EE4800 CMOS Digital IC Design & Analysiszhuofeng/EE4800Slides2010... · EE4800 CMOS Digital IC...

EE4800 CMOS Digital IC Design & Analysis

Lecture 12 Packaging, Power and Clock Distributions

Zhuo Feng

Z. Feng MTU EE4800 CMOS Digital IC Design & Analysis 2010Z. Feng MTU EE4800 CMOS Digital IC Design & Analysis 201012.12.11

Outline■ Packaging■ Power Distribution■ Clock Distribution

Packagesg■ Package functions

►Electrical connection of signals and power from g pchip to board

►Little delay or distortionM h i l i f hi b d►Mechanical connection of chip to board

►Removes heat produced on chip►Protects chip from mechanical damage►Protects chip from mechanical damage►Compatible with thermal expansion► Inexpensive to manufacture and test► Inexpensive to manufacture and test

Package Typesg y■ Through-hole vs. surface mount

Multichip Modules■ Pentium Pro MCM

►Fast connection of CPU to cache►Expensive, requires known good dice

Chip-to-Package BondingChip to Package Bonding■ Traditionally, chip is surrounded by pad frame

► Metal pads on 100 – 200 μm pitchp μ p► Gold bond wires attach pads to package► Lead frame distributes signals in package► Metal heat spreader helps with cooling► Metal heat spreader helps with cooling

Advanced Packagesg■ Bond wires contribute parasitic inductance■ Fancy packages have many signal, power layers■ Fancy packages have many signal, power layers

► Like tiny printed circuit boards

■ Flip-chip places connections across surface of di th th d i hdie rather than around periphery► Top level metal pads covered with solder balls► Chip flips upside downp p p► Carefully aligned to package (done blind!)► Heated to melt balls► Also called C4 (Controlled Collapse Chip Connection)► Also called C4 (Controlled Collapse Chip Connection)

Package ParasiticsgUse many VDD, GND in parallel– Inductance I

Package

– Inductance, IDD

S S ChipVDD

BoardVDD

Bond Wire Lead Frame

ignal Pins

PackageCapacitor

ignal Pads

BoardpGND GND

Heat Dissipation■ 60 W light bulb has surface area of 120 cm2

■ Itanium 2 die dissipates 130 W over 4 cm2p► Chips have enormous power densities► Cooling is a serious challenge

Package spreads heat to larger surface area■ Package spreads heat to larger surface area► Heat sinks may increase surface area further► Fans increase airflow rate over surface area► Liquid cooling used in extreme cases ($$$)

Thermal Resistance■ ΔT = θjaP

► ΔT: temperature rise on chip► θja: thermal resistance of chip junction to ambient► P: power dissipation on chip

■ Thermal resistances combine like resistors■ Thermal resistances combine like resistors► Series and parallel

■ θja = θjp + θpa

► Series combination

Example■ Your chip has a heat sink with a thermal

resistance to the package of 4.0° C/W. ■ The resistance from chip to package is 1° C/W.

■ The system box ambient temperature may reach 55° C55° C.

■ The chip temperature must not exceed 100° C.

■ What is the maximum chip power dissipation?■ What is the maximum chip power dissipation?

■ (100-55 C) / (4 + 1 C/W) = 9 W■ (100 55 C) / (4 1 C/W) 9 W

Temperature Sensor■ Monitor die temperature and throttle performance■ Use a pair of pnp bipolar transistors

► Vertical pnp available in CMOS

lnBEqV

ckTc s BE

IkTI I e VI

= → =

1 2 11 2

ln ln ln ln

c c cBE BE BE

I I IkT kT kTV V V mq I I q I q⎛ ⎞ ⎛ ⎞

Δ = − = − = =⎜ ⎟ ⎜ ⎟⎝ ⎠ ⎝ ⎠

■ Voltage difference is proportional to absolute temp► Measure with on-chip A/D converter

