Safety Certification for Lead Free Flexible and Rigid-Flex PCBs Crystal Vanderpan Underwriters Laboratories Inc. March 27, 2007
Crystal Vanderpan Principal Engineer for Printed Circuit Technologies Joined UL in 1995 Technical rep to UL STP for PCB and CCL standards Active in ASTM - Subcommittee chair D09.07 Electrical Insulating Materials Active in IEC, TC15 - Insulating Materials; and TC91 Printed Boards and Materials Specifications B.S. Chemical Eng/Materials Science, UC Davis Email crystal.e.vanderpan@us.ul.com or visit www.ul.com Page 1
Objective Lead Free Legislation and Industry Research Flexible and Rigid-Flex PCB Certification Material Characteristics Flex PCB Constructions Process and Programs Design Concerns versus Certification Requirements Guidance for accelerating the UL certification process. Ten steps to reduce certification time for PCBs How to find the UL Material Parameters Page 2
PCB Production Environment More complex and sophisticated product designs Communication including entire supply chain is key Smaller, lighter, and environmentally friendly Weight, bendability, multi-dimensional wiring, RoHS/WEEE OEMs turn to flexible circuits to optimize performance and reliability. Demand drivers in automotive, hand held devices, flat panel displays. Page 3
Lead Free Legislation To help protect human health and decrease electronic waste, the EU adopted two Directives WEEE (Waste from Electrical and Electronic Equipment) RoHS (Restriction of Hazardous Substances in Electrical and Electronic Equipment) Similar legislation also adopted in China, Japan, and the US Page 4
RoHS Directive Restriction of Hazardous Substances in Electrical and Electronic Equipment Developed to Support WEEE Directive Eliminate hazardous substances during the disposal and recycling of electronic waste Restrict Substances in Homogeneous Materials Effective Date July 1, 2006 Page 5
RoHS Directive Restricted Substances Cadmium Hexavalent Chromium Mercury Lead Brominated Flame Retardants Polybrominated Biphenyl (PBB) Polybrominated Diphenyl Ether (PBDE) Exempt Tetrabromobisphenol A (TBBPA) Page 6
Electronic Equipment Affected by WEEE & RoHS Large and small appliances Tools IT Equipment Telecommunications Equipment Lighting Toys Sports equipment rrently RoHS exempt Medical Devices Monitoring and Control Instruments Page 7
Industry Research and Testing Lead Free solder alloys used for over 10 years Early research concentrated on Recommending standard alloys SAC, Sn Assessment of basic properties such as Toxicity, wetting and strength behavior Recent research focusing on Additional properties such as Fatigue, production behavior, and process optimization Leadfree solder joints shown to be more reliable than existing eutectic Sn/Pb alloys, however many material problems Page 8
Lead Free Surface Finish Choices Immersion finishes (Gold, Silver and Tin) Electroless Nickel-Immersion Gold (ENIG) Organic Solderability Protectants (OSP) Tin-Silver-Copper (SAC) alloy pastes HASL non-lead containing Tin/Copper and Tin/Silver Each material must be evaluated for its benefits and challenges. Page 9
Processing Temperatures Lead Free materials require 30 C to 45 C higher melting temperatures when compared to tin-lead solder. Soldering Material Melting Points and Associated Solder Pot Temperatures Solder Pot Material Melting Range ( C) Temperature ( C) Sn-Pb 183 188 250 Sn- (99.3Sn/0.7) 227 270 280 Sn-Ag-Bi 206 213 260 Sn-Ag- 217 260 270 Sn-Ag (96.5Sn/2.5Ag) 221 265 275 Page 10
Higher Temperatures Create problems in the PCB Delamination and warping Thermal shock induced cracks Damage to the components, such as Plastic connectors, BGAs, LEDs, electrolytic and ceramic capacitors, etc. Page 11
Material Characteristics Traditional materials used to build flexible and rigid-flex circuits: PCB Construction Component Base dielectric film Conductors Adhesive and bonding films Coverlay Stiffener Copper-clad laminates Material type PI, PET, PEN, PTFE, LCP, epoxy, aramid Rolled annealed (RA) copper foil, electrodeposited (ED) copper foil, aluminum, stainless steel Acrylic, epoxy, PI, PET, adhesiveless PI, PET, PTFE PI, PET, FR-4 laminate, metal substrates FR-4, FR-1, PI, PTFE Page 12
Material Characteristics What to consider? Selection of material types Construction techniques Number of bending cycles Process temperatures Operating temperature Technical requirements Page 13
