Transcript Document

SMTA Wisconsin Chapter
Meeting
PRINTED CIRCUIT BOARD CONSIDERATIONS
FOR LEAD FREE ASSEMBLY
Dale Lee
[email protected]
Bill Barthel
[email protected]
15 May, 2007
RoHS-Overview
What is behind the drive for lead free products?
Directive 2002/95/EC of the European Parliament and of the Council of
27 January 2003 on the restriction of the use of certain hazardous
substances in electrical and electronic equipment. (RoHS-Reduction of Hazardous Substances)
Article 4-Prevention
Member States shall ensure that, from 1 July 2006, new electrical
and electronic equipment put on the market does not contain lead,
mercury, cadmium, hexavalent chromium, Polybrominated
Biphenyls (PBB) or Polybrominated Diphenyl Ethers (PBDE)
specifically Penta PBDE and Octa PBDE
Certain end product categories are exempt until 2010
RoHS & Bromine Free
• Tetrabromobisphenol-A (TBBPA) Is The Leading Flame
Retardant Used In Printed Circuit Boards (95%+) And
Computer Chip Casings
• The Use Of TBBPA Is Currently Not Banned In Any Country
In The World
• There Is Concern That RoHS Or Local Legislation May Be
Revised To Prohibit All BFR’s
– This Is Considered Less Likely Based On Recent Risk Assessment
By The EU
Halogen Free Materials
• All Major Laminate Manufacturers Have A Halogen Free Offering
– Brominated Flame Retardants Replaced Predominately With Phosphorous Based
Material
• These Materials Are More Hydrophilic Than Brominated Materials
– Lower Moisture Sensitivity Level Than Standard FR4 Material
(Some Lead Free FR4 Laminate Materials Are Also Hydrophilic)
• There Are Currently No Low Loss Systems That Are Bromine Free
• These Materials Typically Have A Shorter Shelf Life
– Further Exacerbating Import And Inventory Issues For PCB Fabricator
• These Materials Are More Costly When Compared To Their Brominated
Equivalent
• Halogen Free Materials Are Primarily Manufactured In Asia And Europe
– Very Limited Market For Bromine Free Products In North America
– Off-shore Supply Extends Lead Times Or Create Risk Buy Inventories
Halogen Free Material Usage
• Use Of Halogen-free Materials Will Increase As
Consumer Level Products Push To Be Halide Free
(Design For Environment)
–
–
–
–
Apple Computer
Motorola
Nokia
Others
• iNEMI is sponsoring a project to define
and qualify materials
– Outline key performance characteristics
– Focus on Electrical characteristics, Delamination, and Via
Reliability
– Determine compatibility with higher temperatures
Halide Free Materials
Thermal-Mechanical Data
MATERIAL
Isola IS500
Nelco 4000-7 EF
Polyclad PCL-HF-571
Hitachi BE-67-GH
Nanya NPG-150
Tg TMA Tg DSC
Td TGA
°C
°C
170
180
350
155
165
425
165
340
145
141
150
350
CTE by
TMA
2.8%
3.5%
2.9%*
50
Based on Vendor Data
CTE by TMA = ambient to 288°C
*CTE by TMA = ambient to 260°C
T260
T288
>60
>30
>30
>60
>60
3
3
>60
Materials To Be Assessed
Performance And Test Criteria
PCB Laminate Factors
• Tg – Temperature where laminate changes rate
of thermal expansion. Function of Resin
Chemistry.
• Td – Decomposition temperature of laminate
measured by weight loss by TGA. Function of
Resin Chemistry.
• T260, T288 – Resistance to Delamination at
elevated temperatures. Function of Resin
Chemistry and board Design.
Thermal-Mechanical Properties
• It Is Important To Understand How The Data Provided
By The Material Suppliers Was Generated
– Differences In Test Methods, Samples Or Criteria Used Can
Effect Reported Values
• Comparison Of Data Should Be From Like Conditions
And Methods, Or Differences Understood And
Accounted For
– Decomposition Temperature – Td
• Reported At 2% Or 5%
– Coefficient Of Thermal Expansion – CTE
• Temperature Range; Prefer Room Temp To 288°C
• Resin Content Of Sample
– Time To Delamination – T260 & T288
• Heat Rise: Prefer 100°C Per Minute
• Copper Clad Vs. Unclad Sample
DICY Cure High Tg FR4
TGA: Decomposition Temp
Phenolic Cure High Tg FR4
TGA: Decomposition Temp
Decomposition Temperature
• When Printed Circuit Boards Are Exposed To Multiple
Heating Cycles (Assembly, Rework and Lamination), Small
Repeated Weight Losses Can Degrade The Reliability Of
The PCB.
