As microelectronics manufacturers take efforts for reduction or elimination of Pb from their device packages, characterization of the mechanical properties of different Pb-free solder alloys has become a major industry challenge. The variations in stress and temperature experienced by a solder connection over different time scales during processing and application will significantly affect the resulting microstructure of the material.
Considering the fact that the volume of a solder bump on a microcircuit is very small, the mechanical response of the solder can be directly measured using the nanoindentation technique, which performs quantitative measurement of the creep behavior, modulus, and hardness, as a function of temperature.
Nanoindentation Creep Testing
A conventional (quasi-static) indentation test involves applying a load to force an indenter into the sample surface. The applied load is held constant for some time, and then withdrawn. The hardness and modulus of the material can be estimated with the data on the probe shape, indenter penetration depth, applied force, and unloading stiffness.
Force and displacement are continuously measured to create a Force vs. Displacement curve. The slope of the curve at the initial unloading point yields the contact stiffness. Hence, a single quasi-static test yields a single measurement of hardness and modulus at the maximum penetration depth of the test.
A dynamic indentation test involves superimposing a relatively small sinusoidal oscillation onto the quasi-static loading profile. The contact stiffness is calculated continuously throughout the test, using the values of load amplitude, phase lag and displacement amplitude. Since stiffness is determined at all times, it is also possible to measure the hardness and modulus continuously.
Most materials show some degree of creep while the quasi-static indenter load is kept stable. However, it is challenging task to characterize the creep behavior. Displacement error from thermal drift, while generally small at short durations, becomes prohibitively large after several minutes or hours. The Hysitron nanoDMA® III reference creep testing technique can tackle this problem by applying a dynamic force all through the test, enabling continuous measurement of contact stiffness. Schematic representation of a nanoDMA III creep test is illustrated in Figure 1.
Figure 1. Schematic representation of a nanoDMA III creep test.
At first, the estimation of the modulus of the material (the reference modulus) is performed when error from thermal drift is insignificant. With the knowledge of the modulus, contact stiffness is continuously measured by keeping the quasi-static load as constant. With these data, the contact area and, therefore, contact depth and hardness can be computed without any reliance on the quasi-static displacement measurement, making background thermal drift immaterial. In this manner, creep tests can be reliably performed as long as several hours.
Mechanical Characterization of Pb-Free Solder Bumps
Solder bumps with a composition of 3% Ag and 97% Sn were prepared on top of copper pillars patterned onto a Si wafer. An optical micrograph of the structure is depicted in Figure 2. The solder caps, with a width of approximately 20µm, were domed on top. A Hysitron TI 950 TriboIndenter® coupled with nanoDMA III and xSol High Temperature Stage was applied t measure creep effects at 25, 50, 100, 150, and 175°C, utilizing a diamond Berkovich probe.
Figure 2. Optical micrograph of the silicon, copper, solder structure. (Sample provided by Jürgen Grafe, FHG-IZM-ASSID, Dresden, Germany, and Kong-Boon Yeap, FHG-IFZP, Dresden, Germany.)
Before taking each measurement, the topography of the sample surface imaged using the instrument's in-situ SPM imaging capability to position each test accurately on the top of the dome, where the surface was almost flat and at right angles to the probe (Figure 3). For each test, the force was ramped rapidly to peak force and then held constant for 1000s and the stiffness was continuously measured by a 220Hz oscillation in the mean time. As the load was held, there was an increase in the contact stiffness with the increasing penetration depth of the probe.
Figure 3. Topographical SPM image of a solder bump prior to testing with the target test location indicated by the circle.
The quasi-static force for each test was selected such that the initial depth of the indent would be roughly 500nm, and the dynamic force was chosen to yield a displacement amplitude of roughly 1nm. The target depth was selected as a balance of two considerations.
For a very small indent, the measurement accuracy could be affected by the surface roughness, but if the indent was too large, the free edges around the perimeter of the solder bump could have an influence.
Figure 4. Depth vs. time at each temperature.
Figures 4 and 5 illustrate the continuous changes of contact depth and hardness over time, while Figure 6 delineates modulus as a function of temperature. No difference was observed in the creep curves between the temperature range of 25°C and 150°C, but the difference was readily apparent at 175°C.
Figure 5. Hardness vs. time at each temperature.
Figure 6. Modulus measured at each temperature.
The creep rates were much higher at 175°C, and the hardness deteriorated by 70% over the course of the 1000 second hold. The change in creep behavior indicates a transition in the deformation mechanism between 150°C and 175°C, confirming that the mechanical properties of the solder bumps decay rapidly above 150°C.
This information has been sourced, reviewed and adapted from materials provided by Bruker Nano Surfaces.
For more information on this source, please visit Bruker Nano Surfaces.