Chapter 7: Production of Printed Circuit Boards

1
Chapter 7Production of Printed Circuit Boards

• Focus on automated production of printed circuits
  by Surface Mounting Technology (SMT) and Hole
  Mounting Technology (HMT)

The course material was developed in INSIGTH II,
a project sponsored by the Leonardo da Vinci
program of the European Union
2
Hole Mounting

• Axial components Sequencing and mounting
• Radial components Mounting
• DIP components Mounting
• Odd components Robot or hand mounting

Fig. 7.1The process for production of hole
mounted PCBs
3
Hole Mounting, continued

• Fig. 7.2 a) Schematic example of the most
  efficient sequence of mounting the components of
  a particular PCB.

4
Hole Mounting, continued

• Fig. 7.2 b) The principle of sequencing.

5
Hole Mounting, continued

• Fig. 7.3 Sequencing machine.

6
Hole Mounting, continued

• Fig. 7.4 Axial inserter with two mounting heads.

7
Hole Mounting, continued

• Fig. 7.5 Simplified process in the axial
  inserter
• 1) Cutting the components from the tape
• 2) Lead bending
• 3) - 4) Insertion
• 5) Cut and clinch
• 6) Return to starting position.

8
Hole Mounting, continued

• Fig. 7.6 DIP inserter.

9
Hole Mounting, continued

• Fig. 7.7 Manual mounting board with light guide.

10
Hole Mounting, continued

• Fig. 7.8 Wave soldering machine.

11
Wave Solder Process

• Apply adhesive by dispenser, screen printing or
  pin transfer
• Cure by heat or UV
• Turn board
• Wave solder
• Double-wave soldering machine common for SMT
• Not all SMD components suitable for wave soldering

12
Wave Soldering, continued

• Fluxing
• Pre-heating
• Soldering
• (Cleaning)
• Fig. 7.9
• a) Principle of foam fluxer.
• b) Control system for density and level of the
  flux bath.

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Wave Soldering, continued

• Fig. 7.10 a) Principle of wave soldering.
• b) The real shape of the wave.

14
Wave Soldering, continued

• Fig. 7.11
• a) Industrial in line cleaning machine.
• b) The principle of ultrasound and vapour
  cleaning.

15
ElectroStatic Discharge (ESD) Precautions

• Fig. 7.12 An ESD protected working space. The
  resistors R normally are 100 Kohm - 1 Mohm.

16
Surface Mounting

• Soldering by wave solder process or by reflow
  process
• Fig. 7.13Application of adhesive for SMD
  mounting by
• a) Stencil or screen printing
• b) Dispensing
• c) Pin transfer

17
Surface Mounting, continued

• Fig. 7.14 a) Shadowing in SMD wave soldering.
• b) Solder bridging on fine pitch package.

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Surface Mounting, continued
Lambda wave

• Fig. 7.15 Double wave for SMD soldering. The
  first is a turbulent wave that wets, followed by
  a gentle lambda wave that removes superfluous
  solder.

19
Surface Mounting, continued

• Fig. 7.16 Temperature profile during wave
  soldering in a double wave machine.

20
Reflow Solder Process

• Print solder paste
• Mount components
• Dry solder paste
• Solder by heating to melting of paste

21
Solder Paste

• Consists of
• Solder particles ( 80 by weight)
• Flux
• Solvents and additives to give good printing
  properties (rheology)
• Typical mesh count in screen 80 per inch
• Area ratio Ao a2 /(ab)2
• Paste volume deposited V Vo Ao t
• "Solder ball test" for quality of solder paste
  and solder process

22
Solder Paste, continued

• Fig. 7.17 Microphotograph of Multicore solder
  paste type Sn 62 RMA B 3. The designation means
  62 by weight of Sn, 35.7 Pb, 2, Ag, 0.3 Sb,
  RMA flux, 75 µm average particle size, 85 metal
  content, viscosity 400 000 - 600 000 centipoise.

23
Solder Paste, continued

• Fig. 7.18 Test of solder paste The paste is
  printed through a circular opening with a
  diameter of 5 mm, in a 200 µm thick stencil.
  After reflow, the paste should melt into one
  body, without any particles spreading out.

24
Screen Printing

• Woven screen (stainless steel or polyester) with
  organic photosensitive layer, which is patterned
  with holes (mask).
• Metal stencil with etched or drilled openings.
• Polyester stencil with punched or drilled
  openings.
• Definition and accuracy depends on type, mesh
  count, thickness, tension, squeegee, speed, etc.
  Screen Printing is a complex craft!

25
Screen Printing, continued

• Off-contact for screen printing, contact for
  stencil. Two-step stencil for best definition.
• The most advanced printers are fully automatic
  with vision system for alignment.

