understand copper wire bond...

External Use

TM

Understand Copper Wire

Bond Technology

FTF-SDS-F0022

A P R . 2 0 1 4

Leo M. Higgins III, Ph. D. | Distinguished Member Technical Staff

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External Use 1

Agenda

• What is Wire Bonding?

• Strategy and Background

• Key Development Challenges and Results

• Assembly Optimization for High Volume Mfg

• Sample Reliability Data and Electrical Performance

• FSL Cu Wire Products in Production

• Summary

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Agenda







• Summary

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Cu Bond Wire Strategy

• FSL has converted Consumer / Industrial microcontrollers to Cu wire and are now initiating Automotive conversions.

− Both Gold (Au) and Copper (Cu) wire have been used for wire bonding to Aluminum (Al) bond pads on ICs for many years

Intermetallic compound (IMC) formation provides adhesion between wires and pad

− Recent wire bond (WB) technology advancements are expanding the use of Cu wire.

• Motivation

− Some new products require a high temperature application for which Cu wire is the best solution.

− Avoids the need to pass on increased Au wire costs.

• Introduction / Change Strategy

− All new products will be introduced with Cu wire

− All Fab expansion products will be qualified with Cu wire

− Existing product families will migrate to Cu wire

Some legacy products will migrate to thinner Au wire

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Agenda







• Summary

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Issues Initial Optimized Risk / Specification / Action

Aluminum

push out

(splash) • Spec: Not extending outside the

bond pad

• Action: WB optimization

Cu Al

CuWB Development Challenges: 1st Bond Quality

Challenge: Achieve high reliability bond with optimal Intermetallic

Compound (IMC) coverage without shorts or bond pad damage

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CuWB Development Challenge:

1st Bond without Pad Damage

Challenge: Achieve optimal 1st wire bond without pad damage

Issue Initial Optimized Risk / Specification / Action

Pad peeling

(Cratering

failure)

• Spec: Must consider under-pad structure

‒ Pad stack design: Impact on acceptable bonding

parameters

‒ Best practice learning:

Via location and uniformity

Balanced metal density on each layer

‒ Minimum Al thickness allowed: 0.8 µm

‒ < 0.8 µm, Cu wire + OPM; or use thinner Au wire

‒ Unique bond parameters for tech nodes/pad stacks

Simple example of a Non-optimum Pad

Design – Unbalanced Via Pattern Better Pad Design – Balanced Via Pattern

Highest stress

regions from

Capillary

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CuWB Development Challenge:

1st Bond without Pad Damage

Challenge: Achieve optimal 1st wire bond without pad damage

• Bond Over Active (BOA) Pad Design Optimization to Eliminate Damage:


Pad peeling

or ILD

Delamination

• Spec: FSL Bond Over Active (BOA) rules

• Actions:

‒ BOA rules for Au wire acceptable for Cu wire

– No die design change

‒ Unique bond parameters for technology

nodes and pad stacks

Au Cu

Al Cap

Cu padPassivation

Diodes, transistors, ESD, etc.Active c

ircuitry

Al Cap

Cu padPassivation

Diodes, transistors, ESD, etc.Active c

ircuitry Keep Out

Zone

Al Cap

Passivation

Diodes, transistors, etc.

Active c

ircuitry

Al Cap

Passivation

Diodes, transistors, etc.

Active c

ircuitry

Interconnect Metal

Passivation

vias – Now,

not in bond

area

Evolution of FSL BOA Rules for Au Wire - No BOA (no interconnect under pad)

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Cu-Al

Intermetallic

(IMC) bond layer

corrosion (biased

HAST failure)

• FSL specifies pH and Cl-

• Action: Internal testing demonstrates

higher pH and lower Cl- are best for

CuWB reliability.

