An over-current protection device comprises a PTC material layer, first and second conductive layers, first and second electrodes, and four conductive vias. The first and second conductive layers are in physical contact with first and second surfaces of the PTC material layer, respectively. The first electrode contains a pair of first metal foils, and the second electrode contains a pair of second metal foils. The four conductive vias are formed at the corners each defined by two adjacent planar lateral surfaces. Two conductive vias connect the pair of the first metal foils and the first conductive layer, and the other two conductive vias connect the pair of the second metal foils and the second conductive layer. The ratio of the sum of the cross-sectional areas of the conductive vias to a form factor area of the device is in the range of 7% to 20%.