Tin Whiskers: Cause and Effect

Transcription

1 Tin Whiskers: Cause and Effect A Workshop sponsored by National Electronics Manufacturing Initiative National Institute of Standards and Technology The Minerals, Metals & Materials Society Organizers: Ron Gedney, NEMI Bill Boettinger & Carol Handwerker, NIST NIST Research Team C. E. Johnson, G. R. Stafford, M.E. Williams, K.-W. Moon

2 Sn Whiskers Grow from their Base, not the Tip N. Furuta & K. Hamamura, Jap. J.Appl.Phys. 8(1969) 1404 On inside of a drilled and polished hole in Sn-Al cast alloy

3 Variety of Whisker Types Observed Bright Sn with 1.5 wt% Cu

4 Non-filamentary Eruption on Sn-1.5 wt % Cu electrodeposit

5 Sn Whisker Growth is an: Important scientific problem challenges materials science community Source of stress during crystal growth from aqueous solutions and from vapor phase Interaction of stress, solid diffusion and solid phase precipitation/coarsening Anamolous (fast) diffusion Stress relaxation (creep), recrystallization Important technology problem challenges manufacturing community Why has Pb been an effective whisker retardant for 50 years? Bath chemistry impurities/additives/complexing agents Plating conditions Diffusion barriers Post plating processing (reflow/anneal)

6 Measurement of Stress Evolution in Bimetallic Cu-Sn PVD Thin Films: L. Kabakian, E. Chason and K.S. Kumar Division of Engineering, Brown University, Providence, RI Real-time MOSS (Multi-beam Optical Stress Sensor) measurement for 4 day initial period

7 This workshop is intended to: Bring together key researchers on tin whiskers Compare work that has been done See if a consensus can be developed on a basic model or Define the work that will help us reach such a consensus The end goal is to provide direction to the industry that will assure long-life cycle applications they will not be subject to tin whisker failures.

8 The Morning Session 12:10 Lunch (box lunches will be available for purchase) 9:00 Introduction to the Workshop: W. J. Boettinger and C. A. Handwerker, Metallurgy Division, NIST (Gaithersburg, MD), R. W. Gedney, NEMI (Herndon, VA) 9:20 Stress in Electroplated Sn: Its Measurement and Implication in Spontaneous Whisker Growth: Chen Xu, PW Materials and Chemistry Group, Cookson Electronics (Jersey City, NJ) 10:00 Spontaneous Growth Of Tin Whiskers From Tin Electrodeposits on Phosphor Bronze Sheet: Dong Nyung Lee, School of Materials Science and Engineering, Seoul National University (Seoul, Korea) 10:40 Break 10:50 Tin-Whisker Microstructural Analysis using FIBs / Recrystallization Hypothesis: George T. Galyon, IBM Server Group (Poughkeepsie, NY) 11:30 Focused Ion Beam, Transmission Electron Microscopy, and Synchrotron Radiation Study of Sn Whiskers on Leadframe with Pb-free Surface Finish: K. N. Tu, Department of Materials Science and Engineering, UCLA.

9 The Afternoon Session 1:30 Panel Discussion: Questions posed by moderator and the audience. Moderator: W. J. Boettinger, NIST Panel members: K. N. Tu, UCLA D. N. Lee, Seoul National University G. Galyon, IBM Chen Xu, Cookson Electronics B. Radhakrishnan, Oak Ridge Nat. Lab.

10 The Afternoon Session Topics ~15 minutes each How do the following affect the WGT (Whisker growth Tendency)? Intrinsic plating stress Macroscopic stress relaxation /Recrystallization / Creep Alloying additions to Sn electrodeposit: e.g. Pb, Cu,... Electrodeposit grain size & shape, texture, porosity/inclusions and precipitates Fast Diffusion of Cu, Ni in Sn Stress Molar volume (density) of intermetallics compared to Sn & Cu Stress Dislocation /Vacancy / Interstitial interaction Thermal Cycling Oxide Summarize

11 Intrinsic Plating Stress: Rumpled Bright Sn Plating on Amorphous Carbon Half of Glassy Carbon Disk Half of Plating Deposits > 1 µm thick debond from substrate during plating. Buckling indicates large compressive stress. No Cu 6 Sn 5 intermetallic.

