IEEE Joint CPMT/CAS & SMTA OR Chapter
Seminar
6.00–7.30pm Thursday, April 9th,
PCC Willow Creek, Room 313
Effect of Processing Parameters on the
Microstructural and Reliability of Pb-free Solder
Alloys: Effect of Solder Composition and Sn
Grain Morphology.
Eric Cotts
Physics Department and Materials Science Program,
Binghamton University, Binghamton, NY
Abstract
An improved understanding of how processing affects SnAgCu (SAC)
microstructure provides insight into how to enhance the fatigue performance of Pb
free solder joints. The number and orientation of the β-Sn grains in these solder
joints heavily influence their lifetime. The size, number, and spacing of Ag3Sn
precipitates in a SAC solder are also important factors in determining the
mechanical strength and reliability of SAC solder alloys. Thus the effects of
processing, and material parameters such as solder volume and composition,
metallization chemistry and thermal history on solder joint microstructures were
examined.
Strong correlations between microstructure and both Ag content and
solidification temperature were observed, while thermal history was also shown to
be significant. Both Ag content and cooling rate had a direct effect on Ag 3Sn
precipitate density, while time above the liquidus was also important. The Sn grain
morphology was directly correlated with solidification temperature, providing
insight in to relations between metallization chemistry, solder composition and
volume on Sn grain morphology.
In a companion study led by Universal Instruments Corporation, custom
made ball grid array (BGA) packages were tested in accelerated thermal cycling
(ATC). Various solder alloys doped with Ni (SN100C) and Mn (SAC105-Mn and
SACM), as well as SAC105, SAC 205 SAC 305 and SAC 405 and eutectic SnPb
alloys were reflowed on printed circuit board (PCB) surface finishes, either copper
organic solderability preservative (Cu-OSP) or electroless nickel immersion gold
(ENIG). Precipitate sizes and distributions were analyzed using backscattered
scanning electron microscopy (SEM). Sn grain morphology was characterized by
polarized light microscopy (PLM) and electron backscatter diffraction (EBSD).
The results of the ATC studies were correlated with solder joint
microstructure. Distinct dependences of lifetime on solder composition were
observed, with increases in lifetimes with increases in Ag content. The effect of
variation in solder composition and volume and PCB surface finish on solder joint
microstructure and lifetime was also evaluated. Cu/ENIG surface finishes
generally resulted in better thermal fatigue performances. Results also showed that
solder volume can greatly affect the microstructure and performance of SAC solder
joints in mechanical and ATC tests.
Biography
Eric Cotts is a professor in the Materials
Science program, and the Physics
department, at Binghamton University. He
received his BS degree from Cornell
University in 1978 and his PhD degree from
the University of Illinois at UrbanaChampaign in 1983. His research group
studies phase transformations in metal
systems, with a focus on applications for
microelectronics devices. For instance,
studies of the dependence of nucleation rates
of undercooled Sn on impurity content
provided recommendations for the
processing of interconnects.
All welcome