2 research outputs found
THERMAL CYCLING RELIABILITY OF LEAD-FREE SOLDERS (SAC305 AND SN3.5AG) FOR HIGH TEMPERATURE APPLICATIONS
Eutectic tin lead was the most widely used solder interconnect in the electronics industry before the adoption of lead-free legislation. But eutectic tin lead solder has a low melting point (183oC) and was not suited for some high temperature applications, such as oil and gas exploration, automotive, and defense. Hence, for these applications, the electronics industry had to rely on specialized solders.
In this study, ball grid arrays (BGAs), quad flat packages (QFPs), and surface mount resistors assembled with SAC305 and Sn3.5Ag solder pastes were subjected to thermal cycling from -40oC to 185oC. Commercially available electroless nickel immersion gold (ENIG) board finish was compared to proprietary Sn-based board finish designed for high temperatures. The data analysis showed that the type of solder paste and board finish used did not have an impact on the reliability of BGAs. The failure site was on the package side of the solder joint. The morphology of intermetallic compounds (IMCs) formed after thermal cycling was analyzed
Mechanical behaviour and reliability of Sn3.8AgO.7Cu solder for a surface mount assembly
The demands for compact, light weight and Iow cost electronic products have resulted
in the miniaturisation of solder interconnects to a sub-millimetre scale. With such a
reduction in size, the solder joints cannot be assumed to behave in the same way as
bulk solder in terms of reliability due to the fact that their material behaviours are
influenced by the joint size and microstructure. The complexity of their reliability
assessment is furthermore compounded by the demand for the replacement of
traditional SnPb solder alloys with lead-free alloys, due to the presence of the toxic
and health hazardous element (Pb) in the former alloy. However, these new lead-free
alloys have much less history of industrial applications, and their material and
reliability data is not as well developed as traditional lead-based alloys. In addition,
most previous reliability assessments using finite element analysis have assumed a
uniform distribution of temperature within the electronic assembly, which conflicts the
actual temperature conditions during circuit operation. Therefore, this research was
undertaken to analyse the effect of solder joint size on solder material properties from
which material models were developed, and to determine the effect of an actual (nonuniform)
temperature distribution in an electronic assembly on the reliability of its
solder joints. Following a review of lead-free solders and potential lead-free alloys,
lead-free solder microstructures, and the reliability issues and factors affecting the
reliability of solder joints, the practical aspects of this research were carried out in two
main parts.
The first part consisted of substantial work on the experimental determination of the
temperature distribution in a typical surface mount chip resistor assembly for power
cycling conditions, and the stress-strain and creep behaviour for both Sn3.8AgO.7Cu
solder joints and reflowed bulk solder. This also included building material models
based on the experimental data for the solder joints tested and comparison with that for
bulk solder. Based on the comparison of the material properties, two extreme material
models were selected for the reliability study. Size and microstructure effects on the
solder material properties were also discussed in this part.
The second part comprised of extensive finite element analysis of a surface mount
chip resistor assembly and reliability assessment of its solder joints. The simulation
began with elasto-plastic analysis for 2D and 3D chip resistor assemblies to decide
upon the kind of formulation to be used when the full complexity of both plasticity
and creep is considered. The simulation was carried out considering the determined
non-uniform temperature distribution and idealized or traditional uniform temperature
condition. The solder joint's material properties were modelled using the two material
models determined from the experimental results. The effect of temperature
distribution during thermal cycling and of the selected material models on the solder
joint reliability was demonstrated using finite element analysis and subsequent fatigue
life estimation.
In summary, this research has concluded that the material behaviour of the solder joint
is different from that of bulk solder due to the effect of its size and microstructure. The
anisotropic behaviour of the solder joint cannot be ignored in reliability studies, since
it has a significant effect on the solder joint's fatigue life. The research also showed
the significant effect of an actual (non-uniform) temperature distribution in the
electronic assembly on the solder joint fatigue life