1. Task Group 8-81f Self-mitigation PERM #33 Update
David Pinsky
This document does not contain technology or technical data controlled under either the U.S. International Traffic in Arms Regulations or the U.S. Export Administration Regulations.

2. Background
Components with pure tin finished terminations are at risk for tin whisker growth and potential unreliability.
The addition of lead (Pb) is an effective means to mitigate tin whisker growth. Therefore, eutectic tin lead solder does not pose a meaningful risk of tin whisker growth provided the lead content is at least 3% by weight.
Those portions of pure tin terminations that are replaced by eutectic tin lead solder during SMT processing no longer poses a risk of whisker formation.
Therefore, components where eutectic tin lead solder has fully replaced all tin plating on the terminations are fully mitigated against tin whisker risks. Parts that can be mitigated in this fashion are said to be "self mitigating“.
11/20/201811/20/2018

3. Examples Solder covers the entire termination: self-mitigating
Solder does not cover the entire termination: not self-mitigating
11/20/2018

4. Previous Work
A study was performed in 2010 by Raytheon, the results of which have been published, concluding that:
Component termination geometry could be used to predict reliable self mitigation for components soldered to HASL finished boards, using a specific (fairly typical) SMT process
The study did not address how these results may or may not apply to components soldered to boards using different manufacturing processes, surface finishes, or pad geometries
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5. Aim of the Present Study
If the conditions under which self mitigation is achieved reliably can be determined, it will be possible to use certain tin terminated parts without introducing the risk of whisker shorts, and without employing any other mitigating techniques
Task Group 8-81F was formed under the PERM Council (Committee 8-81) to investigate
This study was performed to evaluate the conditions under which typical SMT components will achieve self mitigation
11/20/2018

6. Design of Experiment
Many potential factors for consideration were considered for inclusion in the study. The four factors shown in the table below were selected. The layout of the board and the components reused from the prior study, to permit direct comparison between the results of the two studies.
Experimental Factor
Settings
Component Packages
16 different part numbers (details below)
Board finish
OSP and Sn Pb HASL
Pad size
Per initial study and 25% smaller
Manufacturing Process
Seven different locations
11/20/2018

7. Design of Experiment
Each of the seven assembly locations were provided with a kit containing eight bare boards (Four types, two replicants) and all of the components necessary to populate them. Each assembler chose process conditions as they would deem appropriate to achieve compliance with J-STD-001, Class 3 requirements
HASL finish/large pads
HASL finish/small pads
OSP finish/large pads
OSP finish/small pads
11/20/2018

8. Assembled Test Vehicle
11/20/2018

9. Status
Phase I results presented at APEX 2016 and published in SMT Magazine in September
Almost finished with Phase II
Results presented at the IPC Reliability Forum: Manufacturing High-Performance Products, Chicago, IL April 26-27
Final report completion this fall
Presentation planned at APEX 2018

10. Phase I Conclusions Package geometry is a significant factor
Some package styles mitigate across all processes
Some do not mitigate for any process
Solder process is a significant factor
Pad size is significant, but not important
Board finish is not significant

11. Termination Length (mils) Termination Height (mils)
Components Used
Part Number
Package Style
Quantity per vehicle
Termination Length (mils)
Termination Height (mils)
06035C103KAT2A
0603 chip 10 31 13
0603YD225KAT2A
A3PN030-ZVQG100
TQFP100-14mm 1 40 16
ADM213EARSZ
SSOP28-5.3mm 2 59
EPM7032AETC44-10N
A-TQFP44-10mm-.8mm
IR2156SPBF
SO14G-3.8mm 41 23
LM2901DG
LTC3703EG_PBF
MBRM140T1G
DO-216AA 5
49/17 20
MC9S08GT16AMFBE
QFP44-.8mm 66 36
MC9S08QE4CLC
LQFP32-7mm-.8mm 42 25
OP482GSZ 46 19
PZT2222AT1G
SOT223
69/73
STAC9200X5TAEB1X
LQFP48-7mm-.5mm
W3L1YC474MAT1AF
0612 chip 12 39
XC9572XL-5TQG100C
LQFP100-14mm-.5mm

12. Assembly Process Details
Setting
Process A
Process B
Process C
Process D
Process E
Process F
Process G
Reflow type
Vapor Phase
Convection Oven
Convection
Oven
Flux
ROL0
ORM0 63/37
Sn62 RMA No-Clean
No clean
Tacky Flux
Stencil thickness
5 mils
4 mils
Laser 5 mil
Stencil aperture
1:1
Time above Liquidus
90s
60-75s
66s
45 sec
60s
70s
Peak temperature
218C
215C
220C
225C
220 ͦC
213C
Atmosphere
Air
Nitrogen
Number of reflow cycles 1 2
Rework
None
Yes

