7 research outputs found
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
Creep damage study at powercycling of lead-free surface mount device
Soldering is extensively used to assemble electronic components to printed circuit boards or
chips to a substrate in microelectronic devices. These solder joints serve as mechanical,
thermal and electrical interconnections, therefore, their integrity is a key reliability concern.
However, newly introduced lead-free solders do not have a long history of applications in the
industry and there is a lack of established material models of their behaviour over the wide
temperature range experienced by electronics systems. Therefore, an extensive reliability
study is required before introducing a new lead-free solder material in the electronic
industries. Moreover, most of the solder materials have low melting temperatures, and are
prone to creep in service. The cyclic temperature operating condition (powercycling) of the
solder joint can result in the creep fatigue failure. Thus, a computational technique is used to
investigate creep damage in solder joints. The present paper deals with creep damage of leadfree
solder joints for powercycling using finite element analysis with the consideration of
experimentally observed non-uniform temperature distributions in the 1206 surface mount
chip resistor. In addition, a comparison is made for inelastic strain accumulation and fatigue
life for creep damage study for spatially uniform and non-uniform temperature powercycling
3D study of thermal stresses in lead-free surface mount devices
The paper presents the study of non-uniform temperature distributions in a flip
chip electronic assembly, and the use of these temperature distributions to analyse the
thermal stresses in lead-free solder joints in surface mount devices. The thermal stresses
in the solder joints are mainly due to the mismatch in the coefficients of thermal
expansions between the component and substrate materials, and temperature gradient in
the electronic assembly. The thermo-elasto-visco-plastic finite element analysis is carried
out to investigate the extent of thermal stresses induced in solder joints between a surface
mount component and a FR4 circuit board (substrate) under conditions of thermal cycling
with the chip resistor operating at its full power condition. Three different cases of spatial
temperature distributions are considered including one with an experimentally obtained
non-uniform temperature distribution. A comparative study of thermal stresses is
performed using a near-eutectic SnAgCu solder material for three different thermal cases
Creep analysis of a lead-free surface mount device
In this paper finite element analysis (FEA) is used to understand the effect of a non-uniform temperature distribution on the creep and fatigue behaviour of lead-free solder joints in an electronic assembly comprising of a chip resistor mounted on printed circuit board (PCB). Solder joints in surface mount devices (SMDs) operate over a temperature range as extreme as -55degC to 125degC, which is high compared to the melting temperature of solder alloys. Exposure of solder joints to these temperatures can result in thermo-mechanical fatigue. Eutectic or near- eutectic tin-lead alloys have previously been used as an interconnection material, but the ban imposed on the use of toxic materials in electronic products demands new lead-free solder materials. This paper presents the experiments carried out using a thermal camera to obtain the real temperature distribution in the electronic assembly. These temperature distributions were used in FEA of the chip resistor under temperature cycling conditions. Unlike accelerated tests for obtaining reliability data, FEA is quick and less expensive
Thermo-mechanical damage accumulation during power cycling of lead-free surface mount solder joints
It is well known that in surface mount technology
(SMT), thermal strains in electronic assemblies are
induced in the solder joints by the mismatch between the
coefficients of thermal expansion (CTE) of the
components, substrate and solder, both during their
processing and in service. Therefore, thermo-mechanical
damage is likely to occur in the solder and the principle
reliability hazard in SMT assemblies is the resulting
fatigue cracking of the solder fillet, caused by cyclic
thermal stresses. These stresses may be caused by both
cyclic variations in power dissipation within equipment
and by external environmental temperature changes. Most
work reported to date has focused on the effects of
environmental temperature changes, although for many
types of equipment power cycling may result in
significant stresses. The present paper describes the
experimental determination of the actual temperature
distribution in a chip resistor assembly when it is
powered. The paper also discusses the significance of
such experimentally determined non-uniform temperature
distributions in electronic assemblies to fatigue damage
accumulation due to both power cycling and to cyclic
variations in the ambient temperature whilst the chip
resistor is powered. This fatigue damage accumulation
study is carried out using finite element analysis
Creep analysis of a lead-free surface mount device
In this paper finite element analysis (FEA) is used to understand the effect of a non-uniform temperature distribution on the creep and fatigue behaviour of lead-free solder joints in an electronic assembly comprising of a chip resistor mounted on printed circuit board (PCB). Solder joints in surface mount devices (SMDs) operate over a temperature range as extreme as -55degC to 125degC, which is high compared to the melting temperature of solder alloys. Exposure of solder joints to these temperatures can result in thermo-mechanical fatigue. Eutectic or near- eutectic tin-lead alloys have previously been used as an interconnection material, but the ban imposed on the use of toxic materials in electronic products demands new lead-free solder materials. This paper presents the experiments carried out using a thermal camera to obtain the real temperature distribution in the electronic assembly. These temperature distributions were used in FEA of the chip resistor under temperature cycling conditions. Unlike accelerated tests for obtaining reliability data, FEA is quick and less expensive
Finite element analysis of lead-free surface mount devices
Transition to lead-free solder materials has raised concerns over the reliability of lead-free
solder joints in the electronic industry. Solder joints provide electrical conduction and
mechanical support for components and may operate over temperature extremes of -55oC to
125oC or greater. These temperatures are relatively high the melting point of the solder. A
mismatch between coefficients of thermal expansion of the component, solder and substrate,
combined with thermal variations during service, results in thermal fatigue that is a common
failure mechanism for solder joints in electronic products. So far most of the studies of this
issue have considered uniform temperature distributions in the electronic assembly. The main
objective of this paper is to investigate the effect of the experimentally observed non-uniform
temperature distribution in the electronic device on the structural response of solder joints in
comparison with that for a uniform temperature distribution