FYS4260/FYS9260: Microsystems and
Electronics Packaging and Interconnect
Interior detail from an
Apple iPhone 5 printed circuit board

Learning objectives
• Understand how printed wiring/circuit boards are constructed
• Types of printed circuit boards
• High density interconnect technology
• Background literature:
– Halbo & Ohlckers Chapter 5
– The HDI handbook
– Malestroem: The printed circuit handbook 6 th ed.

The substrate
• The are several purposes for the substrate for electronic component mounting, including:
– Mechanical support
– Electrical interconnection
– Heat conduction

Printed Circuit Board
A printed circuit board (PCB) mechanically supports and electrically connects electronic components using conductive tracks, pads and other features etched from copper sheets laminated onto a non-conductive substrate.
PCBs can be single sided (one copper layer), double sided (two copper layers) or multi-layer. Conductors on different layers are connected with plated-through holes called vias. Advanced PCBs may contain components - capacitors, resistors or active devices - embedded in the substrate http://en.wikipedia.org/wiki/Printed_circuit_board

Printed Wiring Board vs.
Printed Circuit Board
• Halbo & Ohlckers book emphasises the difference between printed wiring board and printed circuit board
• In principle/by definition, an electrical circuit is not a circuit before it is closed, that is, components have been attached.
• When the board has only copper connections and no embedded components, the term printed wiring board
(PWB) has been recommended.
• Although more accurate, the term printed wiring board has fallen into disuse.
• The lecturer will therefore often use PCB also when
PWB could be more precise.

Some requirements on substrate technology
Function Requirement
Electrical property • High conductivity in conductive wires
• High resistance between wires
Mechanical properties • Stiff and rigid
• Low weight
• Low coefficient of thermal expansion (why?)
Chemical resistance • Must withstand solder fluxes and other chemicals during manufacturing
• Must be stable in usage environment
Fire resistance
Processability
Adhesion
Moisture absorption
• An overheated component may not cause the entire circuit board to catch fire. (FR = flame retardant)
• Compliant with component assembly process
• Possible to shape to fit the application
• Many components are glue attached to the substrate
• Metallization must stick on the substrate
• Organic polymers tend to absorb water from the environment.
Price and availibility • Cost and availability of processing equipment/facilities are always concerns

Classification of circuit boards
1. Base material
2. How wires are defined
3. Physical nature
4. The way wires are formed
5. Number of layers
(single/double/multi layer boards)
6. Plated through holes
7. Production process
Malestrom: Printed Circuit Board Handbook, 6 th ed

RoHS directive:
Impact of lead-free soldering requirement changes FR-4 composition
Malestrom: Printed Circuit Board Handbook, 6 th ed

Another way of classifying laminates
FR-2
FR-3
FR-4
FR-5
FR-6
G-10
CEM-1
CEM-2
CEM-3
CEM-4
CEM-5
CEM-6
CEM-7
CEM-8
*
*
*
*
*
*
Resin
Reinforcement Flame
Cotton Woven Mat Glass retard-
Grade Epoxy Polyester Phenolic paper glass glass veil ant
XXXPC
*
*
*
*
*
*
*
*
*
*
*
*
*
*
*
*
*
*
*
*
*
*
*
*
*
*
*
*
*
*
*
*
*
*
*
*
*
*
*
*
*
Table 5.1: Conventional laminates for printed wiring boards. (The designations are according to National Electrical Manufacturers Association, NEMA, USA.)

Laminate material classification
(alternative to Fig 5.1 in H&O)
Malestrom: Printed Circuit Board Handbook, 6 th ed

Physical properties of some common base materials
Malestrom: Printed Circuit Board Handbook, 6 th ed

FR4 laminate structure
• FR4 is the most widely used starting point for PCBs
• FR4 consists of a woven glass fibre mesh soaked in organic polymer (epoxy) with copper layers laminated – possibly with filler material (similar to steelenforced concrete)
• FR means Flame Retardant
• Organic substrates: Substrates where organic polymers are binders
• Prepreg: The laminate before copper is added and before final curing of laminate
Fig. 5.1: Woven glass fibre for printed wiring board reinforcement

FR4 build-up (two-layer structure)
HDI Handbook p 188

Conventional manufacturing process for
PCB labminates
Printed Circuits Handbook 6 th Edition

Printed Wiring Board technology examples
Fig. 5.2: Printed wiring board structures with varying complexity: a) Single sided and double sided. b) Double sided through hole plated with bare Cu or
Sn/Pb surface. c) Four layer board. d) Six layer board with two Cu/Invar/Cu cores.