13. XRF Data Summary Probability of self-mitigation by package and process
80% Confidence Interval
Probability of self-mitigation by package and process

14. Phase II Results XRF Very old parts Cross-sections
Consistency across multiple leads of a single package
Corroboration with measurements using newer, more advanced instrument
Check on very “thin” readings
Very old parts
Cross-sections

15. Phase II XRF
Two packages indicated as self-mitigating were re-examined, with most leads on all four sides measured, not just two leads as in Phase I
All terminations verified as self-mitigated

16. Example of XRF results (Board 46, U11, Lead 1, Egress)
Raytheon
Example of XRF results (Board 46, U11, Lead 1, Egress)
Spectrum and Fit
Target at lead egress
Results
11/20/2018

17. Phase II XRF
Some measurements were repeated using a new, more powerful instrument (M4 Tornado)
All of board 46 from assembler “A”
Result: All assembler “A” components previously identified as self-mitigating by MXR300 were also identified as self-mitigating by the M4 Tornado
Specific terminations suspected of very thin coatings or with borderline Pb content
Result: Thin coating indications also provided by M4 Tornado, consistent measurements for borderline Pb readings

18. Phase II- old parts
One and a half boards were populated with parts that had been stored in an uncontrolled environment (Tom Hester’s office) for more than five years.
Results of XRF were compared between the old and new parts assembled by the same process at the same time on exact replicant boards

19. Phase II- old parts
Old parts assembled per Process G for packages with >.98 probability with a 90% confidence interval of full coverage for new parts
Package Style
Measured
Mitigated
0603 Chip 62 10
SO-14G 21 19
LQFP-48 4
TQFP-100 8
LQFP-100
Drastic loss of Mitigation
Still Mitigating?

20. Phase II- Sections
Results from SEM/EDS consistent with XRF data for devices with leads, some inconsistency for chip devices
Transitions between solder flow and original tin are abrupt
Undersides of gull-wing leads are often fully covered even when the top sides are not

21. 50X
750X
2000X
Abrupt transition between solder and tin plating

22. Example on fully mitigated lead
Element
Weight%
Atomic%
Sn L
91.47
94.93
Pb M
8.53
5.07
Totals
100.00
750X
2000X

23. Example on non-mitigated lead
Bd 45 U73 left side.
Bd 45 U73 top left spectra.
Bd 45 U73 top left side.
2/25/2017
CER Jim Andorf

24. U1 – Lead 2
50X
750X
2000X
Upper side of lead not fully mitigated

25. Under side of lead fully mitigated
U1 – Lead 2
50X
Element
Weight%
Atomic%
Cu L
1.54
2.89
Sn L
93.70
94.36
Pb M
4.76
2.75
Totals
100.00
750X
2000X
Under side of lead fully mitigated

26. 0603 XRF vs EDS Conflict EDS reading at this site reported 98.5 wt% Sn
Raytheon
0603 XRF vs EDS Conflict
Bd4 C9 right top spectra.
Bd4 C9.
EDS reading at this site
reported 98.5 wt% Sn
(Small amounts of Pb phase
Is seen in image)
XRF reading at this site
reported 66.3 wt% Sn
Bd4 C9 right top.
11/20/2018

27. Raytheon
XRF v EDS conflict
Investigation into cause of the conflict is in progress- outcome is TBD
There is high confidence that the instruments were properly measuring the composition of the regions that were excited by the beams
Possible causes under investigation
Mis-alignment of the XRF spot relative to the termination
Effect of Sn and Pb segregation in joint
11/20/201811/20/2018

28. XRF Data for 0603
Further detailed analysis is being performed on the XRF results for the 0603 components
Represent a “borderline” part
Could provide insight into effect of process variables
Considered:
Oven reflow v Vapor Phase reflow
Parts with longest storage before assembly v other oven-reflowed parts

29. XRF Data for 0603

30. Raytheon
Results from SEM/EDS
Abrupt solder/tin transition verifies that XRF readings are meaningful for mitigation
Bottom surfaces of leads exhibit a higher degree of mitigation than do the upper surfaces
For leaded devices- those indicated as certain to mitigate by XRF was seen to mitigate by SEM/EDS
For chip devices- cause(s) of some disagreements needs resolution
11/20/2018

31. Conclusions Results from Phase I validated
Consistency across leads of large devices
XRF data confirmed by cross-section for devices with leads
XRF results duplicated on alternate instrument
Results for chip devices under investigation
Uncontrolled long term aging of parts can compromise self-mitigating behavior

32. Plans What next?- Let’s discuss Repeat experiment with SAC305?
Consider other parameters of interest?