Case Study: TCT Model Verification for NAND BGAs DfR Conference... · 2019-03-29 · NAND BGAs −12 BGAs monitored per PCBA −2 PCBs, 24 NAND per leg • Resistance Monitoring −

9000 Virginia Manor Rd Ste 290, Beltsville MD 20705 | 301-474-0607 | www.dfrsolutions.com

Case Study: TCT Model Verification for NAND BGAs

March 25 I Ken Symonds


Copyright of DfR Solutions 2019

Abstract

Sample to sample thermal variation during temperature cycle testing can varysignificantly depending on the uniformity of airflow within a chamber.

The Blattau model for solder fatigue indicates that a variation of +/-5C in hot soaktemperature would lead to more than 25% change in solder fatigue stress across thepopulation.

A designed experiment has been performed to evaluate the agreement of the modeledprediction specifically for NAND BGAs used in Solid State Drives.

2



Agenda

12. Thermo-Mechanical Model

13. Experimental vs FEA Comparison

14. Incremental Solder Fatigue Variation

15. Single-Sided vs Double-Sided BGAs

16. Solder Ball Cross Sections

17. Wrap-Up

18. Future Opportunities

19. Acknowledgements

3

1. Overview

2. Thermal Cycle Stress – High Level

3. Hypothetical Scenario

4. Chamber Thermal Variation

5. Modeled Solder Fatigue Variation

6. Designed Experiment

7. Test Hardware

8. NAND BGAs

9. Experimental Results

10. Life-Stress ALTA Analysis

11. Concerns Realized



Overview

Problem Statement• Sample to sample thermal variation during temperature cycle testing (TCT) can be significant due to non-

uniform airflow within chambers • The Blattau model for solder fatigue predicts >25% variation in solder fatigue stress for variation of +/-5C• Question: Is this model accurate for NAND BGAs ?

Designed Experiment• Goal: Determine the change in solder fatigue stress to NAND BGAs due to thermal variation• A controlled thermal cycle test is performed with 3 hot soak temperatures (85C, 105C and 125C )• Test Vehicle: Daisy-chained PCBA and NAND BGAs with resistance-based monitoring

Analysis• Cycles to Failure (CTF) data collected and plotted via Weibull analysis to determine characteristic life • Life-Stress analysis performed in Alta to determine activation energy and incremental thermal acceleration • Thermo-Mechanical Finite Element Analysis (FEA) performed in Sherlock to compare to experimental data

Results• Closed form solder fatigue predictions from Sherlock agree very closely with experimental results • Thermo-Mechanical FEA results from Sherlock agree closely at 125 C, but diverge as temperature

decreases

4



Thermal Cycle Stress - High Level

High Level View

• TCT

− Thermal Cycle Testing is the primary method used to evaluate and qualify the reliability of solder joints between critical Ball Grid Array (BGA) components and PCBAs in Solid State Drives (SSDs).

• CTE Mismatch

− A mismatch in the Coefficient of Thermal Expansion (CTE) causes the PCB to expand more than the component at high temperatures which stresses the solder joints.

• Solder Fatigue

− Over repeated thermal cycles, solder fatigue occurs whereby cracks initiate and propagate through solder balls and can result in electrical failure of the SSD.

5

Note: Image is for a single-sided PCBA. Mirrored BGAs would have less vertical deflection due to opposing stress



Hypothetical Scenario

Chamber Loading

• TCT Qualification plans may have 30 or more samples to begin a test

− Many samples are needed since some are removed at intervals for destructive physical analysis (Dye/Pry, Cross Section)

− Drives may be loaded to accommodate available space on racks

• One possible loading configuration is shown at right

− Is this a good idea?

6



Chamber Thermal Variation

Actual Test Data

• Non-uniform air flow affects drive to drive heat transfer during thermal cycle

• Distribution seen in min/max case temps and ramp rates by chamber location− ∆15C in this case!

• Varies chamber to chamber

7

∆15C

Fast Ramp

Slow Ramp

Good

Airflow

Poor

Airflow

Air Outlet

Air Inlet



Modeled Solder Fatigue Variation

The Blattau Model

• What is the impact of chamber variation on solder fatigue?

• Based on the Blattau model:

− Drives which experience just 5C more than target receive ~19% more stress

− Drives which experience just 5C less than target receive ~16% less stress

• A chamber tolerance of +/- 5C means that drives in a 1,000 cycle qualification could range from 840 to 1,190 cycles

− 35% variation!

8

This change is much too high…

The model must be wrong!

