8.5 Tin Whisker Status Jack McCullen (Intel) File name Jack.t.mccullen@intel.com File name 8.5 J_McCullen.ppt

Accelerated Tin Whisker Test Committee Update Phase 5 Evaluation Special Thanks to Heidi L. Reynolds, Ph.D. Project Chair Accelerated Tin Whisker Test Committee Update Phase 5 Evaluation

Tin whisker test committee Committee Objective: To develop industry standard test methods for predicting tin whiskers. Committee Structure: Richard Parker (current Chair) – Delphi Electronics Heidi L. Reynolds (former Chair) – Sun Microsystems Jack McCullen (Co-Chair) – Intel Mark Kwoka (Co-Chair) – Intersil John Osenbach (Co-Chair) – LSI/Ager Committee is open to non-NEMI members willing to contribute to the work

Accelerated Tin Whisker Test Committee Objective To identify accelerated test methods and to develop industry standard test methods for predicting tin whiskers. R. Parker/iNEMI Phase 5/March 2007

Previous iNEMI Whisker Projects Phase 1 & 2 Evaluations – Data used to develop phase 3 evaluation. Phase 3 Evaluation - (2003- 2004) Validate and verify proposed test methods Compare short-term (1 month) vs. long-term (1 year) testing Results of Phase 3 Evaluation & other Industry Studies provided input for JEDEC standards. JEDEC standard JESD22A121/ Test Methods (May 2005) JEDEC standard JESD201/ Acceptance Criteria (March 2006) Phase 4 Evaluation - Effects of electrical bias on the susceptibility of tin finishes to form and grow whiskers on tin plated components assembled with both lead-free and tin/lead solder.

Phase 5 Evaluation Investigate the effects of temperature and humidity over a wide range of conditions. Hypothesis: Whisker presence and/or length, measured in isothermal environments, is a function of temperature and humidity. Whisker length could be discontinuous at a threshold point or could vary as a function of temperature and humidity over the entire tested range. If such a function exists and becomes known, this function can be used to determine: Optimal whisker test condition(s) Whether whisker behavior measured at accelerated testing conditions can be related to actual storage and/or customer service conditions.

Phase 5 Evaluation Test Matrix X shows JEDEC Std JESD201 Conditions Humidity [% RH] Temperature [C] 10 40 60 85 30 X   45 100

Phase 5 Evaluation (Approximate Total of hours in storage)   C-10R-c 13 C-10-N-c 12 C-10R-b 11 C-10-N-b 10 C-10R-a 9 C-10-N-a 8 C-3-N 7 B-10-R 6 B-10-N 5 B-3-N 4 A-10-R 3 A-10-N 2 A-3-N 1 100/60 60/40 30/10 45/60 30/60 85/85 30/90 60/10 60/60 60/93 Sample Cell Temperature/Relative Humidity ~6550 hours ~11,080 hours ~10,440 hours ~10,080 hours ~15,000 hours* ~10,128 hours ~9890 hours ~4000 hours ~8000 hours ~8000 hours *Inspection to ~5300 hours only due to limited resources.

Observations of Whiskers in Corroded vs. Non-corroded Regions Where possible, whiskers growing in or near (< 200mm) corroded regions were distinguished from whiskers growing in non-corroded regions. This distinction was more difficult in the Test Cells with higher whisker densities. Corrosion 200mm X

Phase 5 Evaluation – Data Measured Used to develop Acceleration Model Incubation Time whisker growth corrosion Maximum whisker lengths # of leads with some whiskers present # of leads some corrosion present Used for Comparison & Analysis Complicated by variations in storage time Data analysis is ongoing

Humidity [% RH] Temperature [C] 10 40 60 85 30 N C,W 45 100 Phase 5 Test Cells exhibiting Corrosion and/or Whisker Growth during the allowed storage time C = Corrosion observed W = Whisker growth observed N = No corrosion or whiskers   Humidity [% RH] Temperature [C] 10 40 60 85 30 N C,W 45 100

30C/60%RH Test Condition No whisker growth or corrosion observed up to 8000 hours in Phase 5 Evaluation Based on industry tin whisker test data submitted over the previous two years, data from 30C/60% JEDEC test condition for 4000 hours does not grow whiskers. Recommend to JEDEC to remove the 30C/60%RH storage test condition from JESD201

Acknowledgement – John Osenbach, LSI/Agere Acceleration Model Development from Phase 5 Whisker Growth/Corrosion Incubation Time Data Acknowledgement – John Osenbach, LSI/Agere

Data Analysis – for Acceleration Model The time to first observation of corrosion or whisker formation for each cell of devices, for each aging condition where corrosion or whisker growth took place (85C/85%RH, 45C/60%RH, 60C/87%RH, 60C/60%RH, and 30C/90%RH) in DOE 5 was fit to three different commonly used Temperature/Relative Humidity acceleration functions: Time = A*exp(Ea/kT)*exp(C%RH) Time = A*exp(Ea/kT)*exp(C/%RH) Time = A*exp(Ea/kT)*(%RH) –F The best fit, as determined both by R2 and by analysis of residual plots, was always equation 1.

