Pb-free issues for the Semiconductor Industry

1. Pb-free issues for the Semiconductor Industry COST531, Vienna, 18th of May 2007 Dr. Ir. Pascal Oberndorff NXP Semiconductors IC Manufacturing Operations Back End Innovation Nijmegen, The Netherlands Pascal.oberndorff@nxp.com Pascal Oberndorff NXP Semiconductors IC Manufacturing Operations Back End Innovation Nijmegen, The Netherlands Pascal.oberndorff@nxp.com

2. Introduction • Pb could be found on the leads in electrolytic finish of semiconductor components • Pb also present in some die – attach, but exemption: alternatives for high Pb solder?? • In case of Ball Grid Arrays (BGA) Pb was present in the solder balls • Semiconductor Industry is also impacted by higher temperature of the soldering process in Pb-free (Moisture Sensitivity)

3. Introduction • Leaded Devices: • Change Electroplating • NiPdAu Preplated Leadframes (costly) • Pure Sn (Whiskers) • BGA • Change Solder balls to Pb-free balls • Sometimes balls go missing?!

4. Leaded Components ElectroplatingPre Plated Leadframes NiPdAu

5. Prevent Pd Oxidation • Prevent Outgas • absorbed by Pd Au or Au Alloy • Prevent Ni Surface Oxidation Pd • Prevent Cu Defusing into Pd Layer Ni Base Material Source : Sumitomo NiPdAu Structure Thicknesses: Ni 0.5- 1.5 m; Pd 5- 60 nm; Au 3-10 nm

6. Au or Au Alloy Pd Au or Au Alloy Ni Pd Molten Solder Molten Solder Base Material Ni Base Material Au Pd Pd Au Soldering NiPdAu Source : Sumitomo Note that when come into contact with molten solder, the Pd and Au/Au Alloy layer dissolves into the matrix structure of the solder, leaving only the Ni Layer.

7. Conclusion for PPF • Only applicable for Small Components due to moisture absorption

8. During storage the component willl absorb moisture from the ambient humidity During board assembly the absorbed moisture will vaporize and escape from the package More critical for Pb-free reflow, due to higher temperatures of Pb-free solder Moisture Sensitivity Source Infineon Adhesion of moulding compound on PPF is worse than on Cu alloys

9. Moisture Sensitivity IC damage risk due to: Popcorn effect

10. Remarks on PPF • Good solution for small packages but costly! • How to improve adhesion to plastic; • Different plastic • Rougher surface of leadframe • Chemicals?

11. Leaded Components ElectroplatingSn based Finishes

12. Comparisonof Various Plating Options ! +/- Matte Sn was selected, mass production within NXP since 2003

13. Problem: Whisker Growth • Most literature agrees whiskers grow because of internal compressive stress in plating layer • Important: whiskers not only grow on pure Sn but also on Sn alloys; even SnPb!

14. Whisker growth not only with pure Sn! Whisker growth on SnBi after 8 months storage at ambient storage Whisker growth on SnCu1 after one year storage at ambient temperature

15. Reasons for Stress in Plating • Irregular intermetallic formation • Mismatch of CTE with leadframe • Incorporation of foreign particles in plating • Mismatch of crystal orientation • Mechanical operations • The growth of whiskers is relieving the present stress

16. Cu6Sn5 Sn Deposit Whisker Cu Substrate Cu6Sn5 Cu L/F Whisker Growth @ Ambient Whiskers grow because of compressive stress in the plating which is caused by irregular growth of intermetallics Tin Whisker is forced out

17. Countermeasure: Postbake (1h, 150 oC) (Within 24 hours of plating) • Because of higher temperature diffusion will shift from grain boundary to bulk diffusion and thus regular intermetallics • Grain Growth of Sn • Diffusion barrier for further intermetallic growth • Possible annealing of stress • Postbake does NOT change CTE mismatch! No whisker! Sn deposit Cu6Sn5/Cu3Sn Cu based LF

18. Cu6Sn5 Sn Deposit Experimental: SelectiveEtching Useful for looking at morphologies of Cu6Sn5 and by differential measuring able to calculate the amount of intermetallic. Cu based LF

19. Intermetallic Growth Directly after Plating 1 hour after plating1 week after plating Intermetallic (Cu6Sn5) grows after plating by grain boundary diffusion of Cu into Sn. After one week the height is more than 3 m.