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Surface Mounting, continued

• Fig. 7.19 Detail of printing stencil (left) and
  printing screen with fine line printing pattern.

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Surface Mounting, continued

• Fig. 7.20 Detail of printing stencil with fine
  pitch printing pattern Cross section of a
  stencil etched from both sides, with an
  acceptable, small amount of offset (40 x
  magnification).

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Surface Mounting, continued

• Fig. 7.21 Two steps printing stencil to give
  less solder paste deposited.

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Surface Mounting, continued

• Fig. 7.22 Printing through 0.3 mm diameter holes
  with Mylar stencil. To obtain the correct amount
  of solder paste two or three small holes may be
  used for each solder land.

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Surface Mounting, continued

• Fig 7.23 a) Screen printer.

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Surface Mounting, continued

• Fig. 7.23 b) The squeegee (DEK).

32
Convection Soldering

• Convection soldering oven.

33
Convection Soldering

• Convection soldering oven Temperature profile on
  PC screen.

34
IR Soldering

• Fig. 7.24 a) IR furnace. Schematically with low
  temperature "area emitter".

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IR Soldering, continued

• Fig. 7.24 b) Industrial IR furnace.

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IR Soldering, continued

• Infrared Soldering
• Plancks law
• W/A k1/l5 exp(k2/lT)-1
• where
• W/A emitted energy pr. second per m2 area per
  micrometer of radiation spectrum
• k1 2 ?hc2 h Plancks constant
• k2 hc/k k Boltzmanns constant
• Wavelength of max. radiation
• lmax k3/T
• Total radiated energy (Stefan Boltzmanns law)
• W/A esT4
• s Stefan Boltzmanns constant
• e emissivity (between 0 and 1)

37
IR Soldering, continued

• Graph of Plancks law

38
IR Soldering, continued

• Fig. 7.25 Typical temperature profile for an IR
  furnace.

39
Vapor Phase Soldering

• Newtons law
• dQ/dt hA (Tf -Ts)
• Where
• dQ/dt energy transferred pr. sec. (W)
• A total area
• h heat transfer coefficient
• Tf vapour temperature (boiling point)
• Ts PCB temperature
• PCB temperature approaches Tf asymptotically
• (Ts -To) Tf -To1 -exp (-t/to)

40
Vapour Phase Soldering

• Fig. 7.26 a) Principle of in-line vapour phase
  soldering machine.

41
Vapour Phase Soldering, continued

• Fig. 7.26 b) Industrial in-line vapour phase
  soldering machine.

42
Vapour Phase Soldering, continued

• Fig. 7.27 Heat transfer coefficient for air and
  fluorocarbons. Boiling fluorocarbons, at the
  bottom, give 200 - 400 times more efficient heat
  transfer than air.

43
Vapour Phase Soldering, continued

• Fig. 7.28 Temperature profile through in-line
  vapour phase soldering machine.

44
Vapour Phase Soldering, continued

• Fig. 7.29 Chemical composition of fluoro carbons
  for vapour phase soldering. Top The liquid
  FC-5311 (3M) C14 F24 is derived from C14 H10.
  Bottom The liquid LS 230 (Galden).

45
Vapour Phase Soldering, continued

• Table 7.1 Physical properties of some primary
  vapours for reflow soldering.

46
Other Soldering Methods

• Impulse (hot bar-, thermode-) soldering
• Hot plate / hot band soldering (thick film
  hybrid)
• Hot air soldering
• Laser soldering

47
Thermode Soldering

• Fig. 7.31 Two types of thermodes for thermode
  soldering.

48
Thermode Soldering, continued

• Fig. 7.32 Temperature profile for thermode
  soldering.

49
Component Placement

• Automatic, dedicated pick-and-place machines
• Manual placement (prototypes, repair)
• Semi-manual (light guided table, etc.)
• Programmable robot
• Elements of Pick-and-Place Machine
• Board magazine/feeder system
• Mounting head(s) (with interchangable grip tools)
• Programming/control unit
• Component "storage" and feeder
• (Vision system)

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Component Mounting

• Fig. 7.33 SMD pick-and-place machine
  (Siemens).The mounting head may also include an
  electronic vision system for very accurate
  placement of fine pitch components.

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Component Mounting, continued

• Fig. 7.34 a) Mechanical gripper in a pick-and
  place machine. b) Detail of the component tape
  when a component is in position for picking.
  c) Vibration feeder.

