The effect of low Au concentrations on the properties of eutectic Sn/Pb
This study was of the effects moderately low Au concentrations ({le} 10 wt%) have on the mechanical properties and microstructure of an eutectic Sn/Pb alloy. Vibration (60--90 Hz swept sine wave for 30 hours) and thermal cycling (0--110C for 1450 cycles) reliability tests were performed on fine pitch leaded chip carriers using eutectic Sn/Pb solder on PCBs (printed circuit boards) with 0, 5, 10, 20, and 50{mu}in nominal Au thicknesses. Testing was also performed on double shear creep specimens consisting of arrays of regular pitch joints. There was a dramatic increase in the number of joints containing voids with increasing Au concentration, an effect more pronounced in the creep joints than in the reliability joints. These voids tended to coalesce and grow during rework simulation of the reliability joints. AuSn{sub 4} intermetallics present in toe of 4.8 wt% (50 {mu}in) Au vibration joints rotated from initial vertical perpendicular to surface of PCB metallization, solidification positions to roughly horizontal (parallel to plating surface) orientations during rework simulation and during aging of the parts. The AuSn{sub 4} intermetallics in the toe of the 4.8 wt% (50{mu}in) Au reflowed joints also rotated after vibration testing. No joint failures were observed in either vibration tested or thermally cycled specimens. Cracks formed in some of the vibration tested specimen joints under the heel of the gull-wing lead at Pb-rich phases. Thermally cycled specimens showed eutectic microstructure and intermetallic coarsening without crack formation. Creep tests showed loss of the superplasticity in eutectic Sn/Pb alloys with even the lowest Au concentration tested of 0.2 wt% Au. Intermetallic rotation was not a factor in crack propagation, but void presence was. Cracks tended to form in joints containing voids before forming in void-free joints. Crack propagation followed Sn/Sn grain boundaries and Sn/Pb phase boundaries from Pb-rich phase to Pb-rich phase.
- Research Organization:
- Lawrence Berkeley National Lab. (LBNL), Berkeley, CA (United States)
- Sponsoring Organization:
- USDOE; USDOE, Washington, DC (United States)
- DOE Contract Number:
- AC03-76SF00098
- OSTI ID:
- 7253786
- Report Number(s):
- LBL-32390; ON: DE92041206
- Resource Relation:
- Other Information: Thesis (M.S.)
- Country of Publication:
- United States
- Language:
- English
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Related Subjects
42 ENGINEERING
GOLD ALLOYS
LEAD ALLOYS
METALLURGICAL FLUX
PRINTED CIRCUITS
SOLDERED JOINTS
SOLDERING
TIN ALLOYS
CRACK PROPAGATION
CREEP
EUTECTICS
INTERMETALLIC COMPOUNDS
MECHANICAL VIBRATIONS
MICROSTRUCTURE
PLATING
THERMAL CYCLING
THICKNESS
ALLOYS
CRYSTAL STRUCTURE
DEPOSITION
DIMENSIONS
ELECTRONIC CIRCUITS
FABRICATION
JOINING
JOINTS
MECHANICAL PROPERTIES
SURFACE COATING
WELDING
360100* - Metals & Alloys
426000 - Engineering- Components
Electron Devices & Circuits- (1990-)