20. Growth of Cu6Sn5 Growth of intermetallic Cu6Sn5 compared @ room temperature and @ 55 oC

21. Growth Curves at Elevated Temperature Growth rate gives idea about amount of intermetallics However, morphology of IMC is different

22. Bulk Diffusion Recrystallization Same amount of intermetallics! 1 h 150 oC 4 h 125 oC Grain Boundary Diffusion Less Recrystallization 1 year RT 42 days 55 oC Morphology of the Intermetallics

23. Calculation of Volume Change 6 Cu + 5 Sn => Cu6Sn5 Calculation of the volume change with help of X-ray data of the unit cell results in: V = 128.08/124.06 = 1.0418 = 3.2% Not taking into account the contribution of the reacting Cu: V = 128.08/81.4= 1.588 = 57.3 % Thus: Cu6Sn5 formation can result in compressive stress.

24. Sn Cu6Sn5 C19400 Grain Growth Before postbake: 3-10 m After postbake: 5-20 m

25. Intermetallic Growth 1 2 Diffusion barrier 1 2 • This graph shows that @ ambient there is no additional intermetallic growth up to 2 years after postbake • No whisker observed after postbake @ ambient for > 4 years, irrespective of Sn thickness

26. CTE Mismatch Difference in CTE Sn= ~ 23 ppm/o FeNi42= ~4 ppm/o Whiskers will grow during temperature cycling This situation is only relevant for assembled components CPD AIT

27. CTE Mismatches No whisker growth on NiFe leadframes in isothermal storage,but after temperature cycling whiskers grow, contrary to Cu-based leadframes. 5 m Sn on NiFe after 500 cycles -55/85oC 5 m Sn on CuFe after 500 cycles -55/85oC

28. Temperature Cycling on Assembly Source: Infineon Whisker length after –40 oC/ 85 oC, 5K/min, 30 minutes dwell time on unsoldered components and modules (SnPbAg and SnAgCu soldered)

29. O2 + 2H2O + 4e- 4OH- O2 in H2O Anode Sn / Sn2+ = -0.14 V Cathode Cu2+ / Cu = 0.34 V Sn Sn2+ + 2e- Cu2+ + 2e- Cu Sn + 2OH-  Sn(OH)2 + 2e- 2Sn + 2H2O + O2 2Sn(OH)2 Whisker in high humidity • Corrosion may happen due to water condensation in association with exposed base material forming a galvanic couple (exposed base material) • Also local defects within the tin may cause localised different nobility forming such a galvanic couple • Presence of water w/o condensation may support the corrosive effect

30. Sn Deposit Cu6Sn5 SnOx Whisker Growth Associated with Corrosion Whiskers grow because of compressive stress in the plating which is caused by irregular growth of intermetallics or oxidation/corrosion products in the Sn layer. Formation of oxide results in volume increase (29/47% SnO and 32% SnO2) Tin Whisker is forced out Cu based LF

31. Relation to Field Life • After 4000 hrs even SnPb shows whisker growth (max length ~ 100 m) • Board assembled units do not show this corrosion or extreme whisker growth during the test time

32. Remarks on Pure Sn finish • Three mechanisms for whisker growth related to field conditions of SMT components have been idenified: • Irregular intermetallic growth at ambient temperatures • CTE mismatch during temperature cycling • Corrosion induced at elevated temperatures & high humidities • Open Questions: • Were does Sn from a whisker come from? • What is mechanism on micro scale? • Why SnPb helps mitigation of whisker growth? • What does board soldering/reflow do to whisker growth? • What can be used as accelerated test with a relation to field life?

