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Comparison of Pb-free Solder on UIC Test Boards and Components

Comparison of Pb-free Solder on UIC Test Boards and Components. Steve Dunford Nokia Mobile Phones. Agenda. UIC Pb-free Test Nokia Configurations Test Conditions X-ray Inspection First Failures Weibull Plots Intermetallic Formations in Pb-free solder Definition and examples

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Comparison of Pb-free Solder on UIC Test Boards and Components

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  1. Comparison of Pb-free Solder on UIC Test Boards and Components Steve Dunford Nokia Mobile Phones

  2. Agenda • UIC Pb-free Test Nokia • Configurations • Test Conditions • X-ray Inspection • First Failures • Weibull Plots • Intermetallic Formations in Pb-free solder • Definition and examples • Reliability Improvements and Concerns • Solder Joint Voiding • Failure Mechanisms • Sn Whiskers • Discussion • Conclusions

  3. Configurations Tested At Nokia • Configuration Sn62 Sn/Ag3.8/Cu 0.7Sn/Ag3.5 Castin OSP 8 32 32 8 Ni/Au 8 32 32 8 • Stencil • .005 inch • 1/1 aperture to pad • Reflow Profile • Peak 244°C (SnAg) • Peak 241°C (Castin, SAC) • 3 Zone Oven • Ambient Conditions

  4. Test Conditions • Temp Cycle • 14 min. dwell+4 min. transition • -40°C to 125°C in single chamber • Components at temperature • 11 minutes cold • 9.5 minutes hot

  5. Assembly Results • Castin • All Solder Joints Good • Popcorned During Reflow - Improperly dried • Significant Voiding in Solder Joints – Results Invalid • Temp Cycle Failure Data Not Used - Invalid • SAC • 1 Open • Polyimide tape on package • Bridged daisy chain around open • SnAg • 1 Open • Bridged daisy chain around open • Sn62 • All Solder Joints Good

  6. Assembly Results SnAg Open SAC Open – Caused by Polyimide Tape on Component

  7. Early Failures – Temp Cycle 0 to 12 SAC – Ni/Au board (461) Package side failure at cycle 12

  8. Pb-Free Ni/Au Failures Up to 2700 Cycles • SnAg – Ni/Au PWB, Failure on PWB at 2700 cycles Shown above. • SAC – Ni/Au PWB, Board Side Failures 180 to 450 cycles

  9. SAC Ni/Au Failures 2700 cycle SnAg – Ni3Sn layer formed above Phosphorous rich Ni layer. Adhered well on most components

  10. Weibull Plots – SAC NiAu Solder fatigue failures “black pad” on PWB or Component contributed to SAC on Ni/Au failures in less than 200 cycles. Other failures not fully investigated at this time.

  11. Weibull Plots – SnAg on NiAu Solder fatigue failures Two Failure Mechanisms – not fully investigated at this time

  12. Package Damage – 2000 cycles Underfill separation, substrate cracks – Likely contributor to different failure mechanism. Sn62 – Ni/Au board, Failure at 2700 cycles

  13. Package Damage – 2700 cycles Underfill separation, substrate cracks – Likely contributor to different failure mechanism. Sn62 – Ni/Au board, Failure at 2700 cycles

  14. Weibull Plots – Best Performers SAC on OSP and Sn62 on OSP

  15. Intermetallic Formations in Pb-free Solder • Necessary for durable interface between solder and solder pads • Can Restrict Evolution of gases • Increased voiding • Increase solder joint strength • Continue to form throughout lifetime • Needles, dendrites, plates and tubes • Cu Pad dissolution • Can re-direct fractures • Not Necessarily the preferred crack propagation path • Potential for Spalling

  16. X-Ray Inspection of Cycled Boards SnAg on OSP 6500 cycles

  17. X-Ray Inspection of Cycled Boards SAC on Ni/Au 6500 cycles

  18. X-Ray Inspection of Cycled Boards CASTIN on OSP 6500 cycles

  19. X-Ray Inspection of Cycled Boards SAC on OSP 6500 cycles

  20. X-Ray Inspection of Cycled Boards SAC on OSP 6500 Cycles

  21. X-Ray Inspection of Cycled Boards SAC OSP 6500 cycles

  22. X-Ray Inspection of Cycled Boards SAC on Ni/Au 6500 cycles

  23. Whisker Growth – Not Typical Sn Whisker SnAg on OSP 6500 Cycles Whisker growth similar to that seen in Previous X-ray

  24. Whisker Growth - Inside Voids SnAg Solder after 6500 cycles, -40 to 125°C

  25. Whisker Growth - Inside Voids Ag/Sn forms on the end of the Sn Whisker

  26. Intermetallic Formations in Pb-free Solder Sn/Ag/Cu on OSP – Intermetallic formations and Void attached to Sn/Ag intermetallic plate

  27. Intermetallic Formations in Pb-free Solder Sn/Ag Plate Redirects Crack, 2700 cycles (-40 to 125°C)

  28. Intermetallic Formations in Pb-free Solder Ag/Sn intermetallic redirects crack propagation, SAC/OSP (2700 cycles, -40 to 125°C)

  29. Intermetallic Formations in Pb-free Solder SPALLING – Portions of Solder Joint Separating (6500 cycles, -40 to 125°C)

  30. Intermetallic Formations in Pb-free Solder Crack Propagation Arrested by Ag/Sn Flake SPALLING – Portions of Solder Joint Separating (6500 cycles, -40 to 125°C)

  31. Intermetallic Formations in Pb-free Solder SPALLING – Portions of Solder Joint Separating (6500 cycles, -40 to 125°C)

  32. Intermetallic Formations in Pb-free Solder After etch

  33. Sink Holes Ag/Sn Spheres Au/Sn Intermetallics Grain Boundary May be associated with continued formation of intermetallic compounds

  34. Intermetallic Formations in Pb-free Solder Unique Failure Mechanisms & Continued Intermetallic Growth (6500 cycles, -40 to 125°C)

  35. Intermetallic Formations in Pb-free Solder SPALLING – Portions of Solder Joint Separating along grain boundaries. (SAC, 6500 cycles, -40 to 125°C)

  36. Intermetallic Formations in Pb-free Solder SAC solder joints 6500 cycles and extended etch

  37. Intermetallic Formations SAC Sn Ag3Sn Plate SAC solder joint 6500 cycles and extended etch

  38. Intermetallic Formations SAC Sn Ag3Sn Plate Ag3Sn spheres and dendrites form in subgrain boundaries

  39. Intermetallic Formations SAC Fractures Form at Grain Boundaries

  40. Intermetallic Formations SAC Fractures Form at Grain Boundaries

  41. Discussions • SAC on OSP shows equivalent or better performance than eutectic Sn/Pb solder • Solder Paste Print • Temperature cycling tests from –40 to 125°C • Intermetallics in Solder Joint • Ag/Sn Plates, Spheres and dendrites in subgrain boundaries. • Can Arrest or Redirect Cracks • Increase Joint life • More Extensive in SAC solder

  42. Discussions • Grain Boundaries Can be Preferred crack Propagation Path • Cracks Any Location in Solder Joint • Vertical or horizontal • Potential for Spalling • Some Concerns Remain • Increased Voiding is Likely to Occur • Increased Intermetallic formation • Spalling Potential • Potential for Sn Whisker Growth

  43. Conclusions • Pb-free processing is Viable for many applications • Can Provide Equivalent or Better Performance over Sn/Pb solders • Our work is not done

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