Several U.S. mobile phone providers, such as AT&T and T-Mobile, use SIM (or Subscriber Identity Module) cards in their phones to identify the owner and store information, including contacts. These SIM cards are small storage devices that slide into slots under the battery. Although SIM cards don't have battery-powered electronics like cellphones do, prolonged exposure to water can damage the card.
SIM cards have delicate electronics, but limited openings to allow water inside to damage the internal components. If you drop your SIM card in water, remove it as quickly as possible. The longer the SIM card is exposed to water, the more likely it is to be damaged. Short exposure is unlikely to cause damage to a SIM card.
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Immediate action is key to saving a wet SIM card. If you drop your cell phone in water, remove the battery and SIM card as soon as possible. The SIM card slot might have filled with water. If the SIM card sits in water too long, it can become useless. Lay the SIM card on a dry paper towel overnight to allow it to dry completely before trying to use it in any phone. If the SIM card sits in water for more than a few minutes, put it in a bowl of uncooked rice overnight to help draw any water out of the card.
If you dropped the SIM card in dirty water, such as when you are washing dishes or in a mud puddle, clean the SIM card before setting it out to dry. Dip a cotton swab in rubbing alcohol and gently wipe the surface of the card. Allow the card to dry overnight before using it in any cellphone.
Before placing the SIM card back in the cellphone, make sure the phone is completely dry, inside and out. If you turn on the phone and engage the electronics -- including the SIM card -- you risk ruining those electronics. The SIM card slot, usually buried below the battery, may take several days to dry out completely. Never place a SIM card into a damp slot.