Baseband chips process and execute the radio functions of smartphones. The iPhone 4S uses a Qualcomm MDM6610 baseband chip, an upgrade from the Qualcomm MDM6600. You must perform a partial teardown on your iPhone to remove and replace the baseband chip. This is a relatively simple endeavor compared to the teardown procedure for previous iPhone models.
Remove the two 5-point Pentalobe screws from the bottom of the iPhone 4S using the Pentalobe screwdriver. Grasp the iPhone with the rear panel facing you and slide the rear panel forward. The rear panel will slide off, exposing the battery and EMI shields.
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Grasp the transparent plastic tab protruding from under the battery and pull up. The battery will release. Remove the 5-point Pentalobe screw securing the Wi-Fi antenna, located at the bottom next to the battery cradle.
Remove the five 5-point Pentalobe screws securing the silver EMI shield at the top of the iPhone 4S. You will expose the rear-facing camera connector and four small ribbon cables.
Insert the flat end of the plastic spudger at the point where the rear-facing camera connects with the logic board. Flip the camera's connector up 1/4 inch with the plastic spudger to disconnect it from the logic board. Remove the rear-facing camera. Remove the ribbon cables at their connection point on the logic board with the flat end of the plastic spudger.
Insert the flat end of the plastic spudger at the point where the Wi-Fi antenna ribbon cable inserts in its socket on the logic board. The ribbon cable socket is at the top of the Wi-Fi antenna. Insert the plastic spudger at the point where the black Wi-Fi antenna cable inserts in the logic board, again, at the bottom of the phone next to the battery cradle. Remove the Wi-Fi antenna.
Lift the logic board from the phone and set it aside. Remove the two silver EMI shields from the logic board by inserting the pointed end of the plastic spudger at the point where they meet the board. Lift up gently with the spudger while working your way around the perimeter of the EMI shields. Remove the EMI shields.
Locate the Qualcomm MDM6610 baseband chip on the logic board. The black baseband chip, marked "Qualcomm MDM6610," is located opposite the chip with the Apple logo labeled "338S0973."
Power up a hot-air soldering iron. Ideally, the soldering iron should reach 545 degrees F, the temperature needed to reflow, or melt, the solder. The baseband chip is soldered to the logic board at 108 points.
Hold the hot-air soldering iron 1/4-inch from the bottom right corner of the baseband chip and rotate it in a small circular motion to distribute the heat evenly. Slowly work your way around the perimeter of the chip twice. Grasp one corner of the chip and lift up gently to determine if the solder has begun reflowing. If not, make another pass around the perimeter.
Lift up on the chip slightly with the tweezers and hold the hot-air soldering iron under the chip until the solder reflows. Remove the faulty baseband chip. Dip a cotton swab in isopropyl alcohol and clean the residue from the logic board.
Place a pea-sized amount of flux paste on the logic board in the center of where you will position the preprogrammed baseband chip. Spread the flux paste over the soldering points that were exposed when you removed the baseband chip. Use the flat end of a metal spudger to spread the paste.
Tin, or melt, the rosin-core solder over the soldering points on the logic board with a soldering iron fitted with a 3/8-inch tip. Spread a thin layer of flux paste over the soldering points with the flat end of the metal spudger. Make sure the flux paste touches each point you soldered.
Position the pre-programmed baseboard chip over the soldered points and press it in place with tweezers or the metal spudger. Hold the hot-air soldering iron 3 inches from the chip and, using a small circular motion, heat it for two minutes. Press the replacement chip in place with the tweezers and allow the solder to cool and set for 15 minutes.
Reassemble the iPhone by performing the teardown steps in reverse.