1. Chapter 3 Materials and Basic Processes
Picture of the chip set of SensoNor’s SP13 Tire Pressure Sensor
The course material was developed in INSIGTH II, a project sponsored by the Leonardo da Vinci program of the European Union
Electronic Pack….. Chapter 3 Materials and Basic Processes

2. Materials: Metals
Right choice, right use and compatibility of materials is the key to good packaging and optimal properties.
Elemental metals:
High electrical conductivity
High thermal conductivity
Higher thermal coefficient of expansion (TCE) than semiconductors and most ceramics
Alloys: taylored to many uses:
Poorer electrical and thermal conductivity than elements
Taylored TCE
Lower melting point
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3. Metals, continued (Table 3.1)
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4. Metal Alloys
Fig. 3.1: Phase diagram for Sn/Pb. The eutectic mixture 63%/37% has a melting point of 183°C.
Alloys have poorer conductivity, both electrical and thermal.
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5. Insulators
(Fig 3.1b)
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6. Semiconductors, Si and GaAs
High thermal conductivity
Electrical conductivity spans many orders of magnitude, depending on doping
Very low TCE
"Machinable" by anisotropic etching (Si)
Excellent protective oxide (Si)
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7. Ceramics
Inorganic, non-metallic materials (Defined by what they not are)
Man made/Synthetic made: Made by powder, compressing or tape casting, and high temperature treatment ( oC)
Generally chemically and thermally very stable
Generally good electrical insulators
Some ceramics are very good thermal conductors
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8. Ceramics, continued
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9. Ceramics, continued Dielectric loss: Main uses: tan d = (1/R)/wC = 1/Q
e = eo (k´ - jk")
tan d = k"/k´.
Main uses:
Substrates for hybrid circuits, component packages, SMD resistors
Multilayer capacitors
Future: Superconductors ?
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10. Materials
Fig 3.1.d
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11. Ceramics, continued
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12. Ceramics, continued
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13. Glasses: Glasses are amorphous, supercooled liquids Uses:
Matrix for thick film pastes
Hermetic seals
Substrates, together with ceramics
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14. Plastics
Organic, synthetic polymer materials with numerous uses in electronics
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15. Plastics, continued Composition, properties:
Monomers derived from benzene
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16. Plastics, continued
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17. Plastics, continued Requirements:
High electrical resistivity, high breakdown field, low dielectric losses, low dielectric constant
Thermal and mechanical stability
Thermal expansion compatible with Si and metals
High mechanical strength/softness and flexibility
Chemical resistance
Good adhesion to other materials
Ease of processing
Low water absorption, small changes of the properties during the effect of moisture.
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18. Plastics, continued Composition, properties:
Linear, branched or crosslinked
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19. Plastics, continued Thermoplastic or thermosetting
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20. Plastics, continued Polymerization: A-, B-, C-stages.
High electrical resistivity , low dielectric constant r, low loss factor tan , high breakdown field Ecrit
Poor thermal conductors
Visco-elastic
Fig 3.7: The structural unit of certain monomers/polymers.
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21. Plastics, continued
"Glass transition": change from glass-like to rubber - like
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22. Plastic Materials: Epoxy Phenolic Polyimide Teflon Polyester Silicone
Polyurethane
Parylene
Acrylic
Polysulphone, polyethersulphone, polyetherimide
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23. Plastics, continued
Fig. 3.9: a):The epoxide group, which is the building block in epoxy, b) - e): Starting materials for epoxy: b): Bisphenol A, which constitutes most of the starting material. The H-atoms in the places X are often replaced with Br to reduce the flammability; c): Epoxy novolac; d): The hardener dicyandiamide; e): The catalyst.
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24. Plastics, continued
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25. Basic Processes
Description of some of the basic processes used in microelectronics, microsystems and electronic packaging.
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26. Photolithography
Fig. 3.10:The steps in photolithographic transfer of patterns and the subsequent etching of metal films with negative photoresist.
If positive resist is used, it is the illuminated part of the photoresist, which is removed during the development.
Positive resist most used today because of better accuracy
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27. Photolithography Colour graph downloaded from Wikipedia (1602-2012:
Self test: Explain the difference between positive and negative resist.
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28. Photolithography, cont
Also, please observe the concept of straight polarity masks and reverse polarity masks:
Straight polarity: In layers with straight polarity, a positive image of the layout will be transferred onto the process layer. In other words, draw the objects that need to be covered with photo-resist after development.
Openings: Mask pattern is repeated on the substrate for additive films etc., like metal patterns. (Assuming positive resist is used)
Reverse polarity: In layers with reverse polarity, draw the areas where photo-resist should be removed. The actual mask will be the negative image of the layout.
Mask pattern is oppositely repeated on the substrate for additive films etc., like openings in oxide for later difussion of dopants. (Assuming positive resist is used)
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29. Screen Printing and Stencil Printing
Fig. 3.11: Screen printing: a) and b): Printing process, c) and d): Details of the screen
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30. Etching Wet, chemical etching "Dry" plasma- or reactive ion etching
Examples, wet etching: Copper: FeCl3 + Cu -> FeCl2 + CuCl In addition: FeCl3 + CuCl -> FeCl2 + CuCl2 Need organic etch resist, not good with PbSn. Gold: KI + I2 -> KI3 + KI (surplus) 3 KI3 + 2 Au -> 2 KAuI4 + KI
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31. Plating Electrolytic plating:
Electric current of ions in electrolyte. External circuit needed. All separate parts of area to be plated must be electrically contacted to external circuit. Example: Cu in CuSO4 /H2SO4 Reaction at anode (Cu supply): Cu -> Cu2+ + 2e- Reaction at catode (substrate): Cu2+ + 2e- -> Cu
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32. Plating, continued Chemical plating:
Takes place without external current
Needed when insulating surfacec are to be plated
Often preceeds electrolytic plating, to make all needed areas electrically conductive
Complex processes of "sensitizing", "activation" and plating
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33. Vacuum Deposition and Sputtering
Vacuum evaporation:
Chamber evacuated to less than 10-6 Torr
Resistance heating
Metal evaporation
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34. Other Methods for Deposition of Conducting or Insulating Films
DC Sputtering (Fig a)
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35. Deposition, continued Radio Frequency AC Sputtering (Fig.3.13.b)
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36. Methods for Electrical and Mechanical Contact
Soldering
Wetting: (Fig. 3.14) Young´s eq.: gls + gl cos Q = gs
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37. Leadless soldering
Leadless soldering replacing lead-based solder due health hazards and environmental issues