‒ FSL tightened MC specification for

Au wire to meet CuWB requirement Corrosion crack

Cu

Al

Solid IMC

Cu

Al

CuWB Development Challenges: 1st Bond IMC Corrosion

Challenge: Avoid 1st bond IMC corrosion to pass Biased HAST / THB reliability tests

• Chloride (Cl-), principally from mold compound, can cause corrosion at the Cu ball to IMC interface resulting in open circuits

• Goal to use same BOM for CuWB and AuWB, including molding compound (MC)

• Internal research developed methods to determine acceptable pH and Cl- levels within ranges specified by suppliers

• Specs often renegotiated with suppliers to allow same MC used for AuWB

• Universal pH and Cl- level spec with reasonable values is not possible

• Acceptable pH and Cl- levels vary among MCs due to other MC attributes

• Acceptable pH and Cl- levels vary with voltage

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Fail

Pas

s

Low Medium High

Low

M

ediu

m

Hig

h

≥ 65V

Pass ≥ 3V

3V

3V

5V

5V Pass ≥ 5V 14V

Pass ≥ 3V

Fails

Pass ≥65V

Pass ≥5V

Pass ≥3V

14V Pass ≥ 3V

Pass ≥ 5V

Biased HAST Test Results with 15 Mold Compounds


• CuWB Mold Compound Biased HAST Compatibility

• pH and Cl- specifications are not constant, and are unique for each MC

• Probability of Cu-Al bHAST corrosion increases with bias voltage

• FSL has developed procedures to assess pH and Cl- safety margins

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4 MCs – 3 fail, 1 pass

Low V Pass, High V Fail

9 lots of a single EMC

2 lots of a single EMC

Fail

Pass


• CuWB Mold Compound Biased HAST Compatibility

• Chart shows results of 98 Biased HAST / THB tests with 19 mold compounds across 5 assembly sites

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Challenge: Pass Biased HAST with both bare Cu and PdCu wire

Initially, corrosion

crack formed under

bare Cu ball bond

with biased HAST

Bare Cu

ball bond

Oxidized Al layer

Al

Cu Proper mold

compound and bond

optimization

eliminated failure

with Bare Cu

Pd-Cu

ball

bond

Corrosion layer

No corrosion

layer

Same BOM, after wire bond

optimization - No bHAST failure

Pd-Cu ball bond Corrosion crack

formed under

PdCu ball bond

with biased

HAST with MC

approved for

bare CuWB

FSL is successful in bHAST with both bare Cu, and PdCu wire

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CuWB Development Challenges: Lead Frame 2nd Bond


2nd Bond

Lead frame

Design

• Spec: Zero heel crack

• Action: Optimize lead frame – no

change to wire looping or length

2nd Bond

Optimization on

LQFP (Ag-spot)

• Spec: Ag thickness – min and max range has been evaluated

• Action: Standard Ag thickness

Cu wire

Ag

Cu leadframe

Fishtail bond

Challenge: Achieve optimal 2nd bond without delamination or broken heel

• Cu work-hardens much more than Au – Effective ductility reduction in 2nd bond region

• 2nd bond optimization + Lead frame design optimization mitigates risk

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Issue Before Optimized Risk / Specification / Action

2nd Bond

Optimization

on BGA (Au-Ni

plated Cu pad)

• Spec: Ni/Au thickness – min and max range has been evaluated

• Action: Standard Ni/Au thickness

Transmission Electron Microscope

(TEM) and Energy Dispersive X-ray

Spectroscopy (EDS) bond phase

analysis shows formation of Au-Cu

IMCs at interface between Cu and

Cu+Au (+Ni) solid solution

Cu wire

Ni

Au – Cu

IMCs

Au – Cu (+Ni) SS

CuWB Development Challenges: Substrate 2nd Bond

Excellent crescent bond and

peel strength achieved

Challenge: Achieve optimal 2nd bond without delamination or broken heel

• Substrate 2nd bonding is performed at lower temperature than on lead frames

• 2nd bond optimization results in excellent bond even at lower temperature

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Decapsulation

process for Au

wire products,

i.e. fuming

nitric acid

• Spec: Developed a new chemical

decapsulation process

• Action: Optimize decap procedure

using modified chemistry and

minimal chemical exposure duration.

‒ 1st Step: Laser milling of MC

‒ 2nd Step: Chemical removal of

mold compound

Challenge: Mold compound (MC) removal by chemical decapsulation for

required post reliability testing and failure analysis can cause

CuWB corrosion, compromising wire pull and ball shear results.

