Presentation on theme: "SMTA Wisconsin Chapter Meeting"— Presentation transcript:
1SMTA Wisconsin Chapter Meeting PRINTED CIRCUIT BOARD CONSIDERATIONS FOR LEAD FREE ASSEMBLYDale LeeBill Barthel15 May, 2007Much of this information was originally presented at SMTAI 2005 Conference. The presentation has been updated to reflect recent changes in the industry.
2RoHS-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-PreventionMember 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 PBDECertain end product categories are exempt until 2010
3RoHS & Bromine FreeTetrabromobisphenol-A (TBBPA) Is The Leading Flame Retardant Used In Printed Circuit Boards (95%+) And Computer Chip CasingsThe Use Of TBBPA Is Currently Not Banned In Any Country In The WorldThere Is Concern That RoHS Or Local Legislation May Be Revised To Prohibit All BFR’sThis Is Considered Less Likely Based On Recent Risk Assessment By The EU
4Halogen Free Materials All Major Laminate Manufacturers Have A Halogen Free OfferingBrominated Flame Retardants Replaced Predominately With Phosphorous Based MaterialThese Materials Are More Hydrophilic Than Brominated MaterialsLower 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 FreeThese Materials Typically Have A Shorter Shelf LifeFurther Exacerbating Import And Inventory Issues For PCB FabricatorThese Materials Are More Costly When Compared To Their Brominated EquivalentHalogen Free Materials Are Primarily Manufactured In Asia And EuropeVery Limited Market For Bromine Free Products In North AmericaOff-shore Supply Extends Lead Times Or Create Risk Buy InventoriesFor FR4 Materials That Are Hydrophilic – You should check with you PCB supplier or their laminate supplier.
5Halogen Free Material Usage Use Of Halogen-free Materials Will Increase As Consumer Level Products Push To Be Halide Free (Design For Environment)Apple ComputerMotorolaNokiaOthersiNEMI is sponsoring a project to define and qualify materialsOutline key performance characteristicsFocus on Electrical characteristics, Delamination, and Via ReliabilityDetermine compatibility with higher temperaturesThe increase in DfE by large OEM’s will advance the availability of halide free laminate materials and drive down costs.
6Halide Free Materials Thermal-Mechanical Data Based on Vendor DataCTE by TMA = ambient to 288°C*CTE by TMA = ambient to 260°CThe 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.
8Performance And Test Criteria FromThis shows the breadth of tests one should consider when assessing these materials. Note most, but not all, have IPC test methods associated with them.
9PCB Laminate FactorsTg – 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.
10Thermal-Mechanical Properties It Is Important To Understand How The Data Provided By The Material Suppliers Was GeneratedDifferences In Test Methods, Samples Or Criteria Used Can Effect Reported ValuesComparison Of Data Should Be From Like Conditions And Methods, Or Differences Understood And Accounted ForDecomposition Temperature – TdReported At 2% Or 5%Coefficient Of Thermal Expansion – CTETemperature Range; Prefer Room Temp To 288°CResin Content Of SampleTime To Delamination – T260 & T288Heat Rise: Prefer 100°C Per MinuteCopper Clad Vs. Unclad Sample
11DICY 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.
12Phenolic Cure High Tg FR4 TGA: Decomposition Temp Note the increase in thermal decomposition temperature from DICY cured material in previous slide.
13Decomposition 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
14Time to DelaminationFor Lead Free Assembly, The IPC Requirement For Time To Delamination Tested At T288 Condition (288C) Should Be The Baseline TemperatureCourtesy of Cookson
15Decomposition 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 ReliabilityCourtesy of Cookson
16Time to DelaminationDelamination May Not Be Visible During Initial Assembly Process But Subsequent Processes Such As Component Rework And Replacement May Result In Visible PCB Delamination Or BlisteringTested 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
17Glass Transition Temperature Sn/PbPb FreeReported In Literature That PTH Reliability Did Not Improve With Increased Tg Normal FR4 MaterialLaminate Z-axis CTE Is Critical To PTH And Via ReliabilityTCE Above Tg Is An Important Factor
18Coefficient of Thermal Expansion Epoxy expansion can be modified by glass and filler contentppm/0C
19TMA Curve 184.92°C Z axis pre Tg 20°C – 185°C 84.3 ppm/°C Z axis post Tg185°C – 288°C315 ppm/°CZ axis Overall20°C – 288°C161 ppm/°CPlot intersect points from graph to determine Tg
20Standard Loss Material Thermal-Mechanical Data 170°C Tg150°C TgStandard Loss: Df Dk 4.1 – 4.4CTE by TMA = ambient to 288°CCourtesy of Merix
21Test 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
22Electrical Properties Dielectric Constant – Dk Or ErDissipation Factor – Df Or Loss TangentChanges In Material Systems To Improve Thermal Resistance Can Alter Electrical PropertiesPhenolic Systems Tend To Have Higher Df And Dk ValuesSome Suppliers Use Fillers To Enhance CTE And Reduce Df, This Typically Increases DkIt Is Important To Understand These Properties When Transitioning To Pb Free MaterialsHowever Subtle, Designs Which Maximize The Performance Of Traditional Epoxy May Be Affected By New Lead Free Compatible Materials
24Material 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
25PCB FinishesMost Finishes are compatible with Lead Free Assembly Process requirementsImmersion SilverEnvironment Exposure (Pollution) SensitiveSulfur, Chlorine, etc.ENIG / Electrolytic Gold/NickelBlack Pad IssueInterest in electroless Ni/Pd/Au as an alternative to ENIGSevere Environmental Exposure (Pollution) SensitiveOrganic Solder Preservative (OSP)Limited number of heating cycle exposuresNew High Temperature Compatible FormulasImpacts Wave Solder Process Solderability
26PCB FinishesMost Finishes are compatible with Lead Free Assembly Process requirementsImmersion TinTin WhiskersLimited Shelf LifeLead Free HASLLimited AvailabilityHigher Cost Than Traditional Tin/Lead Coating
27Lead 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
28PCB Finish Vs Solder Spread OSPImmersion SilverImmersion TinENIGThis 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
29IPC – Lead Free Laminate IPC Laminate/Prepreg Materials Subcommittee Proposal:Glass Transition Temperature ºCDecomposition Temperature ºCZ-Axis Expansion Alpha 1 (max/ ºC) 60 ppmZ-Axis Expansion Alpha 2 (max/ ºC) 300 ppmT260 (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.
