The button cell battery is easily changed without tools or instructions due to the intuitive design. The battery is snapped into the tray and held in place by two plastic tabs. The tabs help to locate the battery properly and keep the battery from falling out even if the tray is inverted upside down. The tray is then glided into the retainer housing that is soldered to the PCB. When a battery tray is inserted into the retainer housing a click is heard assuring the user that the tray has locked securely in place. Since coin cell batteries are nickel plated, MPD battery holders have nickel plated contacts. It is important that the mating surfaces be of the same metal or a more noble metal (e.g. gold) to prevent corrosion build up at the point of contact. Surface oxidation of nickel plating is a naturally occurring event, so it aids solder joint strength to use a solder paste that is compatible with nickel plated parts.