Flexible PCB Construction Layer - non-conductive dielectric material planes Single-Layer Flexible PCBs Singlesided (SS) - one conductor plane Doublesided (DS) - two conductor planes Singlesided (SS) Coverlay Film Conductor () Base Film Adhesive Base Film Conductor () Surface Finish Doublesided (DS) Coverlay Film Conductor () Base Film Adhesive Base Film Base Film Adhesive Coverlay Adhesive Coverlay Film PTH Page 14
Flexible PCB Construction Multilayer Flexible PCB - three or more conductor planes Coverlay Film Conductor () Coverlay Adhesive Base Film Adhesive Base Film Base Film Adhesive Conductor () Coverlay Adhesive Coverlay Film Bondply Coverlay Film Conductor () Coverlay Adhesive Base Film Adhesive Base Film Base Film Adhesive Conductor () Coverlay Adhesive Coverlay Film Page 15
Flexible PCB Construction Multilayer Rigid-Flex PCB - integral combination of single-sided, double-sided and multilayer constructions SR PP Lam PP Bondply Bondply Coverlay Film Base Film Coverlay Film Base Film Coverlay Film Base Film Coverlay Film Page 16
Typical Constructions Flexible PCB* dynamic and repeated flexing applications Bent -to- install or Flex to- install PCB* applications where PCB will be bent for installation or servicing Rigid PCB applications not subject to flexing or bending due to the thickness or characteristics of the material * Ultimate bend capacity or endurance is not established for Flexible and Bent-to-install Construction via the certification test program. Page 17
Typical Constructions Rigid-flex PCB Flexible core base film, base film adhesive, copper, coverfilm, coverfilm adhesive Rigid outer layers single- or multilayer FR-4 material with solder mask on external layer Bonding films or prepreg material Page 18
PCB Certification Standards Flexible films and laminates UL 746F Polymeric Materials Flexible Dielectric Film Materials Flexible circuit boards UL 796F Flexible Materials Interconnect Constructions Page 19
PCB Certification Program Safety certification investigations account for application-focused issues Intended location and environment of PCB in the end product Flammability Only or Full Evaluation Programs Dielectric material types and thickness Number of dielectric layers Conductor weight Platings and surface finishes Overall build-up circuit thickness Page 20
PCB Certification Program "Flammability Only" Recognition End product application location and environment includes flammability only concerns. Thermal, mechanical and electrical capacity is not of concern with regard to the end product certification Flammability only testing Page 21
PCB Certification Program UL ANSI-like Program Flammability Only PCBs evaluated for flammability properties only No re-evaluation (testing) for PCBs of similar construction and performance Flammability testing of one polyimide film with a specific adhesive to represent other polyimide films with the same adhesive UL application still needed Page 22
PCB Certification Program "Full Evaluation" Recognition End product application Location and environment of PCB require flammability classification and maximum operating temperature (MOT) rating Testing includes flammability, conductor adhesion, blistering and delamination testing stiffener bond strength and flexibility testing depends on board application requirements Page 23
End Product UL Certification Program Majority require PCB Recognized with Flammability Rating Maximum Operating Temperature (MOT) Switch to Lead Free may not impact end product certification Assuming PWB used with appropriate Recognized ratings Page 24
Lead Free PCB UL Evaluation Testing required for higher temperature and exposure times Manufacturing process Soldering process Page 25
Lead Free PCB Labeling Unique type designation Identify Lead Free throughout assembly process Board finishes Components Assembly solders Identification is critical for reliable rework Melting points of alternate finishes Possibility of mixing alternate finishes Page 26
Lead Free PCB UL Evaluation Required testing includes Flammability Conductor adhesion (Bond Strength) Delamination and blistering Page 27
Manufacturing Process Evaluation Monitoring includes Board surface finishes Maximum temperature and exposure times Process description includes Solder rrently intended as tin/lead solder Alternate Lead Free solders described by material constituents Page 28
Lead Free Plating Materials Do not require testing to add if UL Recognized process includes plating and Immersion finishes (Gold, Silver and Tin) Electroless Nickel-Immersion Gold (ENIG) Organic Solderability Protectants (OSP) Solder limits are above Leadfree temperatures for assembler reflow/wave processing Page 29