And multilayer structures become increasingly dense
Multilayer boards and high density interconnects are increasingly more common and available
Illustration of multilayer Printed Wiring Board
FYS4260/FYS9260 Frode Strisland 17

Multi-layer PCB
Cross section of 14-layer multilayer printed wiring board, showing a typical inner layer and prepreg material relationship. In this case, to reduce z-axis expansion, the innerlayers are polyimide, while the prepreg material is semicured polyimide.Typical signal, power, and ground layers are also indicated, as well as the thickness of the copper foil for each layer.
Printed Circuits Handbook 6 th Edition
FYS4260/FYS9260 Frode Strisland 18

Single sided boards processing steps
1. Drilling / punching of registration holes
2. Panel cleaning
3. Printing of etch resist
4. Etching
5. Stripping
6. Printing solder resist
7. Curing of solder resist
8. Cleaning of solder areas
Illustration of steps 3-5
9. Deposition of solder coating
10. Punching of holes and edge contour (or drilling/milling)
This is a subtractive process (wires defined by removal of Cu)
Alternative: Additive processes (based on plating Cu)

Single sided boards processing steps
(alternative illustration)
Fig. 5.4: Process steps of "print and etch" process for single sided boards

Printed Wiring Boards:
Manufacturing Process
• Two main classes of wire definition approaches:
– Graphical the image of the master circuit pattern is formed photographically on a photosensitive material, such as treated glass plate or plastic film. The image is then transferred to the circuit board by screening or photoprinting the artwork generated from the master.
– Discrete wire interconnection boards
Discrete-wire boards do not involve an imaging process for the formation of signal conductors. Rather, conductors are formed directly onto the wiring board with insulated copper wire. Wire-wrap® and Multiwire® are the best known discrete-wire interconnection technologies. Because of the allowance of wire crossings, a single layer of wiring can match multiple conductor layers in the graphically produced boards, thus offering very high wiring density. However, the wiring process is sequential in nature and the productivity of discrete-wiring technology is not suitable for mass production.

Discrete wire interconnection board
(example)
Printed Circuits Handbook 6 th Edition

Processing of Double Sided Thru-Hole
Plated Boards
Comment: Process description based on the production of eutectic tin-lead solder. This is not according to industry standard, but illustrates the principles and the way the project prototypes are manufactured
FYS4260/FYS9260 Frode Strisland 23

Graphical illustration of the thru-hole plated boards process steps
Fig. 5.5: Through hole plated PWB, process steps: a) Panel plating. b) Pattern plating. c) Hot air levelling.
FYS4260/FYS9260 Frode Strisland 24

Processing of Double Sided Thru-Hole
Plated Boards (1)
1. Drilling
2. Cleaning of the surfaces and hole ("deburring"), and a mild etch to ensure adhesion in later steps
3. Activation for chemical (electroless) plating.
Dipped into a solution containing Sn 2+ ions, to increase the sensitivity of the surface. The activation takes place in an acidic solution of palladium chloride, that is transformed into metallic Pd. Reaction:
Sn 2+ + Pd 2+ -> Sn 4+ + Pd (met).
In the later plating process, Pd catalyses the deposition of copper.

Processing of Double Sided Thru-Hole
Plated Boards (2)
4. Chemical (electroless) plating of Cu:
Dipped into a reducing bath containing Cu 2+ ions, for example in the form of dissolved CuSO
4
.
Formaldehyde, HCHO, is the common means of reduction. In this bath, Cu 2+ is reduced to Cu that covers the whole surface, including the holes, also where the surface is electrically insulating. At the same time formaldehyde is oxidised into acetic acid.
The plated thickness is approximately 3 µm. The purpose is to create an electrically conducting surface everywhere, for the subsequent step.
5. Electrolytic (electro-) plating of Cu: dipped into an electrolyte that contains Cu2+ ions, such as CuSO
H
2
SO
4
4
dissolved in
. The panel forms the negative electrode (cathode), and a metallic copper plate forms the positive electrode (anode) of an electrolytic cell. At the anode copper is dissolved:
Cu -> Cu 2+ + 2e-.
The reaction at the cathode is the following:
Cu 2+ + 2e- -> Cu, thus, metallic copper is deposited on the panel. Approximately 25 – 30 µm Cu is normally plated, in order to get good coverage in the via holes.