12 Stress vs. time curves for 16.5 µm thick Sn & Sn-Cu Bright deposits on Phosphor Bronze 0-10 Pure Sn (High Purity H 2 O) 219,16.5 µm 221,16.5 µm No whiskers! Stress (MPa) -20 Sn-1.5 Sn-Cu wt%cu 234,16.5 µm 233,16.5 µm Grew whiskers! -30 1E+2 1E+3 1E+4 1E+5 1E+6 1E+7 Time (s)

13 No Whisker/Eruptions Observed in Bright Sn using 18 MΩ cm water Whisker & Eruption Density (mm -2 ) Addition of Cu produced whiskers wt% Cu in deposit (ICP) wppm of Cu +2 in electrolyte 1600 Max. Whisker Length in 1 y. (µm) Moon et al, (2001) NIST wt% Cu in deposit Commercial Sn methanesulfonate bath + Cu 60 ma/cm 2

14 Macroscopic Stress Relaxation by Power Law Creep 1E-8 #219 Pure Sn #245 Sn-Cu low Pb n=5.0 #234 Sn-Cu #248 Sn-Cu high Pb 1E-9 Stain Rate (s -1 ) 1E-10 1E-11 1E-12 1E-13 n=12.3 n=9.5 Knoop YS Pure Sn n= Stress (MPa) McCabe & Fine (Pure Sn): 10-7 at 3 MPa 10-2 at 12 MPa n=6; σ > 8Mpa n=8.6; σ < 8 Mpa Knoop YS Pure Sn alloys

15 Bright Sn & Sn-Cu Deposits on Plated Cu Grain size & shape, texture, IMC shape, location Pure Sn: No Intermetallic on Sn Grain boundaries (684 days) Sn-1.42 wt% Cu: Intermetallic on Sn grain boundaries (219 days)

16 Precipitation of Fine Cu 6 Sn 5 within the Sn grains for Sn-Cu Plating Coarser particles on grain boundaries Fine particles within grains Plating alloyed Sn produces supersaturated solid solution Ppt. of fine particles within the grain; Sn- 1.5 wt% Cu alloy would have 4% Cu 6 Sn 5 independent of Cu 6 Sn 5 formed at substrate. 1 day old Sn-1.5 wt%cu plating on glassy carbon (953 ppm Cu +2 in Electrolyte) TEM by L. Bendersky, NIST Dark field

17 D Nic = 10-5 at 23 C Anomalous (Fast) Diffusion in Sn D Ni a Warburton & Turnbull (1975) Interstitial/substitutional interaction Sn-Ni Yeh and Huntington, Phys. Rev. Lett. (1984)

18 D. W. Stevens & G. W. Powell, Met. Trans 8A(1977) 1531 Stress Generation due to Unequal Diffusion Rates Stress if constrained to remain flat Like Cu Sn Unconstrained Cu Sn

19 Sn-Cu Molar Volume vs. Composition Normal reaction Sn + Cu Cu 6 Sn 5 decrease in molar volume V M (cm 3 /mole of atoms) Cu 6 Sn 5 Cu3 Sn γ β α Sn at % Cu Cu If D Cu >> D Sn ; i.e., Sn atoms immobile and intermetallic forms by addition of Cu to Sn already present, molar volume increases? Change in V M due to ppt. of Cu 6 Sn 5 from supersaturated (Sn,Cu) solid solution formed by plating

20 This workshop is intended to: Bring together key researchers on tin whiskers Compare work that has been done See if a consensus can be developed on a basic model or Define the work that will help us reach such a consensus The end goal is to provide direction to the industry that will assure long-life cycle applications they will not be subject to tin whisker failures.