Analysis of thermal strain of metallized silicon nitride substrate under cyclic thermal loading by digital image correlation

Abstract In this research, a digital image correlation method was employed to measure the thermal strain of a metallized silicon nitride substrate subjected to multiple successive thermal cycles from −40 to 250°C. In all the thermal strain curves observed during the 1000 thermal cycles, consistent hysteresis loops were detected. At the beginning of the heating or cooling process, the curves display slopes nearly identical to the theoretical coefficient of the thermal expansion (CTE) of the metallized composite. Subsequently, the slope gradually decreased until the completion of heating or cooling and eventually reached the CTE of the plain silicon nitride substrate. However, as the number of thermal cycles increased, the deviation temperature from the theoretical CTE during heating increased, whereas the temperature decreased during cooling, but it did so to a lower temperature. The maximum expansion and contraction observed during each cycle increased equally between the initial thermal cycle and the 100th cycle, after which a saturation point was reached. The relationship between the increase, saturation, and cyclic hardening of copper plates on a metallized silicon nitride substrate is further discussed in this study, shedding light on the overall strain range over thermal cycles.