movement of the tool and the workpiece

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b) Drilling

Primary movement: rotation of drill → cutting speed (v, m/min)

Secondary movement: perpendicular to the surface → feedrate (f, mm/rot.)

• the material of workpiece• the material of tool and it's geometry• the speed components of relative movement (main-

and side movements)

Dominant factors:

Composition:

Mechanical technologies in PWB-production

• 88...94 % tungsten carbide (WC)• 6...12 % cobalt (Co)

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Drill package formulation

Entry plate:• preventing of abrasion• preventing of burring• increase of hole position accuracy• increase of hole wall qualityBackup plate:• preventing of burring• protecting of CNC working table

Entry plate

Backup plate

HoleSubstrates

Mechanical technologies in PWB-production

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c) Drilling by UV laser

Laser micro-hole drilling can be used to produce micro-holes in almost any material.

Very high position and diameter tolerances can be achieved.

1. step 2. step

Removing Cu Removing organicwith high intensity with low intensity

Cleaned and coarsed copper

surface

d) Plasma etchingHigh voltage, high energy, rapid rise time electrical pulses are delivered many times per second to an electrode assembly in contact with the material body to generate therein elongate plasma channels which expand rapidly following electrical breakdown of the material causing the material to fracture and fragment.

Mechanical technologies in PWB-production

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2. Brushing

The brushing mashine contains rotating brushes with various corning and a conveyor system for feeding the board into them.

For better efficiency the boards are continuously washed by water spraying

3. Milling (routing)

In contrast to drilling, where the drill is moved exclusively along its axis, the milling operation involves movement of the rotating cutter sideways as well

Milling is the process of cutting away material by feeding a workpiece past a rotating multiple tooth cutter

Mechanical technologies in PWB-production

Click on the figure to start movie!

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3. Printed wiring boards3.3 Chemical technologies in PWB-production

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Chemical technologies in PWB-production

The chemical technologies include the cleaning, layer deposition, layer removal, surface finishing and rinsing processes.

The most important electrochemical and electroless layer deposition processes are based on the same principle: on reduction.

Me n+ + ne - = Me (reduction)

1. Electroplating +_

Men+Can be applied only onto conductive surfaces, for selective coatings it is not suitable.

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2. electroless deposition

3. „direct plating”after deposition of conductive chemical compound onto the insulator surface it can be electroplated

4. immersion deposition

Me1n+

Me2 kekMe2Me2

Me1neMe1n

- for catalytic insulators- purpose is to metallize the isolating wall of drilled holes

Men+ + reducing material = Me

e.g. CuSO4 + 4NaOH + 2HCHO = Cu+2HCOONa + Na2SO4 + H2 + 2H2O

Chemical technologies in PWB-production

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3. Printed wiring boards3.4 Patterning processes of PWBs: Masking technologies

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Masking technologies

1. Screen printing

For patterning (or imaging) with a mask, the dry film photoresist method is the most popular in the PWB industry. Alternatively, the less expensive but lower resolution screen printing imaging technology can be used. Screen printing is the main imaging process used for solder resists, as well.

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Masking technologies

Photoresist applied onto Cu foil,exposed to light and developed

Cu-layer

Photoresist

Cu-layerPhotoresist

3.step: Developing dry film photoresist

2. Dry film photoresist technology

2.step: Photoresist exposure1. step: Lamination

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3. Printed wiring boards3.5 Fabrication of single sided boards using

subtractive or additive technology

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Processing possibilities:

• Subtractive technologyThe raw material is a dielectric plate with copper cladding on one or both sides. The copper layer is removed (usually by chemical etching) where the wiring is

not needed. The resolution is limited by the adhesion of the conducting layer and the

undercutting effect.

• Additive technologyThe conducting layer is deposited on the insulating substrate in a particular

pattern using a mask. It results in finer resolution but worse adhesion.