• The Difference Between Low Td Material With 2% Weight
Loss And High Td Material With 0.5% Weight Loss Per
Assembly Process Cycle Can Be The Difference Between
Scrap And A Reliable Product.
Courtesy of Cookson
Time to Delamination
• For Lead Free Assembly, The IPC Requirement For Time
To Delamination Tested At T288 Condition (288C)
Should Be The Baseline Temperature
Courtesy of Cookson
Decomposition Temperature
• Minimum Decomposition Temperature (Td) Of
340C And Thermal Cycle Reliability
Courtesy of Cookson
Time to Delamination
• Delamination May Not Be Visible During Initial Assembly Process But
Subsequent Processes Such As Component Rework And Replacement
May Result In Visible PCB Delamination Or Blistering
• Tested To Higher Temperatures Comparable To Lead Free Assembly
Temperatures Of T288 Condition, Many Of The Laminates Have
Delamination Times Of Less Than 5 Minutes.
• Visible Carbonization Of The Epoxy Laminate And/Or Charring
Exhibited
Glass Transition Temperature
Sn/Pb
Pb Free
• Reported In Literature That PTH Reliability Did Not
Improve With Increased Tg Normal FR4 Material
• Laminate Z-axis CTE Is Critical To PTH And Via Reliability
• TCE Above Tg Is An Important Factor
Coefficient of Thermal
Expansion
Epoxy expansion can be modified by glass and filler content
60
50
ppm/0C
40
30
20
10
0
Silicon
Alumina
Glass
Copper
Solder
Epoxy
TMA Curve
Z axis pre Tg
20°C – 185°C
84.3 ppm/°C
Z axis post Tg
185°C –
288°C
315 ppm/°C
Z axis Overall
20°C – 288°C
161 ppm/°C
184.92°C
Standard Loss Material
Thermal-Mechanical Data
Tg TMA Tg DSC
Td TGA
MATERIAL
°C
°C
Dicy Cure 170 Tg FR4
160
170
290
Isola IS410
169
179
345
Polyclad 370HR
180
350
MEM 1755
173
177
364
Nelco 4000-11
170
175
360
Nanya NPN170
165
175
369
Isola IS400
145
150
330
Polyclad 250HR
150
350
Nanya NPN150
145
156
367
170°C Tg
CTE by
TMA
4.4%
3.5%
2.7%
3.0%
3.2%
3.5%
3.4%
-
150°C Tg
Standard Loss: Df  0.020 Dk  4.1 – 4.4
CTE by TMA = ambient to 288°C
Courtesy of Merix
T260
T288
<10
>27
>30
>27
30
60
>27
-
5.2
7
3.8
10
4
-
Test Results 2000 IST High Tg FR-4
Standard Dicy based High Tg FR-4 showed about 70% drop in
via fatigue life. Use NEMI Reflow For Lead Free Profile Testing.