Power Distribution■ Power Distribution Network functions

►Carry current from pads to transistors on chip►Maintain stable voltage with low noise►Provide average and peak power demands►Provide current return paths for signals►Avoid electromigration & self-heating wearout►C littl hi d i►Consume little chip area and wire►Easy to lay out

IR Drop in Power Distribution NetworkIR drop: voltage drop due to non-ideal resistive wires

VDD VDD

GNDGND Cadence

Power Delivery NetworkySocket

ChipPackage

C4 Bumps

Entire Power Delivery Network

On-Chip Power Delivery Network

Power System Modely■ Power comes from regulator on system board

► Board and package add parasitic R and L► Bypass capacitors help stabilize supply voltage► But capacitors also have parasitic R and L

■ Simulate system for time and frequency responses■ Simulate system for time and frequency responses

V lt P i t d Ci it P k S ldVoltageRegulator

Printed CircuitBoard Planes

Packageand Pins

SolderBumps

Bulk Ceramic Package On-Chip On-ChipV

BulkCapacitor

CeramicCapacitor

PackageCapacitor

On ChipCapacitor

On ChipCurrent DemandVDD

PackageBoard

Power Delivery Network Modelingy g

Multi-million nodesThousands of nodes

On-Chip Power Grid Modeling & Analysis■ Multi-layer interconnects are modeled as 3D RC network

► Switching gate effects are modeled by time-varying current loadings

Vdd Vdd

■ DC analysis solves linear systemTens of millionsof unknowns !G v b⋅ = :n nG ×∈ℜ Conductance Matrix

■ Transient analysis solves

( )( ) ( )dv tG v t C b t+1

∈ℜ

Capacitance Matrix

Node Voltage Vector

( )( ) ( )G v t C b tdt

⋅ + ⋅ =1 :nb ×∈ℜ Current Loading Vector

Power Requirements■ VDD = VDDnominal – Vdroop

■ Want Vdroop < +/- 10% of VDD■ Want Vdroop / 10% of VDD

■ Sources of Vdroop

► IR drops► L di/dt noise

■ IDD changes on many time scalesPower

clock gatingMax

Average

Important Metrics I■ Voltage Droop.

► Two sources: IR drops and Ldi/dt noises.► Large voltage droop leads to circuit timing failure or function► Large voltage droop leads to circuit timing failure or function

failure.► Usually only 5% rail is allowed for 90nm or beyond

technologiestechnologies.VDDn

+ ( )dn DD DDn SSnV V V V= − −

5%V V≤

Vn5%dn DDV V≤

5%VDDVdn

VDDn -VSSn

IR Drop■ A chip draws 24 W from a 1.2 V supply. The

power supply impedance is 5 mΩ. What is the IR drop?

■ IDD = 24 W / 1.2 V = 20 AIR d (20 A)(5 Ω) 100 V■ IR drop = (20 A)(5 mΩ) = 100 mV

L di/dt Noise■ A 1.2 V chip switches from an idle mode

consuming 5W to a full-power mode consuming 53 W. The transition takes 10 clock cycles at 1 GHz. The supply inductance is 0.1 nH. What is the L di/dt droop?the L di/dt droop?

ΔI = (53 W – 5 W)/(1.2 V) = 40 AΔt = 10 cycles * (1 ns / cycle) = 10 nsL di/dt droop = (0.1 nH) * (40 A / 10 ns) = 0.4 V

Important Metrics II■ Current density.

► High direct current density leads to electromigration phenomenonphenomenon.

► Electromigration decreases reliability or even breaks connections.

/J I w=

, , /avg m avg m mJ I w

,avg mJ σ≤Im

Technology Scaling: Challenges

■ Scaled down supply voltage vs. increased operating frequency and power densities.p g q y p► Higher percentage of voltage droops.► Lower voltage droop tolerance.

■ Shrunk chip area vs. increased gated density.► Less wiring resources.► Larger power delivery network dimensions.

