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SMTA Wisconsin Chapter Meeting

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Presentation on theme: "SMTA Wisconsin Chapter Meeting"— Presentation transcript:

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

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?

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