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Presentation on theme: "SMTA Wisconsin Chapter Meeting PRINTED CIRCUIT BOARD CONSIDERATIONS FOR LEAD FREE ASSEMBLY Dale Lee Bill Barthel"— Presentation transcript:


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 BFRs –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

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

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

7 Materials To Be Assessed

8 Performance And Test Criteria

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

12 Phenolic Cure High Tg FR4 TGA: Decomposition Temp

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 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 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 Sn/Pb Pb Free

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

19 TMA Curve Z axis pre Tg 20°C – 185°C 84.3 ppm/°C °C Z axis post Tg 185°C – 288°C 315 ppm/°C Z axis Overall 20°C – 288°C 161 ppm/°C

20 Standard Loss Material Thermal-Mechanical Data 170°C Tg150°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

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 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 155 ºC –Decomposition Temperature340 ºC –Z-Axis Expansion Alpha 1 (max/ ºC)60 ppm –Z-Axis Expansion Alpha 2 (max/ ºC)300 ppm –T260 (minutes)30 min. –T288 (minutes)15 min. –T300 (minutes) 2 min. Missing from proposal is an assembly and rework process exposure test.

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

31 Industry Stance on Lead Free EMS Forum; A Group Of Leading EMS Providers Has Developed Guidelines For PCB Suppliers 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

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 PCB Thickness Number of Reflow Cycles Networking 10 – 26 Layers Automotive Layers Backplanes 10 – 40 Layers Consumer 2–6 Layers DICY CURE High Tg FR4 Material Matrix Example!

37 Non-DICY 150°C Tg FR4 Material Matrix Example! Number of Reflow Cycles 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 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 Dont 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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