30PCB Laminate Specification Old 4101A Spec Only Had 2 Major Classifications For FR4 Material - /24 & /26New 4101B Spec Has Multiple ClassificationsLow Halide: /92 & /94 (P), /93 & /95 (Al(OH)3)FR4 (w Inorganic Fillers): /97, /99, /101 & /126FR4 (w/o Inorganic Fillers): /121, /124 & /129(Note: All Low Halide Slash Sheets Are Multifunctional. FR4 Are Both Difunctional & Multifunctional By Slash SheetEquivalent 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.
31Industry Stance on Lead Free EMS Forum; A Group Of Leading EMS Providers Has Developed Guidelines For PCB Supplier’s Transitioning To Lead FreeWhile The Guideline Does Not State Specific Pass Fail Performance Criteria For These Materials It Does Identify Critical Reliability Testing Using PreconditioningThis Document Has Identified A Peak Reflow Temp. Of 260°CGeneral Agreement Within Industry For The iNEMI Criteria With Peak Assembly Temperature Of 260°CCustomer surveys specifically geared toward Lead Free Requests from CM’s to collaborate on material evaluationsAt 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 needchose 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
32Lead Free CriteriaMaterials Intended For Use In Lead Free Assembly Must Be Able To Survive Multiple Exposures At 260°C And Have Acceptable Post Assembly ReliabilityMaterials With Improved Thermal Mechanical Properties Are NeededA Variety Of Tests To Should Be Used To Determine Lead Free Capability For Any Given MaterialNo Single Test Or Material Attribute Can Assure Lead Free Assembly CompatibilityNeed To Pass Multiple Tests
33Test Protocol VariantWhen Reviewing And Comparing Data Of Vendor Reports, It Is Important To Understand The Design And Assembly Parameters Of The Test VehiclesBoard Thickness And Layer Count Affect Thermal MassDrilled Hole SizeSurface FinishPreconditioning Methods And Number Of ExposuresCurrently There Is No Industry Standard Test Methodology
34Performance Consideration Performance Requirements May Dictate Material SelectionMission Critical Applications; Military, Aerospace, MedicalDecisions Should Be Conservative To Allow For A Safety ZoneNon Fault Tolerant Systems Where Additional Reliability Requirements Are SpecifiedMainframes, Servers, Communications Equipment, AutomotiveLife Expectancy Of The ProductConsumer Electronics Vs. System ElectronicsSystem EnvironmentAdditional Performance Criteria Such As CAF Resistance May Make Certain Materials More Appropriate
35PCB Assembly Consideration Peak Reflow TemperatureNumber Of Reflow CyclesConsider Build Short ScenariosUnusual Or Special Assembly Processes That Effect Thermal Exposures And Mechanical LoadingPotential Rework Cycles And ProcessesIncluding Up Rev And Part Change OutsMoisture Sensitivity Level & Exposure TimeMore important as value/reliability of the PCA increases
36Number of Reflow Cycles DICY CURE High Tg FR4 Material Matrix “Example!”Number of Reflow Cycles123456Networking 10 – 26 LayersAutomotive LayersBackplanes 10 – 40 LayersConsumer2–6 LayersPCB Thickness
37Non-DICY 150°C Tg FR4 Material Matrix “Example!” Number of Reflow Cycles123456PCB ThicknessNetworking 10 – 26 LayersAutomotive LayersBackplanes 10 – 40 LayersConsumer2–6 Layers
38Non-DICY 170°C Tg FR4 Material Matrix “Example!” Number of Reflow Cycles123456PCB ThicknessNetworking 10 – 26 LayersAutomotive LayersBackplanes 10 – 40 LayersConsumer2–6 Layers
39Failure MechanismsThere Are Several Failure Mechanisms Seen In Materials Failing TestingLack Of Thermal Resistance – Polymer Matrix Degrades, Reducing Reliability Or Leading To DelaminationVolatile 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.
40Failure MechanismsInternal Stress – Thermo-mechanical Properties Don’t Allow The Material To Pass TestsBrittle Material Tends To Relieve Stress By Fracturing“Cratering”Design Elements Impart Additional Stress In LaminationHigh Expansive Material Will Fatigue PTH Causing Cracks
41Failure Mechanisms Surface Finish Challenges Wetting Of OSP After Multiple Thermal CyclesImmersion Silver Voids
42Summary Current Flame Retardants (TBBPA) Is Not A Band Substance Tg Alone Is Not An Indicator Of Lead Free CapabilityT260 And T288 Values Alone Do Not Assure Lead Free Capability Of A Laminate SystemBase Resin Decomposition Temperature (Td) And CTE Values Are Good Indicators Of Lead Free SurvivabilityTraditional DICY Cured FR4 Laminates Are Unable To Survive Multiple Elevated Pb Free Reflow Profiles Or Sustain Impact To ReliabilityReflow Conditions Will Vary Depending Upon Board DesignBoard Design Contributes To The Lead Free Equation (Resin Content)Electrical Properties Need To Be Accounted For In DesignHalide Free And Lead Free Laminate SystemsReliability Impacts Following Assembly Simulation Critical