A surface mountable over-current protection device having upper and lower surfaces comprises a PTC device, first and second electrodes, and first and second circuits. The PTC device comprises a PTC material layer and first and second conductive layers. The PTC material layer is disposed between the conductive layers and comprises crystalline polymer and conductive filler dispersed therein. The first electrode comprises a pair of first metal foils, whereas the second electrode comprises a pair of second metal foils. The first circuit connects the first electrode and conductive layer, and has a first planar line extending horizontally. The second circuit connects the second electrode and conductive layer, and has a second planar line extending horizontally. At least one of the planar lines has a thermal resistance sufficient to mitigate heat dissipation by which the over-current protection device undergoes a test at 25° C. and 8 amperes can trip within 60 seconds.