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Component Mounting, continued

• Fig. 7.35 Fuji CP-II pick-and-place machine. The
  machine has magazine for over 100 types of small
  components, nominal speed up to 15 000 components
  per hour, placement accuracy 0.10 mm. It has a
  rotating head with 12 positions, bottom figure,
  and two alternative tools at each position. There
  are components at all 12 positions at any time,
  with a separate operation being performed. A CCD
  camera shows the accurate position and
  orientation on a CRT screen (Fuji).

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Component Mounting, continued

• Fig. 7.36 Philips large hardware controlled
  pick-and-place machine.

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Solder faults

• Fig. 7.38 Small SMDs standing on edge due to the
  "Manhattan-" or tombstone-" effect.

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Robot System for Placement

• Advantages
• Flexibility Can handle most odd component types
  and boards, in low and high volumes
• Uniform quality
• High placement accuracy ( 0.02 mm)
• Non-manned operation (over night)
• Can work in hostile environments
• Tests and controls can be included in placement
  operation by special sensors on robot

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Robot Mounting

• Fig. 7.39 Example of a programmable placement
  robot for electronics The SCARA robot.

57
Robot System for Placement

• Must be carefully considered
• Cost, including the external equipment, fixtures,
  transport system
• Lower capacity than Pick-and-Place
• Requires careful planning, and often much
  dedicated surrounding equipment

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Robot Mounting, continued

• Fig. 7.40 The main components of a robot system.

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Robot System Components

• Manipulator
• Learning unit
• Control unit
• Types of Manipulator Coordinate Systems
• Cartesian
• Cylindrical (including "Scara")
• Spherical
• "Human-like"

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Robot Mounting, continued

• Fig. 7.41 Types of robot arms a) Cartesian
  motion. b) Cylindrical. c) Spherical. d)
  "Human like". The SCARA robot is a special
  version of the cylindrical type.

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Robot System Components, continued

• Programming
• "Lead-and-learn
• "Jog-and-learn
• "Synthetic programming"

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Robot Mounting, continued

• Fig. 7.42 Multi gripper head.

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Robot Uses in Electronics

• Production
• Component placement
• Production of parts (coils, cables,....
• Board feeding
• Handling of boards, components in testing
• Automatic trimming in test
• Parts assembly for board, rack, chassis, etc.
• Screw and glue operation
• Soldering, welding
• etc.

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Robot Mounting, continued

• Fig. 7.43 Robot cell for electronic component
  placement (Adept)

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Types of Boards SMD and Mixed Assembly

• SMD side A
• SMD side A and hole components side B
• SMD side A and B
• SMD both sides, hole components side B

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Process Sequences

• Fig. 7.44 a -d) Process sequences for boards
  with different types of components on the two
  sides.
• The steps marked "For all processes" on figure a)
  are not repeated on the other figures.

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Process Sequences

• Fig. 7.44 a -d) Process sequences for boards
  with different types of components on the two
  sides.
• The steps marked "For all processes" on figure a)
  are not repeated on this figure.

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Process Sequences

• Fig. 7.44 a -d) Process sequences for boards
  with different types of components on the two
  sides.
• The steps marked "For all processes" on figure a)
  are not repeated on this figure.

69
Process Sequences

• Fig. 7.44 a -d) Process sequences for boards
  with different types of components on the two
  sides.
• The steps marked "For all processes" on figure a)
  are not repeated on this figure.

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Board Testing

• Functional test
• "In-circuit" test
• NB Good designs use one-sided testing
• Test jigs are expensive
• Two-sided jigs very compicated

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Testing of PCBs

• Fig. 7.45 Two methods for single sided test of a
  board with components on both sides.

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Testing of PCBs

• Fig. 7.46 Bed-of-nails test fixture.

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Testing of PCBs

• Fig. 7.47
• a) Detail of single sided test fixture.
• b) Double sided fixture.

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Testing of PCBs

• Fig. 7.48 Two types of test pins.

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Testing of PCBs

• Fig. 7.49 Unacceptable testing. The test point
  should be on the Cu foil on the board, not on the
  component lead.

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End of Chapter 7 Production of Printed Circuit
Boards

• Important issues
• When manufacturing PCBs
• Understand the basic manufacturing steps
• Sequencing and mounting of Hole Mounted
  Components
• Wave soldering Basics. Why we want to avoid
  (yield and reliability problems) When to use it
  for Surface Mount Components (Mixed boards)
• Reflow soldering process Basics. Solder paste.
  Silk screen and stencil printing. Reflow heating
  with hot air, IR, vapor phase, etc.
• Component placement
• Automatic, manual, semi-automatic, and using
  robots
• Types of SMD boards manufactured Understand and
  remember the basic flow diagrams
• SMD side A
• SMD side A and hole components side B
• SMD side A and B
• SMD both sides, hole components side B
• Board testing
• Functional test
• In-circuit test
• Questions and discussions?