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
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.
Examples of Tin-Whiskering for Potential Long-term Reliability Concern
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
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.
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 0.045 inch The pins have a thickness of 0.016 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.
Real-time sequence of whisker formation and growth at ambient temperature in the compressed Sn-Pb layer
The detailed nodular structure formation and growth
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.
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)
EDAX compositional analysis for whiskers and "extruded" nodular structures Whisker is pure Tin Whisker root could be Tin-Pb alloy
EDAX compositional analysis for whiskers and "extruded" nodular structures Extruded Nodular structure could be pure Tin Extruded Nodular structure could be pure Pb
In-Situ SEM whisker formation and growth observation at 70 C
In-Situ SEM whisker formation and growth observation at 70 C
In-Situ SEM whisker formation and growth observation at 70 C
Press-fit-pin mechanical deformation and Compressive Force Complaint pin's load-displacement curve Compliant pins before and after press-fitting
Estimated Compressive Stress The measured contact area length is about 0.036 with thickness of the pins about a 0.016 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.
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 10m 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.