Lead Free Plating Materials No Silver Migration testing required Immersion Silver SAC alloys IPC (3-11g) - Metal Finishes Data Acquisition task group researched silver migration using both Telcordia and UL test methods. Page 30
Leadfree Solder Processing Require higher temperatures than tin/lead SAC (260 C 270 C) Sn-Ag (265 C 275 C) Sn- (270 C 280 C) Affect both PWB and Assemblers Page 31
UL PCB Solder Limits Represent maximum temperature and exposure time during the assembly process Simulated on PWB samples through the thermal shock test Designed to evaluate the physical fatigue of boards exposed to the anticipated assembly soldering temperatures (Solder Limits) 288 C for 20 seconds, industry suggested Page 32
Surveillance (Follow-up Service) Quarterly audit based on UL Follow-Up Service (FUS) Procedure Confirms ongoing compliance Materials base film, coverlay, adhesive, bonding film, and stiffener minimum and maximum material thickness Manufacturing process including maximum temperature, exposure times and the materials used. Page 33
Design Concerns VS Certification Requirements Design #1 Dielectric insulating properties of bonding film or prepreg? SR Lam PP Coverlay Film Base Film Coverlay Film Page 34
Design Concerns VS Certification Requirements Design #1 Design Concern Why? UL s Concerns Certification Testing Bonding Film/Prepreg as insulator between the base film copper and core copper Reduce amount of adhesive to increase reliability Prepreg required to comply with electrical relative temperature indices (RTIs) Long Term Thermal Aging (LTTA) may be required if prepreg is not Recognized with sufficient RTIs. 9-12 months Page 35
Design Concerns VS Certification Requirements End Product with Maximum Operating Temperature (MOT) rating concerns Rigid-Flex PCB with Full Recognition Certification, including MOT Product Category ZPXK2 - Standard UL 796F Prepreg Recognized with sufficient RTIs Page 36
Design Concerns VS Certification Requirements Design #2 Base Film Adhesive Base Film Base Film (PPS) Silver deposition Conductive adhesive Coverlay Film Coverlay Adhesive What are the interfaces of the silver film conductive adhesive layer? For example as the above structure: Base film Adhesive Coverlay Film & Adhesive Copper Page 37
Design Concerns VS Certification Requirements Design #2 Design Concern Why? UL s Concerns Certification Testing Silver conductive paste as shielding Shield electromagnetic interference (EMI) Flammability, silver migration, additional process steps Flammability, conductor adhesion, blistering, delamination, flexibility and silver migration testing Page 38
Reduce Certification Time Ten steps to shorten the certification process 1. Submit request as early as possible 2. Communicate with OEM to determine flammability only or full evaluation program Page 39
Reduce Certification Time 3. Use current UL Follow-Up Service procedure as a template for PCB changes or new construction 4. Describe the critical PCB construction parameters material manufacturer, grade designation, minimum and maximum thickness of each material 5. Submit request with cross sectional diagram Page 40
Reduce Certification Time Critical Parameters (step 4) Single-Layer Flexible PCB Items Base Film Coverlay Covercoat Stiffener Stiffener Adhesive Silver Film or Paste # Constructions Process Application $ Manufacturer (QMTS2); Grade Dsgn.; Film Thickness (Min/Max); Adhesive Thickness (Min/Max)!; External Copper Foil Thickness (Min/Max); Manufacturer (QMTS2); Grade Dsgn.; Film Thickness (Min/Max); Adhesive Thickness (Min/Max); Manufacturer (QMJU2); Grade Dsgn.; Coating Thickness (Min/Max); Manufacturer (QMTS2) or Metal Type; Grade Dsgn.; Thickness (Min); Manufacturer; Grade Dsgn.; Thickness (Min/Max); Manufacturer; Grade Dsgn.; Detail Cross-Sectional Construction Diagrams Detail process descriptions, any step involving temperature higher than 100 o C; Any step involving plating process. Flexible; Flex-to-Install; Rigid Critical Parameters Note:! No need if the base film is adhesiveless # Only for EMI Shielding purpose $ No need for flame only types Page 41
Reduce Certification Time Cross Sectional Construction Diagram (step 5) Single-Layer Flexible PCB Coverlay Film Coverlay Adhesive Coverlay Film Base Film Adhesive Base Film Base Film Adhesive Covercoat or Solder Resist Surface finish PTH Example of covercoat not covering coverlay Keys for UL Base film & Cover film: Grade; Min. Film thk; Max. Film thk Base film & Coverlay adhesive: Min. & Max. thk Covercoat or Solder Resist: Grade; Min. & Max. coating thk Conductor (): Min. & Max. thk Processes? Surface Finish? Page 42