CuWB Development Challenges: Mold Compound Removal

Cu wires

attacked by

fuming nitric acid

No attack after

process

optimization

• Developed benign decapsulation process

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CuWB Development Challenges: Wire Type Verification

Gold Wire Copper Wire

Challenge: Determination of Cu or Au wire bonding in a package independent of Freescale date codes and lot traceability

• X-Ray may be used to determine the wire type

• May require use of a known Au or Cu unit for gray-scale comparison

• Au wires are darker in X-Ray images ( > X-ray attenuation)

• Example of same device with Au (left) and Cu (right) wires in X-Ray photo taken at same kV and current:

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CuWB Development Challenges: Cu Wire Bond Testing

Challenge: CuWB quality testing where failure modes differ vs. AuWB

• Cu wire is stronger than Au wire and Cu wire stretches before breaking

• Wire stretching increases torsion on bond interface

• Torsion may cause pad peel for high strength wire pulls.

Break at hook location

after wire stretch + bend

Wire neck region

High wire strength and

torque on strong IMC

bond can exceed strength

of ILD stack, causing pad

peeling, or ILD cracking

Lower

Torque Higher

Torque

Wire Pull Test

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CuWB Development Challenges: Cu Wire Bond Testing , cont.

• Cu wire bond to Al pad is stronger than Au bond to Al pad

• High bond strength results in different failure modes in high strength bonds

• Cu wire pull and ball shear meets Cpk ≥ 1.67. Variability is > Au wire.

Normal Au wire ball

shear mode: Shears

through Au above the

Au-Al IMC, leaving Au

on the pad.

Normal ball shear

mode for Cu wires.

Shears through the Al

layer, below the very

strong Cu-Al IMC,

leaving no Cu on the

pad.

Ball Shear Test

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Agenda







• Summary

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Process Name/

Operation

Description

Potential Failure

ModeProcess

Product/ Process

Specification

/Tolerance

Evaluation

Measurement

Technique

Sample Size Exit Criteria

Wire Bond Poor Ballbond Integrity Ball ShearMin 6.4g

(t0)

Ball shear Test

(t0)

1 strips/lot,

2 units/strip,

8 balls/unit.

Zero Fails, no

degradation trend

Cpk>1.67

Wire Bond Poor Ballbond Integrity Wire PullMin 4.5g

(t0)Wire Pull Test (t0)

1 strips/lot,

2 units/strip,

8 balls/unit.

Zero Fails, no

degradation trend

Cpk>1.67

Wire BondPoor Wedgebond

IntegrityWire Peel Min 2.5g (t0) Wire Peel Test (t0)

1 strips/lot,

2 units/strip,

8 balls/unit.

Zero Fails, no

degradation trend

Wire Bond Bond pad crateringCratering

TestNo cratering (t0) Cratering Test

1 strips/lot,

2 units/strip,

8 balls/unit.

Zero Fails, no

degradation trend

Wire Bond Poor Ballbond PlacementWirebond

PlacementWithin BPO

Visual Inspection

(min 10x)

1 strips/lot,

2 units/strip

Zero Fails, no

degradation trend

Wire Bond Poor Ballbond Placement Al Push Out Within BPOVisual Inspection

(min 10x)

1 strips/lot,

2 units/strip

Zero Fails, no

degradation trend

Wire Bond WB Yield WB Yield Per Yield ModelVisual Inspection

(min 10x)all units

No yield degradation

trend compare to

yield target (98.5%)

Mold Wire Sw eep Wire Sw eepMax 10% w ire

sw eepXray 10units/lot

Zero Fails, no

degradation trend

Mold Reliability

Mold

Compound

PH and Cl

PH: 5.2-6.5

Cl: 15ppm maxCoC every lot

Zero Fails, no

degradation trend

pH and Cl-

spec

Cu Wire Product Safe Launch Monitor Example 1st 30 production assembly lots (Examples for specific Cu wire, package, etc.)

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Cu Wire Bond Optimization for High Volume Manufacturing

Wire bond specifications and Control Plan have been updated to cover both Gold and Copper wire (both bare Cu and PdCu).

• Cu Wire control items are:

• Pad splash monitor

• Cratering test on every lot

• Cu wire shelf & floor life control

• Rigorous, multi-parameter bonder PM

• Wire pull and shear with set-up, once per shift per bonder, at device change

• 2nd bond wire peel with set-up, once per shift per bonder, at device change

• Mold compound pH and Cl control

• Staging time controls: Pre-wire bond plasma to wire bond; pre-mold plasma to mold

• N2 storage of WIP – Pre-wire bond, post wire bond

• Forming gas flow rate and gas mixture analyzer with auto-alarm

• Ball bond geometry monitor each shift, and with device change

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Agenda






• Summary

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Stress Conditions/Requirements TJN Results

