Chapter 3 Materials and Basic Processes

1. Chapter 3Materials 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 Electronic Pack….. Chapter 3 Materials and Basic Processes

3. Metals, continued • (Table 3.1) Electronic Pack….. Chapter 3 Materials and Basic Processes

4. Metal Alloys • Alloys have poorer conductivity, both electrical and thermal. • Fig. 3.1: Phase diagram for Sn/Pb. The eutectic mixture 63%/37% has a melting point of 183°C. Electronic Pack….. Chapter 3 Materials and Basic Processes

5. Insulators • (Fig 3.1b) Electronic Pack….. Chapter 3 Materials and Basic Processes

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) Electronic Pack….. Chapter 3 Materials and Basic Processes

7. Ceramics • Inorganic, non-metallic • Made by powder, compressing or tape casting, and high temperature treatment (600-1800oC) • Chemically and thermally very stable • Electrical insulators • Some ceramics are very good thermal conductors Electronic Pack….. Chapter 3 Materials and Basic Processes

8. Ceramics, continued Electronic Pack….. Chapter 3 Materials and Basic Processes

9. Ceramics, continued • Dielectric loss: • 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 ? Electronic Pack….. Chapter 3 Materials and Basic Processes

10. Materials • Fig 3.1.d Electronic Pack….. Chapter 3 Materials and Basic Processes

11. Ceramics, continued Electronic Pack….. Chapter 3 Materials and Basic Processes

12. Ceramics, continued Electronic Pack….. Chapter 3 Materials and Basic Processes

13. Glasses: • Glasses are amorphous, supercooled liquids • Uses: • Matrix for thick film pastes • Hermetic seals • Substrates, together with ceramics Electronic Pack….. Chapter 3 Materials and Basic Processes

14. Plastics • Organic, synthetic polymer materials with numerous uses in electronics Electronic Pack….. Chapter 3 Materials and Basic Processes

15. Plastics, continued • Composition, properties: • Monomers derived from benzene Electronic Pack….. Chapter 3 Materials and Basic Processes

16. Plastics, continued Electronic Pack….. Chapter 3 Materials and Basic Processes

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. Electronic Pack….. Chapter 3 Materials and Basic Processes

18. Plastics, continued • Composition, properties: • Linear, branched or crosslinked Electronic Pack….. Chapter 3 Materials and Basic Processes

19. Plastics, continued • Thermoplastic or thermosetting Electronic Pack….. Chapter 3 Materials and Basic Processes

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. Electronic Pack….. Chapter 3 Materials and Basic Processes

21. Plastics, continued • "Glass transition": change from glass-like to rubber - like Electronic Pack….. Chapter 3 Materials and Basic Processes

22. Plastic Materials: • Epoxy • Phenolic • Polyimide • Teflon • Polyester • Silicone • Polyurethane • Parylene • Acrylic • Polysulphone, polyethersulphone, polyetherimide Electronic Pack….. Chapter 3 Materials and Basic Processes

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. Electronic Pack….. Chapter 3 Materials and Basic Processes

24. Plastics, continued Electronic Pack….. Chapter 3 Materials and Basic Processes

25. Basic Processes • Description of some of the basic processes used in microelectronics, microsystems and electronic packaging. Electronic Pack….. Chapter 3 Materials and Basic Processes

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 Electronic Pack….. Chapter 3 Materials and Basic Processes

27. 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) Electronic Pack….. Chapter 3 Materials and Basic Processes

28. Screen Printing and Stencil Printing • Fig. 3.11: Screen printing: a) and b): Printing process, c) and d): Details of the screen Electronic Pack….. Chapter 3 Materials and Basic Processes

29. Etching • Wet, chemical etching • "Dry" plasma- or reactive ion etching • Examples, wet etching:Copper:FeCl3 + Cu -> FeCl2 + CuClIn addition:FeCl3 + CuCl -> FeCl2 + CuCl2Need organic etch resist, not good with PbSn.Gold:KI + I2 -> KI3 + KI (surplus)3 KI3 + 2 Au -> 2 KAuI4 + KI Electronic Pack….. Chapter 3 Materials and Basic Processes

30. 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 Electronic Pack….. Chapter 3 Materials and Basic Processes

31. 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 Electronic Pack….. Chapter 3 Materials and Basic Processes

32. Vacuum Deposition and Sputtering • Vacuum evaporation: • Chamber evacuated toless than 10-6 Torr • Resistance heating • Metal evaporation Electronic Pack….. Chapter 3 Materials and Basic Processes

33. Other Methods for Deposition of Conducting or Insulating Films • DC Sputtering (Fig. 3.13.a) Electronic Pack….. Chapter 3 Materials and Basic Processes

34. Deposition, continued • Radio Frequency AC Sputtering (Fig.3.13.b) Electronic Pack….. Chapter 3 Materials and Basic Processes

35. Methods for Electrical and Mechanical Contact • Soldering • Wetting: (Fig. 3.14) Young´s eq.: gls + gl cos Q = gs Electronic Pack….. Chapter 3 Materials and Basic Processes

36. Leadless soldering • Leadless soldering replacing lead-based solder due health hazards and environmental issues

37. Soldering, continued • Most common solder alloy: 63 % Sn / 37 % Pb (eutectic)Melting point 183 oC Electronic Pack….. Chapter 3 Materials and Basic Processes

38. Soldering, continued • Fatigue: Coffin-Mansons formula:N0.5 x gp = constantwhere 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 Electronic Pack….. Chapter 3 Materials and Basic Processes

39. Soldering, continued • Fig.3.15: Behaviour of solder metal at different temperatures, schematically. [W. Engelmaier]. Electronic Pack….. Chapter 3 Materials and Basic Processes

40. Soldering, continued • Fig. 3.16: Solder joint fatigue in surface mounted assemblies is often caused by power cycling. Electronic Pack….. Chapter 3 Materials and Basic Processes

41. 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 Electronic Pack….. Chapter 3 Materials and Basic Processes

42. Soldering, continued • Fig. 3.18. a) Left: Dissolution rate of Ag in solder metal, and in solder metal with 2 % Ag, as function of temperatureb) Right: Dissolution rate of various metals in solder alloy Electronic Pack….. Chapter 3 Materials and Basic Processes

43. Soldering, continued Electronic Pack….. Chapter 3 Materials and Basic Processes

44. Soldering, continued • Flux and cleaning • Purpose of flux: • 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 Electronic Pack….. Chapter 3 Materials and Basic Processes

45. 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) Electronic Pack….. Chapter 3 Materials and Basic Processes

46. 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 Electronic Pack….. Chapter 3 Materials and Basic Processes

47. Gluing • Purposes: • Mechanical assembly • Electrical contact • Thermal contact • Materials: polymers: • Epoxy, acrylic, phenolic, polyimide • Metal particles for electrical conductivity: r = 1 - 10 x 10 -6 ohm m • Metal or ceramic particles for thermal conductivity: K ≈ 1 - 3 W /m x oC Electronic Pack….. Chapter 3 Materials and Basic Processes

48. 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 %). Electronic Pack….. Chapter 3 Materials and Basic Processes

49. 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. Electronic Pack….. Chapter 3 Materials and Basic Processes

50. 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 Electronic Pack….. Chapter 3 Materials and Basic Processes