Courtesy of Merix
Electrical Properties
Dielectric Constant – Dk Or Er
Dissipation Factor – Df Or Loss Tangent
• Changes In Material Systems To Improve Thermal
Resistance Can Alter Electrical Properties
– Phenolic Systems Tend To Have Higher Df And Dk Values
– Some Suppliers Use Fillers To Enhance CTE And Reduce Df,
This Typically Increases Dk
• It Is Important To Understand These Properties When
Transitioning To Pb Free Materials
• However Subtle, Designs Which Maximize The
Performance Of Traditional Epoxy May Be Affected By
New Lead Free Compatible Materials
Electrical Properties
Dk
MATERIAL
Isola
406406
IsolaFRFR
Isola IS 400
Polyclad 250HR
Isola
410 410
IsolaISFR
MEM 1755
Polyclad 370HR
Polyclad 370HR
Hitachi BE-67-GH
Isola FR 408
Polyclad Getek HR
Nelco 4000-12
Nelco 4000-13
Isola IS 620
Polyclad LD-621
Megtron 5
Df
1 GHz
10 GHZ
1 GHz
10 GHz
3.9
3.8 0.015
0.015 0.017
0.017
3.9
3.8
4.3
0.017
4.65
4.5
0.017
0.019
4.1
4.0
0.020 0.023
0.023
4.1
4.0
0.020
4.4
4.2
0.020
0.023
4.5
4.3
0.017
0.020
4.5
4.3
0.017
0.020
4.4
4.4
0.009
0.014
3.7
3.65
0.012
0.014
3.9
3.8
0.009
0.011
3.8
3.7
0.009
0.008
3.8
3.5
0.010
0.012
3.6
3.6
0.008
0.0085
3.4
3.2
0.006
0.008
3.5
3.4
0.004
0.0085
Material Properties
Courtesy of Isola
PCB Finishes
• Most Finishes are compatible with Lead Free
Assembly Process requirements
– Immersion Silver
• Environment Exposure (Pollution) Sensitive
– Sulfur, Chlorine, etc.
– ENIG / Electrolytic Gold/Nickel
• Black Pad Issue
– Interest in electroless Ni/Pd/Au as an alternative to ENIG
• Severe Environmental Exposure (Pollution) Sensitive
– Organic Solder Preservative (OSP)
• Limited number of heating cycle exposures
• New High Temperature Compatible Formulas
• Impacts Wave Solder Process Solderability
PCB Finishes
• Most Finishes are compatible with Lead
Free Assembly Process requirements
– Immersion Tin
• Tin Whiskers
• Limited Shelf Life
– Lead Free HASL
• Limited Availability
• Higher Cost Than Traditional Tin/Lead Coating
Lead Free Solder Spread
• OSP And Immersion Ag Finishes Do Not
Promote Lead Free Solder Spread (SMT &
PTH)
• ENIG And Immersion Tin Have Similar
Solder Spread To Traditional Tin Lead
PCB Finish Vs Solder Spread
– OSP
– Immersion
Silver
– Immersion Tin
– ENIG
The Amount Of Solder Spread During Reflow Or Wave
Soldering Process Is Dependant Upon Finish Type
IPC – Lead Free Laminate
• IPC Laminate/Prepreg Materials Subcommittee
Proposal:
–
–
–
–
–
–
–
Glass Transition Temperature
Decomposition Temperature
Z-Axis Expansion Alpha 1 (max/ ºC)
Z-Axis Expansion Alpha 2 (max/ ºC)
T260 (minutes)
T288 (minutes)
T300 (minutes)
155 ºC
340 ºC
60 ppm
300 ppm
30 min.
15 min.
2 min.
Missing from proposal is an assembly and rework
process exposure test.
PCB Laminate Specification
• Old 4101A Spec Only Had 2 Major Classifications
For FR4 Material - /24 & /26
• New 4101B Spec Has Multiple Classifications
– Low Halide: /92 & /94 (P), /93 & /95 (Al(OH)3)
– FR4 (w Inorganic Fillers): /97, /99, /101 & /126
– FR4 (w/o Inorganic Fillers): /121, /124 & /129
(Note: All Low Halide Slash Sheets Are Multifunctional. FR4
Are Both Difunctional & Multifunctional By Slash Sheet
Industry Stance on Lead Free
• EMS Forum; A Group Of Leading EMS Providers Has
Developed Guidelines For PCB Supplier’s Transitioning To
Lead Free
– While The Guideline Does Not State Specific Pass Fail Performance
Criteria For These Materials It Does Identify Critical Reliability
Testing Using Preconditioning
– This Document Has Identified A Peak Reflow Temp. Of 260°C
• General Agreement Within Industry For The iNEMI Criteria
With Peak Assembly Temperature Of 260°C
Lead Free Criteria
• Materials Intended For Use In Lead Free Assembly
Must Be Able To Survive Multiple Exposures At
260°C And Have Acceptable Post Assembly
Reliability
– Materials With Improved Thermal Mechanical
Properties Are Needed
