Currently, there is intense competition in the industry for the development of new portable handsets. Maximizing the screen size and enhancing the performance are of utmost importance for the design of contemporary cellular phones. In particular, a sliding phone has a greater number of functions than other types of cellular phones, and the screen size of a sliding phone is relatively larger. However, because the existing spring units have limitations in terms of their mechanical performance, it is impossible for the display size to be maximized by transferring shortcut buttons, called navi-keys, which are located on the same plane of the display unit. The life cycle of a sliding phone is inversely proportional to the degree of spring deformation, which in turn depends on the sliding stroke. Therefore, a long-stroke sliding mechanism was investigated, and the manufacturing process for a novel sliding spring unit was proposed without increasing the thickness of the existing cellular phone. The possibility of performing semi-auto sliding strokes (>60 mm) was verified by flatness tests, life cycle tests, and sliding-force measurements. The performance of the designed spring unit was verified by carrying out reliability tests such as life cycle tests under the condition that the number of strokes was more than 100,000 and measurement tests to check whether the sliding force exceeded 250 gf.