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• 1 Design considerations

• 1.1 Board Material Properties

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• Why are board material properties a critically important element of a printed circuit design?

  • Mechanical support of both the circuit conductors and the components which those conductors interconnect.
  • electric performance:
    the dielectric constant and loss tangent of the board material can significantly effect board design. 
  • from manufacturing perspective:
    dimensional stability => yield , cost
    difficulty to process
  • Thermal performance: both in assembly and in use
    hand soldering, rework, component replacement => higher temp

  
  

• Give two broad categories of base material (substrates):

  • organic substrates:
    - layers of paper impregnated with phenolic resin
    - layers of woven or nonwoven glass cloth impregnated
       with epoxy resin, polyimide, cyanate ester, BT resin, etc.
  • inorganic substrates:
    ceramic and metallic materials such as aluminium, soft iron,
    and copper-invar-copper. 

• Which are the construction elements of Board Materials?

  • Resin systems
  • Reinforcements
  • Metal foils 
    

• Which are the most commonly used resin systems? #4

  • Epoxies: a good blend of physical, electrical and processing properties at reasonable cost
  • Polyimide: highest operating temperature => rework & repair friendly
  • Cyanate Ester:
    low dielectric constant =>high speed design
    good thermal performance
  • Bismaleimide Triazine(BT resin):
    high temperature at reasonable cost => BGA packages

• What are the most commonly used reinforcements?What is their function?

  Reinforcements provide the dimensional stability and the bulk of the mechanical properties.

  • glass cloth: E type glass is most used.
  • Glass Felt: nonwoven glass mat, most used in combination with fluoroplastic resins
  • Aramid cloth: negative CTE in X, Y but high in Z 
  • Aramid paper

• Which are the common thicknesses for copper foil?

  • half ounce (17 µm)
  • one ounce (35 µm)
  • two ounce (70 µm)

  Thinner foils are commonly used to facilitate the manufacture of fine line circuits.

  Thicker copper foils are used for power and ground distribution

• Which material for flexible laminates is regularly used?

  • Polyimide films: high temperature, high speed
    
  • polyester films: low cost

• Which are the key environmental material properties? #4

  • Thermal expansion:
    changes in the XY-plane, controlled by reinforcement 
    CTE [ppm/°C]
  • Glass transition temperature: Tg
    above Tg expansion in the Z-axis, at rate much faster than XY-axis
    determined by the resin system
  • Moisture Absorption:
    most organic materials are hygroscopic
  • Temperature at decomposition Td

• From which two perspectives expansion is a concern to the designer?

  1. Potential warpage of the assembly
  2. damage to the plated through holes

  High glass transition materials tend to be more forgiving.

• Discuss Thermal expansion in relation to XYZ-axis and Tg?

  Thermal expansion [ppm/°C] in Z-axis is several times greater than expansion in XY-plane.
  Above Tg, expansion in Z-axis can be as much as four times greater than below Tg.