Processing of Double Sided Thru-Hole
Plated Boards (3)
6. Pattern definition
Dry film photoresist is laminated on to both sides, normally negative resist. The resist is illuminated through a positive photographic mask and is developed. The pattern is therefore black on the photomask, and the photoresist will dissolve where there is a pattern, during the development.
7. Tin/lead plating for etch masking:
The panel is connected to the cathode of an electrolytic bath containing
Sn 2+ and Pb 2+ ions. The anode is metallic Sn/Pb alloy. The electrolyte is based on fluoroboric acid, HBF
4
. The ratio between the concentration of the ions in the bath and on the anode, is such that the deposited layer of metal on the panel will be approximately the eutectic mixture 63Sn/37Pb
(percent by weight). The normal thickness is about 7 µm. After this the photoresist is dissolved in a suitable solvent, for instance methylene chloride.

Processing of Double Sided Through
Hole Plated Boards (4)
8. Etching:
The Cu foil is etched simultaneously on both sides, analogous to step 4, Section 5.5, but with an ammonia-based etch bath, which does not attack Sn/Pb. The plated Sn/Pb serves as an etch resist.
After the etching, the Cu is covered with Sn/Pb where we want conductor pattern and solder lands, as well as in the holes through the board.
9. Fusing:
If it is desired to have Sn/Pb on the completed board, a "fusing" step follows. It consists in heating of the board to a temperature where the alloy melts and changes its crystalline structure. It flows and covers the nearly vertical edges of the etched copper. We get an intermetallic copper/tin interfacing layer. The heating may take place in hot air or oil, by IR radiation heating, etc.
10. Organic solder resist may be added by screen printing

Graphical illustration of the thru-hole plated boards process steps
Fig. 5.5: Through hole plated PWB, process steps: a) Panel plating. b) Pattern plating. c) Hot air levelling.
FYS4260/FYS9260 Frode Strisland 29

Double Sided Through Hole Plated Boards:
Choice of Surface Metallisation and Solder
Resist
Fig. 5.6.a: Selective Sn/Pb surface coverage with hot air levelling. The alternatives, bare Cu or Sn/Pb on all Cu surface, are shown in Figure 5.2 b).

Definition of patterns
2000 
Lasers are used to define pattern on photographic plate or to expose photolitography film
Pattern definition anno 1960

Choice of Surface Metallisation and Solder
Resist, continued
Fig. 5.6.b: "Tenting", i.e. covering of the via holes by dry film solder resist.
If tenting is only partially succesful, chemical entrapment from surface finish preclean lines is probable

Multilayer Printed
Wiring Boards
1. Drilling
2. Rinse, Photo process for inner layers
3. Etch inner layers
4. Black oxidation for adhesion promotion
5. Baking
6. Lamination
7. Drilling of through holes
Further process as for double layer boards

Fig. 5.7:
Process steps for multilayer printed wiring boards with holes only through the board.

Via Holes in Multilayer Printed Wiring
Boards
Fig. 5.8: Types of via holes: a) Through hole. b) Buried hole. c) Blind hole.
Figure d) shows a microscope section of a drilled blind via. (Contrave´s
"Denstrate" process).

Fine Line Printed Wiring Boards, Additive
Process
Fig. 5.9 a): The development of minimum line width from 1965 until
1990. The figures in the ovals tell how many conductors can be positioned between the leads of DIP-components with a lead pitch of
0.1" (number of "channels").

Fine Line Printed Wiring Boards,
Additive Process, continued
• Etch control: Under etch/etch factor
• Accuracies in subtractive processes limited by underetching.
• Additive process avoids this problem, but requires cleanroom processing and collimated light
Fig. 5.9 b): Underetch and etch factor.

Fine Line Printed Wiring Boards:
Photolithographic Process
Fig. 5.10.a:
Machine for double sided illumination with parallel light, for pattern transfer from photographic film for fine line printed wiring boards.
Take-home message from this figure from the 90's: Alignment is still important in two-sided photolitographic processes

Fine Line Printed Wiring Boards:
Photolithographic Process, continued
Fig. 5.10.b :
Automatic in-line system for lamination of photoresist, illumination and development, in an enclosed clean room atmosphere.

Metal Core Printed Wiring Boards
• Better heat conduction
• TCE matching with ceramic packages
• Most common:
Cu/Invar/Cu
Fig. 5.2.d) Six layer board with two Cu/Invar/Cu cores.

Metal Core Boards, continued
Invar Cu
Fig. 5.12 a): Cross section of metal core board with one
Cu/Invar/Cu core (Texas
Instruments).
Fig. 5.12 b):
Thermal coefficient of expansion of
Cu/Invar/Cu, as function of the composition (Texas
Instruments).