64 LQFP Grouper

KLM Results

100 LQFP Grouper

WBS Wire Bond Shear (Avg min 17g / ind. min

9.5g), 30 bonds from 5 units per lot, 3 lots

Passed, Total 15 units

3 lots Cpk > 1.67


3 lots Cpk > 1.67

WBP Wire Bond Pull (min 3g)

30 bonds from 5 units per lot, 3 lots


3 lots Cpk > 1.67


3 lots Cpk > 1.67

Stress Conditions/Requirements TJN Results

64 LQFP Grouper

KLM Results

100 LQFP Grouper

bHAST Preconditioning before biased Highly Accelerated

Stress Test:

bHAST = 130°C/85%RH for 96 hrs

Passed 2x reqt

192 hrs

3 lots 0/231

Passed 2x reqt

192 hrs

3 lots 0/231

uHAST Preconditioning before unbiased Highly

Accelerated Stress Test:

uHAST = 130°C/85%RH for 96 hrs

Passed 2x reqt

192 hrs

3 lots 0/231

Passed 2x reqt

192 hrs

3 lots 0/231

TC Preconditioning before Temperature Cycle: TC = -

65°C to 150°C for 500 cycles

WBP after TC on 5 units per lot, 3 lots

Minimum 3g force

Passed 4x reqt

2000 cycles

TC - 3 lots 0/231

WBP Cpk > 1.67

Passed 4x reqt

2000 cycles

TC - 3 lots 0/231

WBP Cpk > 1.67

HTSL High Temperature Storage Life:150°C for 1008 hrs Passed 2x reqt

2016 hrs

3 lots 0/231

Passed 2x reqt

2016 hrs

3 lots 0/231

Test Group C – Package Assembly Integrity Tests

0.18um S12G Product Family Extended Reliability Results Meet or exceed AEC1 requirements

Test Group A – Accelerated Environment Stress Tests

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External Use 26

Cu Wire High Temperature Application: High Temp Bake

• C90TFS Technology with Bond Over Active Pads – MAPBGA, 23µm bare Cu wire, Freescale TJN assembly site

Exceeds AEC grade 0 for HTB (150C for 2016 hr, or 175C for 1008 hr)

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Meets 2x AEC Grade 1 TC requirement (-65 to 150C for 500 cycles)

MSL3/260

Min Ave Max Min Ave Max Min Ave Max Min Ave Max

0/80a 0/79

0/80b 0/77 * 7.564 9.401 * 20.481 26.016

0/80c 0/77 * 6.516 9.373 22.777 28.235 31.029

0/80d 0/77 5.347 8.,681 9.912 20.329 26.769 33.479

0/80e 0/79

0/79

0/79

0/78

0/79i

0/79

n/an/a

5.96 8.48 11.26 14 27.2

n/a

47

n/a

500 Cycles

Electrical Electrical ElectricalWire Pull Ball Shear

AATC (sample 4, -65 to 150)

n/a

n/a

n/a

n/a

Wire Pull (122393) Ball Shear

1000 Cycles (FYI)

Wire Pull (spec. > 1.8g) Wire Pull (spec. > 1.8g)

Cu Wire High Temperature Application:

Temperature Cycling

• C90 technology, with Bond Over Active (BOA) LQFP, 0.9 mil

bare Cu wires, Freescale Malaysia assembly site (KLM):

− Passed electrical test after AATC 1000 cycles (-65 to 150 C):

All wire breaks for wire pull, and all acceptable ball shear modes

for ball shear tests

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External Use 28

Electrical Performance of Cu Wire vs. Au Wire

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

1.1

1.2

0.5 0.6 0.7 0.8 0.9 1

Wire Diameter (mil)

R o

f 4

mm

Wir

e

Au (500 MHz)

Au ( 1 GHz)

Cu (500 MHz)

Cu (1 GHz)

Resistance of a 4 mm wire

Most

product

s

55nm

products

45nm

products

• DC resistivity: − Cu wire (4N): 1.7 micro-ohm-cm,

− Au wire (3N - 4N): 2.3 micro-ohm-cm

− Au wire (2N): 3.2 micro-ohm-cm

• Due to skin effect, resistance of a 4 mm long Cu bond wire can be ~10-20% lower than Au depending on wire diameter

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Cu vs. Au Electrical Test Comparison Ring Oscillator Speed CZ study

• Cu wire • Au wire

Electrically Equivalent

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Cu vs. Au Electrical Test Comparison IDD dynamic current with Vdd supply

• Cu wire • Au wire

Electrically Equivalent

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Cu Wire Bond Improves High Temperature Reliability

(a) Cu-Al (b) Au-Al

Wire bonds after 2016 hrs at 150C (a) Cu-Al (b) Au-Al

Wire bonds after 0 hrs of Aging

Comparison of Cu and Au wire bonded to

an Al pad after aging.

