Coefficient of thermal expansion (CTE)

• Soldered parts must have similar CTE

  • Else solder joints fail over time
    • Heating/cooling causes stress fractures

• Silicon CTE is 3ppm/ºC

  • BGA packages are 6ppm/ºC
    • Alumina ceramic (Al2O3)
  • Silicon/Alumina CTE match is close
    • Flip-chip with underfill spreads the stress

• PCB CTE is 20ppm/ºC

  • BGA/PCB CTE match is poor
    • Solder balls fail when BGA is large ( > 40mm on a side)

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