Measurements and Simulations of the Creep Strain in Flip Chip Solder Balls

The paper reports the in-situ characterization of the creep behavior of flip chip solder balls during 4 point bending test by means of the IForce stress chip. The stress chip technology can measure mechanical stress at the surface of the silicon die. In flip chip configuration, the solder balls are only a few micrometer away from the n-and p-MOS current mirror cells of the stress chip. Hence, relaxation in the solder balls should directly cause measurable effects in these sensor cells. This is demonstrated by 4-point-bending of the flip chip stress chips. A simulation with and without creep material parameter proves the validity of the approach. A time-dependent analysis of the relaxation within the stress cell closest to the solder ball allows the deduction of the relaxation properties from the measurement.