35%



Designed Experiment

Thermal Profiles

• 3 DOE Legs

− High soak temperatures: 85,105,125C

− Low soak temperature: 0C

• Resistance-based test

− Initial resistances: 5 to 8Ω

− Failure criteria:10Ω

− Test until all NAND have failed

• Plot Weibull by Cycles to Failure (CTF)

− Determine characteristic life for each profile

9

Three Thermal Profiles

Ramp Rate Soak Durations

10C/Min 10 Min



Test Hardware

• Daisy-Chained NAND / PCBA− Design based on an actual SSD product; however,

modified to incorporate direct resistance-based measurement across all solder balls in discrete NAND BGAs

− 12 BGAs monitored per PCBA

− 2 PCBs, 24 NAND per leg

• Resistance Monitoring − PCBAs connected with wire specifically used for resistance

monitoring

• Chamber Configuration− Test boards mounted with additional fans to provide

uniform airflow

• Data Acquisition− Data acquisition unit records both resistance and BGA

case temperature in 10s increments

10

Data Acquisition Pod and Controller

Test Module, Chamber Arrangement



NAND BGAs

Experimental vs Modeled

• Experimental vs modeled configurations for NAND BGAs

Experimental NAND Layout

w/ Daisy Chained Solder Balls Modeled NAND Layout

11



Experimental Results

Weibull Analysis

• Experimental cycles to failure are evaluated per component when the resistance measurement first exceeds 10Ω

• 2P Weibull plots are generated for each profile

− Beautiful data!

• Characteristic life (Eta)

− Relative acceleration between conditions is determined from the ratios of Eta

› 125C to 105C: 1.9x

› 105C to 85C: 1.9x

• This is why I chose 105C rather than 100C for the mid-point even though 0 to 100C is a primary standard

12



Experimental Data Analysis

ALTA Analysis

• A Life – Stress analysis was performed in ALTA using data from hot soak conditions of 85, 105 and 125C

− From this, the thermal activation energy (Ea) and acceleration (AF) between conditions is determined

• This information can be used to determine a data in 5C increments using the Arrhenius relationship:

13

𝐴𝐹 = 𝑒𝐸𝑎𝐾

1𝑇𝑢𝑠𝑒−

1𝑇𝑆𝑡𝑟𝑒𝑠𝑠

Where:

Ea = Thermal activation energy

K = Boltzman’s constant



Concerns Realized

Solder Fatigue Variation Verified

• The incremental extrapolation from Alta allows a direct comparison of experimental to modeled data

• The agreement is curiously excellent! − The reason is that both approaches heavily

leverage the same Arrhenius equation!

• This data confirms the initial concern: − +/- 5C thermal variation leads to ~ 35%

variation in solder fatigue!

− Test environments need to be carefully characterized and designed

• WDC has developed proprietary hardware capable of controlling thermal variation to < 2.5C!

14



Thermo-Mechanical Model

15

Mesh Strain Stress

Finite Element Analysis

• Thermo-mech module in Sherlock enables fatigue life prediction for BGA components due to system level effects such as double-sided BGA configuration.

• Incremental static steps are solved to determine thermally induces stress and strain in each solder joint in every package.

• Fatigue prediction is provided by identifying the critical solder joint for each component and calculating the energy density at highest temperature increment.



Experimental vs. FEA Comparison

• Experimental and predicted failure rates are matched well for the 0 to 125°C profile and deviate for the lower ΔT profiles

• Assumptions such as the Weibull beta, BGA mold, substrate and PCB properties could all contribute to deviation at the more benign stress states

• FEA predictions provide good rough prediction. This allows for model tuning to better match the NAND BGA for variety of thermal profiles

16



Incremental Solder Fatigue Variation

FEA vs Experimental

• Incremental experimental vs FEA modeled solder fatigue is determined and compared

• Since the Weibull overlay shows 125C is nearly equal and diverges at lower temperatures, the deltas are referenced to an assumed 125C equivalency

• At 100C, +/- 5C thermal chamber variation projects to fatigue variation of 12% for FEA vs 15% experimental

17

+/-5C

Note: These percentages around 100C differ from slide



Single-sided vs. Double-sided BGAs

• Predictions can be extended for single-sided BGA configuration using the same Thermo-mechanical model approach.

• Double-sided component stiffen the PCB resulting in larger thermally induced shear strain in solder joints compared to single sided components.

• Strain distribution at 125°C shows higher strain in both solder joints and PCB for the double-sided configuration as expected.

18

Single-sided

Double-sided

Equivalent strain distribution at 125°C



Single-sided vs. Double-sided

Experimental Results

• All BGAs were depopulated from 1 side of a set of boards prior to running them through the 125C thermal profile

• The data sets matched fairly well in slope and demonstrated an acceleration of 3.2X for the double-sided configuration

19



Single-sided vs. Double-sided

FEA Results

• Thermo-Mechanical FEA modeling of single- vs double-sided configurations showed an acceleration of ~ 1.8X for the double-sided case

20



Solder Ball Cross Sections

• Cross-section performed on the perimeter of the NAND BGA package that include two of the four corner anchor balls.

