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).

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.

13
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. 7.13 Application of adhesive for
  SMD mounting by
• a) 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.

18
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.
  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.

26
Surface Mounting, continued

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

27
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).

28
Surface Mounting, continued

• Fig. 7.21 Two steps printing stencil.

29
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.

30
Surface Mounting, continued

• Fig 7.23 a) Screen printer.

31
Surface Mounting, continued

• Fig. 7.23 b) The squeegee (DEK).

32
IR Soldering

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

33
IR Soldering, continued

• Fig. 7.24 b) Industrial IR furnace.

34
IR Soldering, continued

• Infrared Soldering
• Plancks law
• W/A k1l-5 exp(k2/lT)-1)-1,
• where
• W/A emitted energy pr. second per m2 area
• 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)

35
IR Soldering, continued

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

36
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 -To)(1 -exp (-t/to))

37
Vapour Phase Soldering

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

38
Vapour Phase Soldering, continued

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

39
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.

40
Vapour Phase Soldering, continued

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

41
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).

42
Vapour Phase Soldering, continued

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

43
Other Soldering Methods

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

44
Thermode Soldering

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

45
Thermode Soldering, continued

• Fig. 7.32 Temperature profile for thermode
  soldering.

46
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)

47
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.

48
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.

49
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).

50
Component Mounting, continued

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

51
Solder faults

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

52
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

53
Robot Mounting

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

54
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

55
Robot Mounting, continued

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

56
Robot System Components

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

57
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.

58
Robot System Components, continued

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

59
Robot Mounting, continued

• Fig. 7.42 Multi gripper head.

60
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.

61
Robot Mounting, continued

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

62
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

63
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.

64
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.

65
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.

66
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.

67
Board Testing

• Functional test
• "In-circuit" test
• NB Good designs use one-sided testing
• Test jigs

68
Testing of PCBs

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

69
Testing of PCBs

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

70
Testing of PCBs

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

71
Testing of PCBs

• Fig. 7.48 Two types of test pins.

72
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.