The Effect of Coating and Potting on the Reliability of QFN Devices

1. The Effect of Coating and Potting on the Reliability of QFN DevicesCheryl Tulkoff and Dr. Nathan Blattau 2. © 2004 - 200720102QFN: What is it?oQuad Flat Pack No Lead or Quad Flat Non-Leadedo‘The poor man’s ball grid array’oAlso known asoLeadframe Chip Scale Package (LF-CSP)oMicroLeadFrame (MLF)oOthers (MLP, LPCC, QLP, HVQFN, etc.)oOvermolded leadframe with bond pads exposed on the bottom and arranged along the periphery of the packageoDeveloped in the early to mid-1990’s by Motorola, Toshiba, Amkor, etc.oStandardized by JEDEC/EIAJ in late-1990’soFastest growing package type 3. © 2004 - 200720103Quad Flat No-Leads (QFN)oElimination of leadsoProvides lower resistanceoLower inductanceoHigher performanceoHigher package densitiesoTradeoffsoIncreased power densityoManufacturabilityoMore susceptible to thermal mechanical fatigueCycles to failure-40 to 125CQFP: >10,000BGA: 3,000 to 8,000QFN: 1,000 to 3,000 4. © 2004 - 200720104Advantages: Thermal PerformanceoMore direct thermal path with larger areaoDie  Die Attach  Thermal Pad  Solder  Board Bond PadoqJa for the QFN is about half of a leaded counterpart (as per JESD-51)oAllows for 2X increase in power dissipation 5. © 2004 - 200720105Advantages: InductanceoAt higher operating frequencies, inductance of the gold wire and long lead-frame traces will affect performanceoInductance of QFN is half its leaded counterpart because it eliminates gullwing leads and shortens wire lengthshttp://ap.pennnet.com/display_article/153955/36/ARTCL/none/none/1/The-back-end-process:-Step-9-QFN-Singulation/Popular for RF Designs 6. 6© 2004 - 200107Disadvantage: Thermal Mechanical Fatigue (Solder)o Design change: More silicon, less plastico Increases mismatch in coefficient of thermal expansion (CTE)BOARD LEVEL ASSEMBLY AND RELIABILITYCONSIDERATIONS FOR QFN TYPE PACKAGES,Ahmer Syed and WonJoon Kang, Amkor Technology.Does the increasedsusceptibility of QFNdevices make themmore sensitive toconformal coating andpotting effects? 7. © 2004 - 20072010oThe use of underfills, potting compounds and thick conformal coatings can greatly influence the failure behavior under thermal cyclingoAny time a material goes through its glass transition temperature problems tend to occuroConformal coating should not bridge between the PCB and the componentoUnderfills designed for enhancing shock robustness do not tend to enhance thermal cycling robustnessoPotting materials can cause PCB warpage and tensile stresses on electronic packages that greatly reduce time to failureRules of Thumb 8. 9© 2004 - 200107Thermal Cycling: Conformal Coatingo Care must be taken when using conformal coating over QFNo Coating can infiltrate under the QFNo Small standoff height allows coating to cause lifto Hamilton Sundstrand found a significant reduction in time to failure(-55 / 125C)o Uncoated: 2000 to 2500 cycleso Coated: 300 to 700 cycleso Also driven by solder jointsensitivity to tensile stresseso Damage evolution is farhigher than for shear stressesWrightson, SMTA Pan Pac 2007 9. © 2004 - 2007201010oWhy did conformal coating effect thermal cycling performance?oVerification and determination of mechanical propertiesoElastic Modulus as a function of temperatureoGlass Transition TemperatureoCoefficient of Thermal ExpansionYoung’s Modulus Datasheet 1260 psi (8.7 MPa) Coefficient of thermal Expansion Datasheet 55 ppm/°CConformal Coating Properties (Glass Transition Temperature) 10. © 2004 - 2007201011Elastic Modulus – DMA - TensileDatasheet - Specification (8.7 MPa, 1260 psi), Tg = 15°CAcrylic conformal coating 11. © 2004 - 2007201012Glass Transition Temperature Tg ≈ 5 to 15°CCoefficient of Thermal Expansion - TMABelow Tg CTE – 170 ppm/°C Above Tg : CTE – 340 ppm/°CAcrylic conformal coating 12. © 2004 - 20072010oIdeally the CTE of the potting should be as close to the CCA as possibleoUsually in the 20 to 30 ppm/°CoThe larger the CTE, the more compliant the potting must be to limit the stresses imparted to the CCAoPotting should the generate hydrostatic pressure (equal on all sides) of the circuit cardoThis prevents warping of the CCA as the potting expandsoExcessive warping will greatly reduce time to failureoMay cause overstress failures.oThis may require modification to the housingoHousing may need to be relatively stiffPotting 13. © 2004 - 20072010oQFN failures occurring very rapidly during temperature cycling with urethane based potting materialoAll units were failed at the 100 cycle inspection (-40 to 105C)oGood quality joints with sufficient solder thicknessRigid Housing with Free Surface 14. © 2004 - 20072010Material PropertiesPotting Compound Isotropic Material CTEx,y = 120 ppm Significant increase in modulus or stiffness below with high CTE 15. © 2004 - 20072010PCB Warpage due to Potting Shrinkage 16. © 2004 - 20072010QFN WarpageUnpottedPottedOrder of magnitude higher deformation and deformation concentrated over corner solder joints 17. © 2004 - 20072010oVery high stresses during cold dwell of thermal cycleSolder StressesHOTCOLD 18. © 2004 - 20072010oThe higher the creep strains the shorter the time to failureCreep StrainsUnpottedPottedoExcessive creep occurring at cold temperatureoMore energy required to cause cold temperature creep (more damaging) 19. © 2004 - 20072010oThe lack of a compliant lead structure makes QFN devices more susceptible to PCB warpage related failuresoMechanical properties of the potting materialoGlass transition temperature (Tg)oModulus should be specified above and below the TgoCTE should be specified above and below the TgoThe design of the housingoMay provide a surface to which the potting material can pull against when shrinking causing PCB warpageoShould be designed to provide as close to a hydrostatic pressure as possible (equal pressure on all sides)Conclusions