Packaging

• Packages
  • Electrical connection of signals & power from chip to board
  • Mechanical connection of chip to board
  • Little delay or distortion
• Benefits
  • Protects chip from mechanical damage
  • Removes heat produced on chip
  • Compatible with thermal expansion
  • Inexpensive to manufacture & test

• Types
  • Through-hole
  • Surface mount
  • ...

Chip-to-Package Bonding

• Traditionally, chip is surrounded by pad frame
  • Metal pads
  • Gold bond wires
    • Attach pads to package
    • Contribute parasitic inductance
  • Lead frame
    • Distributes signals in package
  • Metal heat spreader
    • Helps cooling
• Advanced: flip-chip (C4 - Controlled Collapse Chip Connection)
  • Steps
    • Metal pads covered with solder balls
    • Chip flips upside down, aligned to package
    • Melt balls
  • Advantage
    • Smaller resulting chip
    • Short wires reduce inductance, allow higher-speed signals, & conduct heat better
  • Disadvantage
    • Not suitable for easy replacement or manual installation
    • Requires very flat surfaces to mount to
    • Short connections are stiff, thermal expansion on the chip may crack the connections

Package Parasitics

Power Distribution

• Power distribution network
  • Carry current from pads to transistors on chip
  • Maintain stable voltage with low noise
  • Provide average & peak power demands
  • Provide current return paths for signals
  • Avoid electro-migration & self-heating wearout
  • Consume little chip area & wire
  • Easy to lay out

Power Requirements

• $$V{DD} = V{DDnominal} - V_{droop}$$
• $$V{droop}$$ < +- 10% of $$V{DD}$$
  • IR drops
  • $$L \frac{di}{dt}$$ noise
• $$I_{DD}$$ changes on many time scales
  • Rapid current variation -> huge voltage drop
  • Performance v.s. voltage stability tradeoff

Power System Model

• Power comes from regulator on system board
  • Board & package add parasitic R & L
  • Bypass capacitors help stabilize supply voltage
    • Also have parasitic R & L
• Multiple capacitors in parallel
  • Large capacitor near regulator - low impedance at low frequencies
    • But also low self-resonant frequency
  • Small capacitor near chip & on chip - low impedance at high frequencies
  • Choose caps to get low impedance at all frequencies