In the electronics assembly arena, lead is being targeted due to the concern regarding environmental pollutants.  Thus, the lead-free solder and its reliability are getting highlighted.  During qualification testing, lead-free solders in electronic assemblies are typically subjected to thermal cycling.  In the lead-free solder material, microstructural evolution and material property degradation occurs due the thermal aging phenomena during dwell periods at the high temperature extreme of thermal cycling.  In addition, during ramping between low and high temperature extreme, lead-free solders can also experience aging phenomena.  In our prior work, we have compared material properties (stiffness and strength) degradation of lead-free solder materials in isothermal aging and five different thermal cycling exposures.  Changes in material properties were higher for all the thermal cycling exposures compared to the aging.  But microstructural evolution and material property degradation exacerbated as the ramp rate decreases in thermal cycling.

 

In this current study, the creep behavior evolutions occurring in SAC305 lead free solder subjected to thermal cycling exposures have been investigated.  Uniaxial test specimens were prepared by reflowing solder in rectangular cross-section glass tubes with a controlled temperature profile.  The samples were thermally cycled from -40 C to +125 C under a stress-free condition (no load) in an environmental chamber after reflow solidification.  The thermal cycling profile consisted of a 150 minutes cycle with 45 minutes ramps and 30 minutes dwells.  It was chosen for this study as in our previous study it showed most detrimental effects on mechanical behavior evolution.  In this condition, 10-15 samples were cycled for various durations of cycling (e.g.  48, 96, and 240 cycles), which were equivalent to various aging times at the high temperature extreme of T = 125 C.

 

After cycling, creep testing was performed on all the cycled samples at three different stress levels (10, 12 ,15 MPa) for 0, 1, 2, and 5 days of equivalent aging time.  All the tests were performed at room temperature.   For the specified thermal cycling profile, the evolution of the creep rate was obtained as a function of the stress level, as well as the net aging time at the high temperature extreme.  Results showed that the creep rate increases with the increase in stress level for 0, 1, 2, and 5 days of aging at high temperature extreme.


Evaluation of the Creep Response of Lead-Free Solder Materials Subjected to Thermal Cycling

Division / Sponsors: 
Electronic & Photonic Packaging Division (EPPD)

Paper Proceedings: https://asmedigitalcollection.asme.org/InterPACK/InterPACK2020/volume/84041

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