38. Soldering, continued
Most common solder alloy: 63 % Sn / 37 % Pb (eutectic) Melting point 183 oC
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39. Soldering, continued
Fatigue: Coffin-Mansons formula: N0.5 x gp = constant where N is number of stress cycles, and gp is the relative deformation amplitude, meaning that both number of cycles and stress level determine lifetime
Useful adition : 2 % Ag (Surface mount), to reduce leaching (dissolution of the termination metal that leads to deterioration of mechanical and electrical properties)
Harmful contaminant: Au, will increase brittleness because of AuSn intermetallics
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40. Soldering, continued
Fig.3.15: Behaviour of solder metal at different temperatures, schematically. [W. Engelmaier].
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41. Soldering, continued
Fig. 3.16: Solder joint fatigue in surface mounted assemblies is often caused by power cycling.
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42. Soldering, continued
Fig. 3.17: Experimental data for fatigue in Sn/Pb solder fillet by cyclical mechanical stress. High temperature and low cycling frequency gives the fastest failure, because the grain structure relaxes most and is damaged
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43. Soldering, continued
Fig a) Left: Dissolution rate of Ag in solder metal, and in solder metal with 2 % Ag, as function of temperature b) Right: Dissolution rate of various metals in solder alloy
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44. Soldering, continued
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45. Soldering, continued Flux and cleaning Purpose of flux: Categories:
Dissolve and remove oxides etc.
Protect surface
Improve wetting
Categories:
Soluble in organic liquids
Water soluble
Types:
Organic resin fluxes ("rosin")
Organic non resin based fluxes
Inorganic fluxes
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46. Soldering: Flux and cleaning
Fig. 3.19: Time for solder alloy to wet a pure Cu surface, depending on the activation of the solder flux. The degree of activation is given by the concentration of Cl- ions in the flux (temperature: 230 °C)
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47. Soldering: Flux and Cleaning
Designations:
R (Rosin, non-activated): No clorine added.
RMA (Rosin mildly activated): < 0.5 % Cl
RA (Rosin, activated): > 0.5 % Cl
Cleaning
Freon (TCTFE) now forbidden. Replaced by alcohol etc.
Trend: No cleaning
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48. Gluing Purposes: Materials: polymers: Mechanical assembly
Electrical contact
Thermal contact
Materials: polymers:
Epoxy, acrylic, phenolic, polyimide
Metal particles for electrical conductivity: r = x ohm m
Metal or ceramic particles for thermal conductivity: K ≈ W /m x oC
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49. Gluing, continued
Fig. 3.20: Thermal conductivity of epoxy adhesive with various amounts of Ag [3.16 a)]. The concentration is in volume % Ag. (23 vol. % corresponds to approximately 80 weight %).
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50. Gluing, continued
Fig. 3.21: The thermal resistance from the electronically active part, on top of the Si chip (¨junction¨) through a bonding layer of glue or soft solder and a thin alumina ceramic layer covered with Cu to heat sink. The samples with chips bonded by gluing, C and A, have approximately twice as high total thermal resistance as those which are soft soldered, D and B.
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51. Chip Mounting: Die Bonding
Fig. 3.22: Thermal resistance from junction to heat sink through adhesive of various thicknesses. For thick layers the resistance approaches the value calculated, based on the bulk thermal conductivity of the adhesive. For thin layers the resistance is higher, approaching a constant value, which indicates an "interface thermal resistance" caused by defects in the adhesive layer
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52. Chip Mounting: Die Bonding, continued
Eutectic die bonding:
Au/Si (363 oC), Au/Sn (280 oC)
Soft soldering: Sn/Pb, Ag/Pb
Glueing
Adhesive cracking, fig. 3.23: Thermal cycling induces defects giving increased thermal resistance.
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53. Chip Mounting: Die Bonding, continued
Fig. 3.24: Use of adhesive for contacting IC-chips with small pitch, schematically: a): Anisotropic conductive adhesive, the conduction is through the metal particles in the adhesive; b): Electrically insulating adhesive, the conduction is through point contacts where the adhesive has been squeezed out.
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54. Si Chip Electrical Contact
Wire bonding
Tape Automated Bonding (TAB)
Flip chip
Planar bonding
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55. Wire Bonding Ultrasonic Thermo- compression Thermosonic Geometry Types
Ball - wedge: Shown in illustration
Wedge - wedge
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56. Wire Bonding From Small Precision Tools
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57. Electronic Pack….. Chapter 3 Materials and Basic Processes