■ Fine-grained power management■ Fine-grained power management.► Power gating.► Multiple power domains.

Technology Scaling: Implications■ Simulation:

► Stricter accuracy requirement.M ffi i t i t f ti d► More efficient in terms of runtime and memory.

► Full chip simulation with package and integrated components.

■ Verification:► Power-gated power delivery network with multiple power► Power gated power delivery network with multiple power

gating configurations.

D i■ Design:► Optimization for wire sizing.► Detailed tradeoff analysis for on-chip voltage regulation.

► Optimization for on-chip voltage regulation.

Frequency Responsey■ Multiple capacitors in parallel

► Large capacitor near regulator has low impedance at low frequencies

► But also has a low self-resonant frequency► Small capacitors near chip and on chip have low impedance at

high frequencies

■ Choose caps to get low impedance at all frequencies

impeda

f (H )

frequency (Hz)

Example: Pentium 4■ Power supply impedance for Pentium 4

► Spike near 100 MHz caused by package Ly g

■ Step response to sudden supply current chain► 1st droop: on-chip bypass caps► 2nd d k it► 2nd droop: package capacitance► 3rd droop: board capacitance

[Wong06]

Clock Distribution■ On a small chip, the clock distribution network is just a wire

► And possibly an inverter for clkb

■ On practical chips, the RC delay of the wire resistance and gate load is very long► Variations in this delay cause clock to get to different elements at

different times► This is called clock skew

■ Most chips use repeaters to buffer the clock and equalize the delay► Reduces but doesn’t eliminate skew

Example■ Skew comes from differences in gate and wire delay

► With right buffer sizing, clk1 and clk2 could ideally arrive at the same time.

► But power supply noise changes buffer delays► clk2 and clk3 will always see RC skew2 3

3 mm 3.1 mmgclk

0.5 mm

1.3 pF

clk1 clk2clk3

0 4 pF 0 4 pF0.4 pF 0.4 pF

Review: Skew Impact

clk clk

Combinational Logic

Q1 D2■ Ideally full cycle isavailable for work

tsetup

■ Skew adds sequencingoverhead

■ Increases hold time too

( )setup skew

sequencing overhead

pd c pcqt T t t t≤ − + +

hold skewcd ccqt t t t≥ − +

D2 tcd

Solutions■ Reduce clock skew

►Careful clock distribution network design►Plenty of metal wiring resources

■ Analyze clock skew►Only budget actual, not worst case skews►Local vs. global skew budgets

T l t l k k■ Tolerate clock skew►Choose circuit structures insensitive to skew

Clock Dist. Networks■ Ad hoc■ Grids■ Grids■ H-tree

Hybrid■ Hybrid

Clock Grids■ Use grid on two or more levels to carry

clock■ Make wires wide to reduce RC delay■ Ensures low skew between nearby points■ But possibly large skew across die

Alpha Clock GridsAlpha 21064 Alpha 21164 Alpha 21264

gclk gridgclk grid

Alpha 21064 Alpha 21164 Alpha 21264

H-Trees■ Fractal structure

► Gets clock arbitrarily close to any point► Matched delay along all paths► Matched delay along all paths

■ Delay variations cause skew■ A and B might see big skew

Itanium 2 H-Tree■ Four levels of buffering:

►Primary driverRepeaters

►Repeater►Second-level

l k b ffclock buffer►Gater

■ Route aroundTypical SLCBLocations■ Route around

obstructions Primary Buffer

Hybrid Networksy■ Use H-tree to distribute clock to many points■ Tie these points together with a gridp g g

■ Ex: IBM Power4, PowerPC► H-tree drives 16-64 sector buffers► Buffers drive total of 1024 points► All points shorted together with gridp g g

EE4800 CMOS Digital IC Design & Analysiszhuofeng/EE4800Slides2010... · EE4800 CMOS Digital IC...

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