33. Leadfree solder ball adhesion improvement in BGA-packages Philips Applied Technologies Update of May 2007 Jo Caers, Zhao Xiujuan, Jan Kloosterman

34. BGA Soldering

35. (CuNi)6Sn5 (CuNi)3Sn4 Ni Image of 0hr SAC-ball overview Root Cause Description of Missing Ball Issue • Present used Pb-free solder (SAC405) is very sensitive for impact loading because of: • Very high strength of the bulk solder • Formation of brittle intermetallic layer between solder ball and package substrate. • Formation of large Ag3Sn needles

36. Ni Ni Ni Ni SEM/BSE images of PSK sample (missing bump), showing almost no intermetallic residues Cross sections of products showing missing SAC405 solder balls on NiAu pad

37. Solution • Controlling growth and structure of the IMC layer • Reducing the strength, while improving the ductility of the Pb-free solder used ￫ modification of the SAC solder ball composition from SAC405 to SAC101 • Composition of SAC101 SAC405 • Ag : 1% 4% • Cu : 0.1% 0.5% • Ni : dopant no • X : dopant no • Sn : balance balance

38. Effects of Ag and Cu concentration Effect of Cu-content (Supplier Presentation) Effect of Ag-content (Supplier presentation)

39. Effect of Ag on liquidus temperature

40. High speed shear test • Test equipment : Instron micro tester • Test location : Instron Singapore • Instruction and test set up by IME-Instron-Apptech Singapore • Test speed : 0.45m/s • Shear height : 50m • Ball diameter : 600m High speed ball shear tester

41. High speed shear test-Failure modes • Three typical failure modes were found: • Fracture in IMC • Fracture in Bulk solder • Pad peel Fracture in Bulk solder Fracture of Pad peel Fracture in IMC

42. High Speed Shear testing

43. High speed shear testFailure modes on Supplier A finish • Bulk failure happens in SnPb and in SAC101 solder joints • The SAC0535 (SAC-X) alloy, SAC305 and SAC405 mainly fail in the intermetallic compounds • In general more intermetallic failure than for Supplier B finish

44. High speed shear test – Failure modes on Supplier B finish • Bulk failure only happen in SnPb solder joints • SAC101 only failed with Pad peel • Assume the IMC strength > the strength in Pad interface if failure in pad peel, SAC101 is better than other solder alloys to resist the high speed impact.

45. BLR tests: JEDEC Drop testing • Test board : standard JEDEC test board according to JESD22-B111

46. BLR tests: JEDEC Drop testing • Test packages:

47. BLR tests: JEDEC Drop testing Method • Drop test set-up To event detector (AnaTech: Model 128-256 STD)

48. BLR tests: JEDEC Drop testing Method • Test conditions • Acceleration: 1500g, 0.5ms half-sine pulse • Test duration: 30 drops • Failure criteria: daisy chain resistance larger than 500 and the first event of intermittent discontinuity followed by 3 additional such events during 5 subsequent drops • Sample size: 6 boards per type of package Acceleration measured from base plate

49. BLR tests: JEDEC Drop testing results.SAC 101 versus SAC405 Supplier B-NiAu SAC101 Supplier A-NiAu SAC101 Supplier A-OSP SAC101 Supplier A-NiAu SAC405 • SAC 101 clearly shows improved drop test performance compared to SAC405. • Failure distribution of supplier B is more narrow than that of supplier A • Following table shows the characteristic lifetime at 63.2% failure with 95% 2-sided confidence level, for SAC 101.

50. ReliaSoft's Weibull++ 6.0 - www.Weibull.com Probability 99. Weibull NiAu 90. W2 RRX - SRM MED F=9 / S=36 50. CB[FM]@95.00% 2-Sided-B [T1] 10. Cumulative failures (%) 5. Philips Philips CFT 1. 2005-11-02 13:17 1. 10. 100. N-drops b=2.9551, h=51.6013, r=0.9898 BLR tests: JEDEC Drop testing results.PbSnversus SAC 101 PbSn SAC 101 • Drop test performance of SAC 101 on NiAu is comparable to that of PbSn on NiAu!