58. Tape Automated Bonding (TAB)
Connection made in two steps:
Inner Lead Bonding
Connecting tape to chip
Outer Lead Bonding
Connecting tape to substrate
Connection made by thermocompression
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59. Tape Automated Bonding (TAB)
Standard process:
Fabrication of gold bumps (Fig. 3.28):
Deposition of contact/barrier metals
Photolithography
Electroplating
Strip and etch barrier metals
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60. TAB, continued
Fig. 3.26: A picture of a TAB film with the Cu pattern, as well as the holes in the film for excising the circuits, and the sprocket holes for moving the film during processing.
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61. TAB, continued Fig. 3.27: The main steps in TAB processing.
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62. Tape Automated Bonding (TAB)
Wafer cutting
Fabrication of TAB film
Hole punching
Cu foil lamination
Lithography + etch of Cu pattern
Tinning of Cu
Inner lead bonding (ILB)
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63. TAB, continued Protection (glob top) Testing Outer lead bonding:
Excising, lead bending
Placement/thermode soldering
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64. Advantages of TAB: High packaging density
Can contact chips with >1000 I/O
Excellent electrical properties (high frequency)
Robust mounting
Pre-testable (contrary to COB)
Gold bumps give hermetic seal to chip
Gang bonding gives high yield, is less time consuming than wirebonding
TAB film can be used as daughter board
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65. Disadvantages of TAB: Non-standard wafer processes
Special custom design film for chip
Needs special machine/tool for OLB
Demanding repair
Low availability of std. chips and TAB service
Little standardization
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66. Flip chip Active face of chip is flipped towards substrate
Substrate pads are identical to chip pads
Area array connections possible
All connections done simultaneously
Smallest possible footprint (1:1)
Short interconnections
Low inductance and resistance
Excellent electrical properties
Little flexibility
Change of chip pad configuration implies redesign of substrate
Small, but increasing amount of interconnections are flip chip
To be dealt with in much more detail…

67. Flip Chip Process: Deposit barrier metals
Deposit solder bump metals (solder) by photolithography/metal mask and sputter or plating
Reflow
Cut wafer
Turn chip and mount on substrate
Heat substrate to reflow solder
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68. Flip Chip, history Introduced by IBM 1962
Flip chip has been used for decades, but with little impact
Wire bonding is far more common
Flip chip technology has not been considered mature
The industrial infrastructure has been small
The market share of flip chip connections is believed to increase significantly
Wire bonding will remain dominating for many years
Flip chip especially for ”advanced packaging”

69. Flip Chip, continued Advantages: Disadvantages:
Highest packing density
Excellent hi freq. properties
Up to I/O
Disadvantages:
Very difficult placement and reliable solder/cleaning
Lack of thermal flexibility
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70. Flip Chip consists of: Chip Substrate Interconnection system:
Si, GaAs, etc.
Substrate
Ceramic, organic, dielectic-covered metal, silicon, etc.
Interconnection system:
Metallization on chip and substrate pads
Chip (or substrate) bumps
Bonding material
Underfill encapsulant

71. Different Flip Chip technologies
Flip chip is not standardized!
From C. Lee, ESTC 2006, Dresden

72. Wire Bonding, TAB and Flip Chip
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73. Planar Bonding with Adaptive Routing
Fig. 3.32: Planar bonding with laser-assisted adaptive conductor routing. The top two figures a) and b) show a substrate cross section with details of the mounting of the chip in an etched through-hole. Figure c) shows the conductor layers and polyimide insulation on top of the substrate. The bottom figures show an exploded view of all the layers.
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74. End of Chapter 3 Materials and Basic Processes
Important issues:
Materials:
Distinguish between metals, ceramics, glasses and plastics
Important mechanical and thermal parameters like modulus of eleasticity, thermal expansion coefficient and thermal conductivity.
Important electrical parameters like dielectric constant and resistivity or conductivity
Have a basic understand of the importance and value of the most important materials parameter, and why they are important for the use of the specific material in specific applications.
For instance knowing the electrical conductivity of copper or thermal conductivity of epoxy within an accuracy of 25%
Basic processes
Lithographics, screen and stencil printing, etching, plating, vacuum deposition, sputtering, soldering, gluing, wire bonding, TAB, and flip chip
Other basic processes described in other chapters, like surface mount technology
Questions and discussions?
Electronic Pack….. Chapter 3 Materials and Basic Processes