Characterization of the Creep Behavior of Lead Free Solder Bumps

The 9th International Conference on the Mechanics of Time Dependent Materials
Characterization of the Creep Behavior of Lead Free Solder Bumps using
Nanoindentation
Yingjie Dua, Huiyang Luoa, Yong Hea, and Hongbing Lua, Xiao Hu Liub, Minhua Lub,
aDepartment
of Mechanical Engineering, the University of Texas at Dallas, Richardson, TX 75080, USA,
[email protected], [email protected], [email protected], [email protected];
bMaterials and Reliability Sciences, IBM TJ Watson Research Center, 1101 Kichawan Rd, Yorktown Heights, NY
10520, USA, [email protected], [email protected]
Keywords: nanoindentation, steady state creep exponent, activation energy, flip chip, solder bump,
correction, finite element analysis, elastic effect.
Introduction
In electronic packaging, solder bumps are used for electrical interconnects and for use as
mechanical bonds that often serve as conduit to remove heat from joined device. Creep behavior of
solder bumps is very critical to the mechanical reliability of interconnects. Solder bumps are small,
and their properties change from bump to bump, as they depend on microstructures which vary
significantly from one bump to another. To characterize the creep behavior, nanoindentation is an
effective tool. It is noted that the existing nanoindentation techniques for measurement of creep
properties are only applicable to viscoplastic materials with negligible elastic deformation.
Objectives
The objectives of this study are (1)to evaluate the elastic effect on the measurement of creep
properties using nanoindentation; and (2) to measure the creep parameters, specifically the steady
state creep exponent and activation energy of two types of reflowed lead free solder bumps for flip
chips(controlled collapse chip connection).
Methodology
The elastic effect is evaluated using commercial finite element analysis (FEA) software, Abaqus 6.11.
The indenter tip was modeled as a rigid axis-symmetric cone with half angle of 70.3o and the sample
was modeled as a cylinder. For materials with non-negligible elastic deformations, elastic-powercreep model with following constitutive equation was used in simulation
𝜎̇
𝜀̇ = 𝐸 + 𝛼𝜎 𝑛
(1)
where 𝐸 is Young’s modulus. Coefficient 𝛼 is a constant and 𝑛 is steady state creep exponent.
Different combinations of 𝐸, 𝛼 and 𝑛 were used in simulations to investigate the elastic effect.
Nanoindentation tests were conducted on Agilent G200 Nano Indenter. Creep parameters of lead
free solder bumps were measured by constant loading (CL) and constant rate loading (CRL)
nanoindentation. In CL nanoindentation, the load increases to maximum in 1 second and keeps
constant for 800 seconds. Creep power exponent can be extracted from a single test using following
equation
0.5
𝑛=
(2)
𝜕lnℎ/𝜕ln𝑡
where t is holding time and h is indenter tip displacement. In CRL nanoindentation, load increases
linearly with time, 𝑃 = 𝑘𝑡, where 𝑘 is loading rate. The creep power exponent can be extracted from
a group of nanoindentations with same maximum load and different loading rate
−0.5
𝑛=
(3)
𝜕lnℎ/𝜕ln(𝑘)
CRL nanoindentations of identical loading conditions were conducted at different temperature to
obtain activation energy.
𝑄 = −2𝑅𝑛
𝜕lnℎ
𝜕(1/𝑇)
where R is gas constant and T is thermodynamic temperature.
(4)
Results and analysis
It was determined by FEA that without considering the contribution of elastic deformation, steady
state creep exponent obtained from nanoindentation tends to be larger than the actual value as
shown by solid circular dots in figure 1. Equation 5 is used to extract effective plastic deformation from
nanoindentation experiment on an elastic-power-creep material.
𝑃
𝐸𝑒𝑓𝑓
=
2𝑡𝑎𝑛𝜃
(ℎ2
𝜋
− ℎ𝑝 2 )
(5)
where 𝜃 is half angle of the conical tip, 𝑃 is applied load and ℎ is indenter tip displacement. 𝐸𝑒𝑓𝑓 is
the effective modulus defined by 𝐸𝑒𝑓𝑓 = 𝐸/(1 − 𝑣 2 ) . ℎ𝑝 represents the effective plastic
displacement. The elastic effect can be eliminated by using the effective plastic displacement, ℎ𝑝 ,
instead of the actual indenter tip displacement in creep component calculation. After correction, the
creep exponents can be obtained with high precision as shown by solid square dots in figure 1.
The new correction approach was applied to extract the creep exponents and activation energy
from the nanoindentation data. Figure 2 shows that the creep exponents obtained from CL
nanoindentation experiments on pure Sn solder and Sn-1%Ag solder are 9.3 and 10 respectively. CRL
nanoindentation experiments yield same creep exponents. Activation energy obtained from CRL
nanoindentation experiments on pure Sn and Sn-1%Ag solder are 30kJ/mol and 37.5kJ/mol
respectively.
Figure 1, FEA result of creep exponents before and Figure 2, log-log plot of displacement vs. time from CL
after correction. Creep exponents can be obtained nanoindentation. The determined creep exponents of
with high precision after correction.
pure Sn and Sn-1%Ag solder are 9.3 and 10
respectively.
References
[1] M. Mayo and W. Nix, “A Micro Indentation Study of Superplasticity in Pb, Sn, and Sn-38 Wt% Pb”, Acta
Materialia, 36, 2183–2192, 1988.
[2] Ian N. Sneddon, “The Relation Between Load and Penetration in The Axisymmetric Boussinesq Problem
For a Punch of Arbitrary Profile.” Int. J. Engng Sci. 3: 47–57, 1965.

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