• A Variety Of Tests To Should Be Used To
Determine Lead Free Capability For Any Given
Material
• No Single Test Or Material Attribute Can Assure
Lead Free Assembly Compatibility
– Need To Pass Multiple Tests
Test Protocol Variant
• When Reviewing And Comparing Data Of Vendor
Reports, It Is Important To Understand The Design
And Assembly Parameters Of The Test Vehicles
–
–
–
–
Board Thickness And Layer Count Affect Thermal Mass
Drilled Hole Size
Surface Finish
Preconditioning Methods And Number Of Exposures
• Currently There Is No Industry Standard Test
Methodology
Performance Consideration
• Performance Requirements May Dictate Material
Selection
– Mission Critical Applications; Military, Aerospace,
Medical
• Decisions Should Be Conservative To Allow For A Safety Zone
– Non Fault Tolerant Systems Where Additional Reliability
Requirements Are Specified
• Mainframes, Servers, Communications Equipment, Automotive
– Life Expectancy Of The Product
• Consumer Electronics Vs. System Electronics
– System Environment
• Additional Performance Criteria Such As CAF
Resistance May Make Certain Materials More
Appropriate
PCB Assembly Consideration
• Peak Reflow Temperature
• Number Of Reflow Cycles
– Consider Build Short Scenarios
• Unusual Or Special Assembly Processes That
Effect Thermal Exposures And Mechanical
Loading
• Potential Rework Cycles And Processes
– Including Up Rev And Part Change Outs
– Moisture Sensitivity Level & Exposure Time
– More important as value/reliability of the PCA
increases
DICY CURE High Tg FR4
Material Matrix “Example!”
Number of Reflow Cycles
6
5
Networking
10 – 26 Layers
4
3
Automotive
4 - 8 Layers
2
1
Backplanes
10 – 40 Layers
Consumer
2–6 Layers
.031
.062
.093
.125
.157
.188
PCB Thickness
.212
.244
.275
Non-DICY 150°C Tg FR4
Material Matrix “Example!”
Number of Reflow Cycles
6
5
Networking
10 – 26 Layers
4
3
Automotive
4 - 8 Layers
2
1
Backplanes
10 – 40 Layers
Consumer
2–6 Layers
.031
.062
.093
.125
.157
.188
PCB Thickness
.212
.244
.275
Non-DICY 170°C Tg FR4
Material Matrix “Example!”
Number of Reflow Cycles
6
5
Networking
10 – 26 Layers
4
3
Automotive
4 - 8 Layers
2
1
Backplanes
10 – 40 Layers
Consumer
2–6 Layers
.031
.062
.093
.125
.157
.188
PCB Thickness
.212
.244
.275
Failure Mechanisms
There Are Several Failure Mechanisms Seen In
Materials Failing Testing
• Lack Of Thermal Resistance – Polymer Matrix
Degrades, Reducing Reliability Or Leading To
Delamination
• Volatile Outgassing – The Extra 30-40º C In Reflow
Temperature Greatly Increases Vapor Pressure.
• Increased Moisture Sensitivity – The Extra 30-40º C
In Reflow Temperature Greatly Increases Moisture Vapor
Pressure.
Failure Mechanisms
• Internal Stress – Thermomechanical Properties Don’t
Allow The Material To Pass
Tests
– Brittle Material Tends To
Relieve Stress By
Fracturing
• “Cratering”
– Design Elements Impart
Additional Stress In
Lamination
– High Expansive Material
Will Fatigue PTH Causing
Cracks
Failure Mechanisms
• Surface Finish Challenges
– Wetting Of OSP After Multiple Thermal Cycles
– Immersion Silver Voids
Summary
• Current Flame Retardants (TBBPA) Is Not A Band Substance
• Tg Alone Is Not An Indicator Of Lead Free Capability
• T260 And T288 Values Alone Do Not Assure Lead Free
Capability Of A Laminate System
• Base Resin Decomposition Temperature (Td) And CTE Values
Are Good Indicators Of Lead Free Survivability
• Traditional DICY Cured FR4 Laminates Are Unable To Survive
Multiple Elevated Pb Free Reflow Profiles Or Sustain Impact To
Reliability
• Reflow Conditions Will Vary Depending Upon Board Design
• Board Design Contributes To The Lead Free Equation (Resin
Content)
• Electrical Properties Need To Be Accounted For In Design
– Halide Free And Lead Free Laminate Systems
• Reliability Impacts Following Assembly Simulation Critical
Thank You
Questions?