1. Jin Liang，G. Morand, N. Dariavach
In-Situ Observation of Nucleation and Growth of Whiskers on Sn-Pb HASL Coating Layer Inside Plated Through Holes with Press-Fit Pins
Jin Liang，G. Morand, N. Dariavach
EMC Corporation
Hopkinton, MA 01748
Dongkai Shangguan
Flextronics
San Jose, CA 9513

2. STUDY OUTLINE Motivations Experimentals
Whisker formation and growth at ambient temperature.
Whisker formation and growth at elevated temperature.
Compressive Stress Estimation and Finite Element Modeling Press-fit-in Pin Systems.

3. Examples of Tin-Whiskering for Potential Long-term Reliability Concern

4. Possible Causes for Tin Whisking
National Electronics Manufacturing Initiative (NEMI), the National Institute of Standards and Technology (NIST) and The Minerals, Metals & Materials Society (TMS) agree that whisker growth is function of:
Grain size
Residual stress
External stress
Organic inclusion – carbon, sulfur, oxygen, …
Inorganic inclusion
Hydrogen, water incorporation
Thermal stresses
Substrate material
Electric and magnetic fields
Nucleation: Shear of surface oxide due to elastic anisotropy of the grain
Electrolyte
Plating conditions: current density, pulse plating
Temperature
Atmosphere
Time

5. MOTIVATIONS
Whether an externally applied stress field will induce whisker formation in Sn-Pb eutectic coating?
At what compressive stress level do the tin whiskers nucleate and grow?
Does elevated temperature increase or decrease whisker initiation and growth.

6. Test Couples and Experimental
HASL Sn-Pb eutectic layer of 0.2 mils to 0.4 mil with a thin Ni over-plating on 1.0 mil plated copper, inside PCB PTH with nominal diameter of inch
The pins have a thickness of inch, were mechanically pressed into plated through holes (PTHs).
The whisker test coupons were carefully cut from PCBs and mounted in a slow cure mounting media for cross-sectioning.
In-situ SEM Observation.

7. Real-time sequence of whisker formation and growth at ambient temperature in the compressed Sn-Pb layer

8. The detailed nodular structure formation and growth

9. Detailed Structure at Root of Whiskers after 1008 Hours
Tin whiskers and “extruded” nodular structures after 1008 hrs。
High magnification picture for Area in the write box left. detailed structure at the root of that whisker.

10. Detailed Structure at Root of Whiskers after 26.5 Hours
Detailed structural features at the root of the nodules
High mag. photo showing whisker initiation site and surrounding area. (26.5hrs)

11. EDAX compositional analysis for whiskers and "extruded" nodular structures
Whisker is pure Tin
Whisker root could be Tin-Pb alloy

12. EDAX compositional analysis for whiskers and "extruded" nodular structures
Extruded Nodular structure could be pure Tin
Extruded Nodular structure could be pure Pb

13. In-Situ SEM whisker formation and growth observation at 70 C

14. In-Situ SEM whisker formation and growth observation at 70 C

15. In-Situ SEM whisker formation and growth observation at 70 C

16. Press-fit-pin mechanical deformation and Compressive Force
Complaint pin's load-displacement curve
Compliant pins before and after press-fitting

17. Estimated Compressive Stress
The measured contact area length is about with thickness of the pins about a inch
The average stress level is calculated to be around 14 ksi inside the Sn-Pb coating layers.
Detailed Elasto-plasticity FEM modeling of the whole press-fit-pin system is in progress.

18. Conclusions
Highly localized compressive stress on thin HASL Sn-Pb eutectic layer inside PCB PTH wall with press-fit complaint pins promotes rapid formation and growth of nodular structure out of Sn-Pb solder layers. Long Tin whiskers can form from these nodules.
The nodules can be pure tin, Pb, or Sn-Pb alloy, their growth is dependent on the location stress conditions and on temperature.
Elevated temperature promotes the nodule formation and growth, but few long whiskers were observed.
At ambient temperature, under right conditions, whiskers with length of more than 10m are seen in less than 30 minutes.
The average compressive stress level inside the Sn-Pb HASL layer is around 14 ksi. High stress level may promote nodule formation & growth, but not necessarily all favor tin whisker formation and growth.