Solder Joint Reliability from Material Properties
‘Best Poster Paper at IPC APEX EXPO 2010’
Chris Hunt
National Physical Laboratory, Teddington, Middlesex, TW11 0LW
[email protected]
n Exposes samples to the following applied
Developments in
loading conditions:
characterisation of interconnect
– Mechanical displacement of sample under
mechanical properties
Cu support
Solder joint
Time-Lapse Photography & DIC
Applications
25
30
n This is an example
of a study that can
be carried out with
the IPTM. Here the
plastic deformation
behaviour of
a SAC alloy is
characterised.
n Sufficient
displacement
amplitude required
to obtain plastic
deformation in
solder.
n 2400 second
waveform – 800
second ramp, 400
second dwells,
Effect of different
displacement
amplitudes.
isothermal conditions
– Temperature control of sample
– Combined mechanical displacement with
temperature control
– Combine with applied current in sample:
electromigration studies
n Independent programmable control for
all parameters
n Electrical resistance measurement of
sample under loaded conditions
n Viewable sample area provides optical
access for imaging
– Strain mapping using digital imaging
correlation (DIC) or similar techniques.
(Applicable for certain configurations)
n Designed for easy sample mounting
n Displacement control with laser or
LVDT sensors
n Interconnect mechanical
properties
n Lifetime prediction
n Materials data
– Scaling issue
– Interconnect heterogeneity
– Intermetallics and
microstructure effects
– Time dependent, fatigue
80
IPTM – Waveform
Amplitude
15
20
10
6.6
5
60
Load [N]
40
20
25°C
80°C
15000
60°C
0
-20
0
5000
10000
20000
25000
-40
-60
-80
30
25
20
15
10
6.6
5
Time [s]
IPTM can be used to collect creep properties
Effect of reducing dwell time on SAC
305, at lower temperatures impact of
dwell time is stronger
Isothermal Fatigue Tests
Fit to Coffin Manson
Isothermal Tests
Run isothermal test
to characterise temperature
properties.
Following results are for
SAC 305
Microstructural examples from isothermal tests
ISOTHERMAL TESTS
MATERIAL PROPERTIES DETERMINATION
10000
1000
100
Material: 96.5Sn 3.0Ag 0.5Cu
Temperatures: 30 °C, 60 °C,
125 °C
Displacement: ±15 µm;
displacement control
Cycle: 40 minutes, trapezoidal
shape; dwells 6’40’’
Failure Criteria: 25% load drop
Coffin Manson 3.0Ag
Sn 0.5Cu
Experimental dat
1
0.01
0.1
1
_ _p
Data shows failure occurring at earlier at higher temperature. Failure criteria
can be defined from various parameters. Here we choose to use load drop.
Furthermore, we show there is an approximate linear relationship between
%load drop and increasing Nf. This limited data can be fitted to a Coffin
Manson behaviour.
Displacement [µm]
40
Temperature
80
30
60
20
40
10
20
0
0
-10
-20
Displacement
Load
-20
-40
-30
-60
0
800
1600
2400
3200
4000
Load [N], Temperature [°C]
Thermal cycle:
-10 to 80 °C
n
n
n
n
PCB and device convergence
Miniaturisation is driving up current density in
the interconnect
Above a critical level mass diffusion effects occur
Voiding a and electromigration effects
30 °C sample
n Cracks appearing in
different regions
(e.g. near interface)
n Recrystallisation of Sn
near crack.
n Similar IMC as untested
20µm
20µm
20µm
-80
4800
Time [s]
Displacement
in copper arms,
±29 µm, is
added to the
desired solder
displacement,
±10 µm = 39 µm.
125°C, 50A
Force drop (%)
Capability of various sample designs
– Configuration representative of real interconnects
Can measure degradation in joint
Failure criteria can be tailored
Can run isothermal or full thermo-mechanical testing
Simultaneously study fatigue and electromigration
Strain mapping of joint
125 °C sample
n Recrystallisation of Sn
in larger areas
n Larger IMCs, tend to
develop perpendicular
to the interface
40°C, 50A
40°C, 0A
40-125°C, 0-50A
40-125°C 0A
30
Summary
60 °C sample
n Cracks appearing in
different regions
(e.g. near interface)
n Clearer recrystallisation
of Sn near cracks.
n Larger IMCs, randomly
scattered
125°C, 0A
40
35
n
n
n
n
n
n
Untested sample
n Long dendrites with
main axis perpendicular
to surface
n IMC Cu6Sn5 randomly
scattered
Electromigration
-40
n Controlled tests on single solder joints
can be performed in thermal cycles using
the IPTM. Temperatures below 0 °C are
obtained using liquid nitrogen.
n The displacement can be varied to
simulate the thermal expansion of
the material at the corresponding
temperatures used, but higher or lower
values can be used.
n Tests performed on SAC305 and SnPb at
-20/+80 °C, with ±15 µm show a much
better performance of the lead-free alloy.
20µm
20µm
10
Thermomechanical Fatigue Tests
Conditions:
Materials: 96.5Sn 3.0Ag 0.5Cu; 63Sn 37Pb
Temperatures: cycle -20/+80 °C
Displacement: ±15 µm; displacement control
Cycle: 40 minutes, trapezoidal shape; dwells 6’40’’
Deformation for the whole
solder joint is measured by
displacement transducers. A
detailed strain map can be
got from feeding time lapse
photographs to a digital imaging
correlation programme to
compute the micro-strain in
the solder joint. These data
can indicate where the stress
maximises, indicate where
subtle differences in joint format
are important, help explain
difference in material properties,
and confirm FEA models.
Conclusions from isothermal tests
25
20
15
10
5
0
0
5
10
15
20
25
Cycle
30
35
40
45
n Controlled tests on single solder joints can be performed
at different temperatures using the IPTM.
n Weibull statistics of fatigue data helps in the
interpretation of the results, which can then be used in
empirical prediction models or in constitutive models.
n Time-Lapse photography and DIC help in determining the
higher strain areas during the test and predict well, where
it is more likely to find cracks.