1. SMTA Wisconsin Chapter Meeting
PRINTED CIRCUIT BOARD CONSIDERATIONS FOR LEAD FREE ASSEMBLY
Dale Lee
Bill Barthel
15 May, 2007
Much of this information was originally presented at SMTAI 2005 Conference. The presentation has been updated to reflect recent changes in the industry.

2. 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

3. 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

4. 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
For FR4 Materials That Are Hydrophilic – You should check with you PCB supplier or their laminate supplier.

5. 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
The increase in DfE by large OEM’s will advance the availability of halide free laminate materials and drive down costs.

6. Halide Free Materials Thermal-Mechanical Data
Based on Vendor Data
CTE by TMA = ambient to 288°C
*CTE by TMA = ambient to 260°C
The physical properties of similar classes of laminate material can vary greatly from manufacturer to manufacturer. During the design process, the critical parameters should be addressed and specified to ensure that the correct materials are utilized during PCB fabrication.

7. Materials To Be Assessed
From

8. Performance And Test Criteria
From
This shows the breadth of tests one should consider when assessing these materials. Note most, but not all, have IPC test methods associated with them.

9. 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.

10. 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

11. DICY Cure High Tg FR4 TGA: Decomposition Temp
Typical DICY cure thermal decomposition temperature curve. Note that material decay actually begins (dip in curve) before reported temperature.

12. Phenolic Cure High Tg FR4 TGA: Decomposition Temp
Note the increase in thermal decomposition temperature from DICY cured material in previous slide.

13. 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

14. 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

15. Decomposition Temperature
Increase in Td improves reliability of PCB even after multiple exposures to assembly process temperatures.
Minimum Decomposition Temperature (Td) Of 340C And Thermal Cycle Reliability
Courtesy of Cookson

16. 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

17. 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

18. Coefficient of Thermal Expansion
Epoxy expansion can be modified by glass and filler content
ppm/0C

19. TMA Curve 184.92°C 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
Plot intersect points from graph to determine Tg

20. Standard Loss Material Thermal-Mechanical Data
170°C Tg
150°C Tg
Standard Loss: Df  Dk  4.1 – 4.4
CTE by TMA = ambient to 288°C
Courtesy of Merix

21. 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

22. 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

23. Electrical Properties
Isola FR
Isola FR
Polyclad 370HR

24. Material Properties Courtesy of Isola
Note that physical properties of materials are not only affected by manufacturer but also by type of material utilized (core) for fabrication of PCB. Example – Dk difference between and 106/1080 stackup for nearly the same thickness. Generally, the higher the resin content, the lower the Dk.
Courtesy of Isola

25. 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

26. 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

27. 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

28. PCB Finish Vs Solder Spread
OSP
Immersion Silver
Immersion Tin
ENIG
This is a comparison of the same variable spacing solder paste print pattern on different PCB finishes and reflowed in nitrogen. Note the differences between the spacing between pads that are bridged (spread) of OSP & Im-Ag versus Im-Sn and ENIG finished. In other words, where the solder paste in printed on the pad with OSP & Im-Ag will be where it will stay during reflow (It will not center onto pad if printed offset).
The Amount Of Solder Spread During Reflow Or Wave Soldering Process Is Dependant Upon Finish Type

29. IPC – Lead Free Laminate
IPC Laminate/Prepreg Materials Subcommittee Proposal:
Glass Transition Temperature ºC
Decomposition Temperature ºC
Z-Axis Expansion Alpha 1 (max/ ºC) 60 ppm
Z-Axis Expansion Alpha 2 (max/ ºC) 300 ppm
T260 (minutes) min.
T288 (minutes) min.
T300 (minutes) min.
Missing from proposal is an assembly and rework process exposure test.
This was the recommendation of the IPC subcommittee for lead free laminates which was the input into the recent slash sheet additions to IPC-4101B specification.

30. 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
Equivalent Tg inorganic filled and unfilled laminate slash sheet classes (i.e.- /99 & /124) can have different failure modes (mechanical shock) or CTE (Z-axis) and electrical properties (Dk) and should be address during design process for functional impacts.

31. 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
Customer surveys specifically geared toward Lead Free Requests from CM’s to collaborate on material evaluations
At the annual IPC Printed Circuits Expo in March not only did we see evidence that our industry was recovering, but the attendance on classes for all aspects of Pb Free were highly attended.
Motorola Boca performing Pb Free for quite some time stating 235C peak all you need
chose the NEMI 260 C peak due to the large form factor and high layer count that represents the most challenging portion of our portfolio Last Bullet – being able to survive a 260 C reflow is not enough, the inherent long term reliability of the PCBA must be preserved

32. 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

33. 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

34. 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

35. 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

36. Number of Reflow Cycles
DICY CURE High Tg FR4 Material Matrix “Example!”
Number of Reflow Cycles 1 2 3 4 5 6
Networking 10 – 26 Layers
Automotive Layers
Backplanes 10 – 40 Layers
Consumer
2–6 Layers
PCB Thickness

37. Non-DICY 150°C Tg FR4 Material Matrix “Example!”
Number of Reflow Cycles 1 2 3 4 5 6
PCB Thickness
Networking 10 – 26 Layers
Automotive Layers
Backplanes 10 – 40 Layers
Consumer
2–6 Layers

38. Non-DICY 170°C Tg FR4 Material Matrix “Example!”
Number of Reflow Cycles 1 2 3 4 5 6
PCB Thickness
Networking 10 – 26 Layers
Automotive Layers
Backplanes 10 – 40 Layers
Consumer
2–6 Layers

39. 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.

40. Failure Mechanisms
Internal Stress – Thermo-mechanical 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

41. Failure Mechanisms Surface Finish Challenges
Wetting Of OSP After Multiple Thermal Cycles
Immersion Silver Voids

42. 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

43. Thank You
Questions?