The use of synchrotron radiation x-rays is a powerful and novel technique for determining the strain distribution in flip-chip dies. In this study, thermal and electrical effects on the strain of such dies were investigated in situ using synchrotron radiation x-rays. Intense light and small beam size allow precise measurement of minute strain variations in the dies. Subtracting from these variations the strains associated with the thermal expansion of silicon yields the real variations of the strain in dies caused by interactions among the various layers of materials in the flip chip. The prominent warpage of the flip chip at various temperatures is associated with changes in the strain in the dies. The values measured using synchrotron x-rays agreed closely with simulation results. The strains in dies of different thicknesses were compared; thinner dies exhibited large variations in strain under various test conditions.