• Semi-additive technologyIt combines the advantages of both previous technologies

Fabrication of single sided boards usingsubtractive or additive technology

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The subtractive and additive technologies

Subtractive Additive

Cu foil covered insulating board Insulating board

Electroless metallization, Positive mask(screenprinting, photoresist, metal)

Negative mask(screenprinting, photoresist)

Etching, mask stripping Electroless metallization, mask stripping

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Subtractive technology of single sided PWBs

Base material: copper foil covered insulating board

Undercutting Mushroom effectMask removal

[Removing metal mask („orange effect”)]

Negative mask(photoresist, screen printing)

Positive mask(photoresist, screen printing)

Etching Positive metal mask (Sn, Sn/Pb…)

Etching

Solder mask

wire pad

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3. Printed wiring boards3.6 Subtractive technology of double sided, through-

hole metallized boards

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Subtractive technology of double sided, through-hole metallized boards

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The subtractive and additive technologies I.

Subtractive Additive

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Electroless copper

The subtractive and additive technologies II.

Subtractive Additive

Sn plating

Sn reflow

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Semi-additive technology

A possible way of producing the substrate

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3. Printed wiring boards3.7 Technology of co-laminated multilayer printed

wiring boards

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Technology of co-laminated multilayer printed wiring boards

Each inner layer must be patterned and the surface of the Cu must be prepared for gluing (Sn stripping if necessary, oxidization)

The inner boards can have through holes that will become buried vias in the multilayer board.

Co-lamination technology: the boards are glued together with preimpregnated (prepreg) B-stage epoxy foil.

Needs at least 30...60 minutes on 170 oC, at 150 N/cm2 pressure.

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Cu plating

Sn/Pb plating

resist stripping

Cu etching

Technology of multilayer boards II.

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Via types and layers of multilayer boards

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Sequential technology with wall-metallized vias

• Sequential build-up (SBU) technology:

A multilayer board is created by applying conductive and insulating layers one after each other.

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Sequential build up (SBU) technology

Comparison of structure of different microvias

Laser

Plasma

Drilling

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3. Printed wiring boards3.8 Special printed wiring boards and their technologies

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Technology of Metal Substrate PWB-s

•Metal core covered with insulating layer and a copper

foil on the outer surface.

Aim: to achieve better heat conductivity• epoxy-woven fiber glass board: 0.2 W/mK,• IMS boards: 1.3 W/mK.

IMS = Insulated Metal Substrate

Formulation of the pattern by subtractive

technology:

drilling of the metal plate, filling the hole with

epoxy enriched with Al2O3,

drilling of the epoxy, metallization of the holes

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•Aim: to minimize the thermal expansion mismatch between the substrate and the components / to adjust the thermal expansion coefficient of the substrate to that of the components

Thermal expansion coefficient:• epoxy-woven fiberglass 12..16 ppm/°C• CCC encapsulation 5.9…7.4 ppm/°C

Metal Core PWB

CCC = Ceramic Chip Carrier

V=0.12..1.5 mm

Core materials ( 5 ppm/°C):• Cu-Mo-Cu (CMC)• Cu-Invar-Cu (CIC)

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Flexible PWBs

Two possible ways of manufacturing:1. Cu foil laminated onto the plastic,2. Plastic is deposited onto the Cu foil(the more

up-to-date technology).

Materials: plastic (polyimide - Kapton, polyester - Mylar,PTFE - Teflon)foil without rreinforcement

Available in single-, double- and multilayer construction as well.

Application: connecting moving elements, vibration tolerating devices (due to smaller mass), 3D interconnection systems.

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Rigid-flex combined PWB

Adhesive foil

Wiring pattern

Metallized holes

Metallized through holes

Flexible substrate

Rigid substrateRigid

substrate

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3D MIDs (Molded Interconnection Devices)

Interconnection is produced by applying the wiring onto the surface of the plastic devices.• Metallization of the plastic:

electroless Cu deposition.• Applying the photoresist:

by electrophoresis• Exposure: direct writing by laser exposure applying of 3D fotomask

The 3D wiring may replace some mechanical components, like touch

switch components

SMD

Shock resistant plastic, e.g. PEI=polyetherimid

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Multiwire PWB

Multiwire PWB: combining the PWB technology with the conventional wiring. Metallised through holes are made after sticking insulation covered wires into the cover prepreg film.