Data Analysis (cont.) – For AF Models Corrosion Incubation Models: the data were grouped by; film thickness and if the devices were exposed to a simulated reflow Individual empirical models were developed for Film thickness/reflow: 3um thick, no reflow, data for all three suppliers (A,B,C) was fit 10um thick, no reflow, data for all three suppliers (A,B,C) was fit 10um thick, reflowed, data for all three suppliers (A,B,C) was fit

Data Analysis (cont.) – For AF Models Whisker Growth Incubation Models: the data were grouped by; film thickness, if the devices were exposed to reflow, and if whiskers were outside of a 200um radius of a corroded region or within a 200um radius of a corroded region or in the corroded region itself Individual empirical models were developed for Film thickness/reflow/corrosion: 3um thick/no reflow/no corrosion, data for all three suppliers (A,B,C) was fit 10um thick/no reflow/no corrosion, data for all three suppliers (A,B,C) was fit 10um thick/reflow/no corrosion, data for all three suppliers (A,B,C) was fit 3um thick/no reflow/corrosion, data for all three suppliers (A,B,C) was fit 10um thick/no reflow/corrosion, data for all three suppliers (A,B,C) was fit 10um thick/reflow/corrosion. data for all three suppliers (A,B,C) was fit

Summary of fitting parameters for acceleration functions Incubation time for corrosion = A*exp(Ea/KT)*exp(C%RH) Film Type A (hrs) Ea (eV) C (%RH) R2 3mm-N 0.0587 0.38 -0.0294 0.91 10mm-N 0.117 (2xA3um) 0.74 10mm-R 1.31 0.28 -0.015 0.79 Incubation time for whiskers = A*exp(Ea/KT)*exp(C%RH) Film Type Corrosion A (hrs) Ea (eV) C (%RH) R2 3mm-N N 1.15 0.31 -0.031 0.94 10mm-N 1.16 0.28 -0.017 0.72 10mm-R 0.0014 0.41 -0.012 0.69 Y 5.16 0.23 -0.018 0.86 1.97 0.3 0.9 Whisker prone films: ref: J. Osenbach et. al, J. Mater. Sci.: Mater. Electron, pp 283-305 (2007) A = 0.007hrs; Ea = 0.44eV; C = -0.044 (%RH)

Corrosion Incubation Time (Key) Model: Corrosion/Whisker Incubation time = A*exp(Ea/KT)*exp(C*%RH); A , Ea, and C are empirically determined fitting functions Empirical Model Experimentally Determined Time to Corrosion Spread in experimental data “time distribution” at that test condition Regression line: Provides information on “goodness of fit of model to data Ideally line would have: a slope of 1 and intercept of 0 Residual: Ideally a plot of the residual versus time would produce a random data plot, e.g. no trend x position/y position: Measured/calculated # of cells that have corrosion, @Temp/RH condition

Temperature Effect- non-corroded regions

Relative Humidity Effect-non corroded regions Approximately 4X longer incubation time than 60C/87%RH

Saturation or no Saturation in Whisker Growth Rate (Based data from all test conditions) Some of the films have growth rates that appear to be saturating (sub-linear whisker length versus time) followed increasing growth rate (super-linear whisker length versus time) Some films have growth rates that appear to be saturating (sub-linear whisker length versus time) Some films have whisker growth rates that are linear in time No clear evidence for or against ultimate saturation of whisker growth rate over time at the conditions where whisker were observed

Ongoing Analysis Plating thickness Reflow vs. Non-reflowed Supplier Differences Lot variations for Supplier C AF for Corrosion Incubation time.

Modeling of maximum whisker lengths? Issue is still under technical debate With respect to Phase 5 Evaluation, the ability to model maximum whisker lengths was limited by the length of the experiment itself Some test cells had only just begun to growth whiskers Some researchers believe that because, statistically the maximum whisker length is the “outlier” of the entire whisker population, it cannot be modeled as a function of temperature and humidity.

Conclusions Whisker presence and the initiation of corrosion can be represented by a function of temperature and humidity. Two temperature/humidity conditions are not necessary. 60C/87%RH appears to be the optimal high temperature/high humidity test condition at this time for Sn over Cu substrates The iNEMI tests can be used to indicate behaviour at other temperature/humidity points that could be relevant storage or service conditions within the limits of the whisker and corrosion (incubation) acceleration functions developed in this study. Whisker formation differs in corroded and non-corroded regions, but it appears that the incubation times for both regions can be modeled. Data are still under analysis and review regarding the effects of reflow and plating thickness’