• The red line indicates sectioning plane on the package

21

From 125C sample

• Two of the corner joints are found with cracks along the diagonal of the solder joint.

• Other two corner joints are found to have cracks along both the PCB and component side through the bulk of the solder.

• Variation in solder joint geometry and microstructure contribute to variation in observed crack path formation.

Top side BGA

Bottom side BGA

PCB

Package

Package



Solder Ball Cross Sections

• Solder joints under the die (critical to function) demonstrate cracking along the package/solder interface.

• Cross-polarized microscope image indicates three grains in the solder joint.

• Recrystallized region is observed at both the package and board interface.

• Crack location at the die shadow indicates higher strain localized region.

22



Wrap Up

• The initial concern of significant variation in solder fatigue stress applied during thermal cycle qualification has been confirmed and verified − Resistance-based data for 3 thermal profiles was analyzed using Weibull and Life-Stress analysis− BGA and PCBA design information was analyzed using both closed-form and thermo-mechanical

finite element analysis− These analytical techniques transformed data from 3 DOE legs 20C apart to incremental resolution

of 5C

• Thermal variation of +/- 5C in hot soak temperature WILL lead to a range of ~850 to 1,200 effective cycles for a 1,000 cycle qualification − This level of variation can render results inconclusive at best or, worse, misinterpreted

• Analysis of single- vs double-sided BGA mounting was also performed− Double-sided or mirrored mounting shows an acceleration of 3.2x (experimental) vs 1.8x (modeled)

over single-sided configurations

• The experimental data obtained in this work will be critical in refining existing FEA models so that more complex problems can be confidently addressed − Most importantly, the ability to extend this methodology to accurately predict reliability of

functional balls toward the inner region of the array will be highly valuable

23



Future Opportunities

The current presentation has referenced the first ball to fail in NAND BGAs which by design is one of anumber of non-functional so called ‘anchor balls’ used to absorb the brunt of the thermo-mechanical stress.In this manner, the effect of thermal variation on characteristic life for different profiles has beenexperimentally verified.

• First Functional Ball CTF

What is perhaps more important to the reliability community is to better quantify the expected cyclesto failure for the first functional ball. During the course of this work, data on the first functional ball tofail has also been gathered which could be analyzed to better characterize true operational life forNAND BGAs.

• Alternate Profiles / Mechanisms

Each of the data sets for mirrored NAND in this work show Weibull Betas of ~5; however, datacollected for a -40 to 85C profile shows a distinctly steeper slope, perhaps due to an alternate failuremechanism or increased acceleration. Additional work to understand this potential difference could beinteresting.

24



Acknowledgements

• Special thanks to Maxim for detailed support and guidance on FEA analyses and physics of solder fatigue failure.

• Appreciation to Craig whose memorable exclamation of “yay, physics wins again!” -when an FEA analysis correctly identified the cap of buried via design to be susceptible to cracking under excessive design-related stress - was an inspiration as to the fun of figuring out the underlying physical mechanisms for reliability failures.

• Nathan of course as the ‘founding father’ of the Blattau model for solder fatigue failure and for numerous impromptu consults as interesting issues arise.

• David for FEA support and Natalie for patience:^)

• Nisha and Bernhard for understanding.

25



Speaker Bio

Ken Symonds, Western Digital

Currently with the Advanced Reliability Engineering (ARE) team at

Western Digital, Ken has leveraged his academic foundation in physics

and material science in over 30 year’s of experience in reliability test

and analysis with emphasis on semiconductor and storage technologies.

Beginning with hands-on failure analysis and investigation of device

physics and wear out mechanisms in semiconductors, Ken has extended

this focus to reliability modeling of Solid State Drives.

Maxim Serebreni, Research Engineer at DfR Solutions

Maxim currently works as a research engineer with DfR

Solutions. He has a background in experimental mechanics,

material characterization and numerical modeling. His

current research involves integration of Pb-free solder

alloys in harsh use environments. He has consulted in the

fields of electronics reliability, electronic packaging design

and solder alloy metallurgy. He is currently completing his

PhD in Mechanical Engineering at the University of

Maryland, College Park under the supervision of Dr.

Patrick McCluskey.

26

Documents

Case Study: TCT Model Verification for NAND BGAs DfR Conference... · 2019-03-29 · NAND BGAs −12 BGAs monitored per PCBA −2 PCBs, 24 NAND per leg • Resistance Monitoring −