New Materials for PWBs
• Higher Tg
• Better dimensional stability
• ε r
low, not dependent on T, f, or moisture
• Low losses
• Lower TCE
• Purpose
– High frequency use
– Controlled characteristic impedance
– High reliability
• Materials
– Cyanate ester
– PTFE (Teflon)
– Polyimide
– and others

New Materials for PWBs, continued
Fig. 5.13: TCE for FR-4 below and above T g
in a): the x or y direction, b): the z-direction.

New Materials for PWBs, continued
Table 5.2: Material parameters for polymers for printed wiring boards
Material ε r
Tan δ
(at 1 MHz)
Paper/phenolic
Bisphenol epoxy (FR-4)
Multifunctional epoxy
Tetrafunctional epoxy
BT/epoxy
Cyanate ester
Polyimide (Pi)
PTFE (Teflon)
*) Melts, no regular glass-transition
4.7
4.3 - 5
0.025
0.02
4.3 -4.5
0.02
4.3 -4.6
0.02
3.5 - 4.2
0.012
2.8 - 3.6
.002 - .005
3.0 - 4.6
.002 - .01
2.1
.001
α (T < Tg)
[ppm/ oC]
33 -60
140
55
100
50 -100
35 - 80
70 - 120
Tg
[oC]
95
130
145 - 180
> 150
185 - 225
250 -290
230 - 315
250 *)

New Materials for PWBs, continued
ε r tan δ
Fig. 5.14: Frequency dependence of ε r
ε r
and tan δ for FR-4.
:Relative dielectric constant. tan δ : Loss tangent.

Commercial Products, continued
Fig. 5.15 a): Structure of Rogers material RO2800.

Commercial Products, continued
Fig. 5.15 b): Combination of Gore-Ply and FR-4 gives a simple process, and at the same time low dielectric losses and reduced capacitance to ground.

Commercial Products, continued
Fig. 5.16: Attenuation in (dB) as function of frequency for a one meter long stripline, for the high performance materials Gore, Nelco and polyimide, compared to FR-4.

Commercial Products, continued
Fig. 5.17: Top: Microwire from PCK, with conductors insulated with organic insulation, and a metal foil as ground plane. Bottom: Next generation technology, where each conductor has its own metal shield.

Commercial Products, continued
Fig. 5.18: The equipment head that deposits the conductors on the laminate for
Microwire.

Special Boards
• Flexible printed wiring boards
– Dynamic or static bending.
– Uses: Movable parts and odd shaped, cramped places

Flexible Printed Wiring Boards, continued
Fig. 5.19: Flexible printed wiring boards:
Most of the electronics in Minoltas camera Maxxum
9000 is on two flexible printed circuit boards.

Table 5.4:
Properties for materials used for flexible printed wiring boards.
Typical values
Flexible Printed Wiring
Boards, continued
Unit Glass Epoxy
Solderability
Max. continuous operating temperature
Tensile strength
Moisture absorption
Coefficient of linear expansion
°C/s
°C
Peel strength to copper kp
260/10
150 kp cm-2 1750
%
4,5
0,5
°C-1
%
1,1 10-5
0,2 - 0,8
Etch shrinkage:
Machine direction
/transverse direction
Dielectric constant
(60 Hz)
Dissipation factor
(1 kHz)
Resistivity
Cost ratio
(laminate only)
Comments
3,4
0,037 ohm cm 1,6 1013
1,4/2
Not suitable for continuous folding use.
Max. peel strength to copper and minimum elongation.
Polyester base laminate
230/1
110
1500
1,8
0,8
1,5 10-5
1,0 - 0,55
3,25
0,006
1017
1
Sensitive to solder heat.
Lowest cost.
Good physical and electrical properties
Polyimide
base laminate
260/10
220
1700
1,3
2,5
2,0 10-5
0,45 - 0,25
3,5
0,003
4 1016
2/3
Non-flammable.
Outstanding physical and electrical properties.

Flexible Printed Wiring Boards, continued
Fig. 5.20: Cross section of flexible PWB:
Top: Single layer conductor foil.
Bottom: Double layer conductors with through hole plating.

Membrane Switch Panels
Purpose: Switches and informative instrument fronts.
Fig. 5.21 a): Membrane switch panel, schematically.
Top: Structure
Bottom: Cross section of a normal panel and a panel with metal dome.

Membrane Switch Panels, continued
Fig. 5.21 b): Exploded view of simple switch panel

End of Chapter 5:
Technologies for Electronics – Overview
• Important issues:
– Many circuit board technologies are used.
– You should understand and be able to explain the main processing steps in order to develop a PCB
• Please comment and discuss!