• Cu-Al intermetallic has grown, but has

not completely consumed the Al pad.

No Kirkendall voiding is seen.

• Au-Al intermetallic has grown to

consume the entire thickness of the Al

pad, and large voids have formed in the

IMC region.

Comparison of Cu and Au wire bonded to an

Al pad with no aging.

• Very thin Cu-Al IMC layer vs. thick Au-Al

IMC layer with early Kirkendall voiding

Voids

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External Use 32


• Here, the IMC growth rate is approximated to inversely correlate with reliability

• 175 C 1000 hr: Bare Cu Wire - Cu-Al IMC growth rate is ~1/10 the Au-Al rate

− Reliability of Cu-Al bond is 10X that of Au-Al bond 175 C

• Freescale work: 150 C 1000 hr: PdCu Wire - Cu-Al IMC growth rate ~1/38 the rate of Au-Al IMC

− Reliability of PdCu-Al bond is 38X that of Au-Al bond at 150 C

100 hrs.

500 hrs.

1000 hrs.

1600 hrs.

Calculated ball bond IMC thickness vs.

Square root of time at 175C

ref. C. Breach, The Great Debate: Copper vs. Gold Ball Bonding,

Advanced Packaging, Oct. 2008. (Calculated from data from H. J.

Kim, et al., IEEE Transactions on Components and Packaging

Technologies, Vol. 26, No. 2, June 2003).

IMC Diffusion Coefficient

(D; cm2/sec) from Freescale Studies

Wire Type on Al Pad Temp(150C)

Au - Al 1.10 X 10-14

PdCu - Al 2.89 X 10-16

(Au-Al IMC growth 38x faster vs. PdCu

wire; ~10x faster vs. bare Cu wire)

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• Freescale Cu-Al IMC Bond Interface Studies (PdCu Wire) up to

225 C to determine IMC growth activation energy

200C - 288 hrs

PdCu Ball

IMC growth with no voids

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Agenda






• Summary

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Cumulative Analog Product Copper Wire Bond Volume: Automotive HSOP Package (AEC grade 1), under-hood application

No Copper wire related customers returns with > 35M units shipped by end 2013

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HSOP Package Overview

• In production with Cu wire

bonding since 2004

• 50µm bare Cu wire

• NiPd PPF lead frame

• Attached Cu heat slug

• 11x16mm molded body

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No Cu wire related customers returns with

> 300M units shipped by end December 2013

Cumulative Copper Wire Bond Volume by Package Type Consumer and Industrial Microcontroller & Digital Networking Products

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Freescale Cu Wire

Conversion Summary

Current status:

Total of 476 devices qualified

• Automotive qual: 96 completed

• Scope:

− 17 wafer technologies

− 6 wafer fab locations

− 6 assembly sites

− 7 package platforms:

Lead frame and Substrate

• Bare Cu and PdCu wire

• Reliability:

− 2x AEC G1

− AEC G0

Feb. 2014

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External Use 39

Agenda


• FSL Cu Wire Strategy

• Development Methodology Flow





• Summary

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External Use 40

Cu Bond Wire Summary

• Gold (Au) and Copper (Cu) wire have been used in ICs for the interconnection of silicon die to package terminals for many years.

• FSL has shipped > 60M automotive analog components from 2004, to date with no customer issues

• Recent wire bond technology advancements are expanding the use of Cu wire.

• FSL has converted Consumer / Industrial microcontrollers to Cu wire and now initiating Automotive conversions.

− > 230M units shipped to date with no customer issues

Motivation

• Some new products require a high temperature application in which Cu wire is the best solution.

• Avoids the need to pass on increased Au wire costs.

Introduction / Change Strategy • All new products will be introduced with Cu wire

• All Fab expansion products will be qualified with Cu wire

• Existing product families will migrate to Cu wire

− Note: Some legacy products will migrate to thinner Au wire

TM

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