Reduce Certification Time Critical Parameters (step 4) Rigid-Flex PCB Critical Parameters Items Critical Parameters Base Film Coverlay Bondply b/w Flex & Flex Covercoat on Flex Bonding Sheet b/w Rigid & Flex Rigid Laminate Prepreg Solder Resist on Rigid Silver Film or Paste # Constructions Process Manufacturer (QMTS2); Grade Dsgn.; Film Thickness (Min/Max); Adhesive Thickness (Min/Max)!; External Copper Foil Thickness (Min/Max); Manufacturer (QMTS2); Grade Dsgn.; Film Thickness (Min/Max); Adhesive Thickness (Min/Max); Type (Free Film or Film with Adhesive); Manufacturer; Grade Dsgn.; Film Thickness (Min/Max)$; Adhesive Thickness (Min/Max); Manufacturer (QMJU2); Grade Dsgn.; Coating Thickness (Min/Max); Type (Free Film or Prepreg or Film with Adhesive); Manufacturer; Grade Dsgn.; Film Thickness (Min/Max)$; Adhesive Thickness (Min/Max); Prepreg Thickness (Min) Manufacturer (QMTS2); Grade Dsgn.; Thickness (Min); Manufacturer (QMTS2); Grade Dsgn.; Thickness (Min); Manufacturer (QMJU2); Grade Dsgn.; Manufacturer; Grade Dsgn.; Detail Cross-Sectional Construction Diagrams Detail process descriptions, any step involving temperature higher than 100 o C; Any step involving plating process. Note:! No need if the base film is adhesiveless # Only for EMI Shielding purpose $ Only for film with adhesive type bondply Page 43
Reduce Certification Time Cross Sectional Construction Diagram (step 5) Rigid-Flex PCB Material Type Grade Dsgn. Thickness (mic) Min. Max. Solder Resist Solder Resist Taiyo XXXX Copper Copper 18 102 Prepreg Prepreg Nan Ya NP-140B 50 Copper Copper 18 34 Rigid Laminate Rigid Laminate Nan Ya NP-140TL 50 Copper Copper 18 34 Bonding Sheet Bonding Sheet Isola FR402++ 50 Coverlay 12.5 25 Taiflex FD-1 Coverlay Adhesive 15 35 Copper 18 18 Base Film Adhesive 15 20 Base Film Taiflex N 12.5 25 Base Film Adhesive 15 20 Copper 18 18 Coverlay Adhesive 15 35 Taiflex FD-1 Coverlay 12.5 25 Bondply Microcosm PEAP# 25 25 Coverlay 12.5 25 Taiflex FD-1 Coverlay Adhesive 15 35 Copper 18 18 Base Film Adhesive 15 20 Base Film Taiflex N 12.5 25 Base Film Adhesive 15 20 Copper 18 18 Coverlay Adhesive 15 35 Taiflex FD-1 Coverlay 12.5 25 Bonding Sheet Bonding Sheet Isola FR402++ 50 Copper Copper 18 34 Laminate Laminate Nan Ya NP-140TL 50 Copper Copper 18 34 Prepreg Prepreg Nan Ya NP-140B 50 Copper Copper 18 102 Solder Resist Solder Resist Taiyo XXXX Rigid Interconnect Construction Flexible Interconnect Construction Rigid Interconnect Construction Page 44
Reduce Certification Time 6. Request manufacturing process description Include maximum temperature and time exposure (ie. 275 ± 5 C, test 280 C) Page 45
Reduce Certification Time Process Description Example - for reference only 1. May cut and then bake at o C max. for hours max. 2. May drill or punch holes. 3. May desmear, electroless and then electroplate copper. 4. May laminate or print photo-resist and then dry at o C max. for minutes max. 5. May UV exposure and develop. 6. May etch by any etchant except chromic-sulfuric acid. 7. May strip the photo-resist and water rinse and then dry at o C max. for minutes max. 8. May apply brown or black oxide and water rinse, and then dry at o C max. for minutes max. 9. May pre-laminate Coverlay at kg/cm 2 max. at temperature o C max. for hours max. 10. May laminate Coverlay at kg/cm 2 max. at temperature o C max. for hours max. 11. May post bake at temperature o C max. for hours 12. May print legend and then bake at temperature o C max. for hours max. 13. May HASL at temperature o C max. for seconds max. 14. May ENIG (Electroless Ni/ Immersion Au) or Immersion Sn. 15. May Electroplate Ni/Au. 16. May route or punch. 17. May water rinse and then dry at temperature o C max. for hours max. 18. No other plating operations performed and no other temperatures greater than 100 o C encountered. Page 46
Reduce Certification Time 7. Use preselection criteria to choose materials with sufficient parameters for the PCB construction 8. Request multiple solder limits reflecting the assembly reflow process Example: 180 C/120 sec, 217 C/100 sec, 265 C/20 sec, cool to room temp. 9. Sample preparation should be verified with UL staff 10. Notify UL of the expected shipping date Page 47
How to find UL Recognized Materials? www.ul.com Certifications Input Info Link to File UL Category Codes (CCN) Flexible PWB ZPXK2 Flexible base material QMTS2 Plastic Component (PI, PET or PEN Film) QMFZ2 Coatings of PWB QMJU2 Rigid PWB ZPMV2 Page 48
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