Electronic substrates on which precise circuits were already formed can be cleaved without leaving any micro-cracks on the cleaved surface by a thermal stress cleaving using a line heat source. In order to understand the crack extension behavior; time-dependent stress intensity factors of long and short cracks pre-induced from the end of a thin plate are analyzed when a thin plate is heated by a continual line heat source on the crack line. In the case of a long crack, the stress intensity factor is almost independent from the crack length but, in the case of a short crack, the factor is strongly dependent on the crack length. Accordingly, two different modes of crack extension appear. The condition, which mode appears, can be clarified from the comparison of stress intensity factors of two cases. With high heating power, a strip with narrow width can be cleaved stably. For stable cleaving, therefore, it is essential to use high heating power so long as the heat damage doesn't appear. This conclusion agrees well with experiments using glass plates.
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