Practical: Replacing GSM IC – JSS3 Computer Hardware & GSM Repair Lesson Note (First Term)

Replacing a GSM IC is the process of removing a faulty chip from a phone motherboard with hot air and soldering a new chip in its place.

Behavioural objectives

1. State what solder, flux and a hot air rework station are.
2. List the tools used to remove and replace an IC.
3. Arrange in order the steps for removing an IC.
4. Explain how to clean the pads and align a new IC.
5. Follow the safety rules for working with heat and fumes.

What is solder and why do we use it on ICs?

Solder is a metal that melts at a low temperature and joins parts to a circuit board.

• IC (Integrated Circuit): a small chip that does a job in the phone, such as the Power IC or the Audio IC.
• Fixing method: ICs are held on the board by tiny beads of melted solder, not by screws.
• Pads: the small metal spots on the motherboard where the legs of the chip are soldered.
• Legs: phone chips have hundreds of tiny contacts underneath, so we cannot melt them one by one.

What is advanced soldering?

Advanced soldering is the use of a hot air rework station to melt all the solder under a chip at the same time.

Basic soldering uses a soldering iron, which looks like a pen and heats one point at a time.

What are the three essentials of advanced soldering?

The three essentials are heat, air and flux.

1. Heat: melts the solder.
2. Air: carries the heat to the chip.
3. Flux: a special paste that helps the solder melt and flow smoothly.

What tools are needed to replace an IC?

You need a hot air rework station, flux, tweezers, solder wick and a PCB holder, as well as a soldering iron and Kapton tape.

• Hot Air Rework Station: a machine that blows very hot air, usually between 300°C and 400°C, through a nozzle.
• Flux Paste: a yellow or clear grease applied before heating. Never solder without flux.
• Anti-Magnetic Tweezers: used to nudge and lift the hot chip safely.
• Solder Wick (Desoldering Braid): a copper braid that soaks up old, dirty solder from the board.
• PCB Holder: a metal clamp that holds the motherboard steady while you work.
• Soldering Iron: used with the wick to clean the pads.
• Kapton Tape: heat-resistant tape that protects nearby parts.

What are the steps for removing an IC?

An IC is removed by heating it evenly until the solder melts, then lifting it straight up.

Work with care. If you are too fast, you may rip the pads off the board. If you are too slow, you may overheat and damage the chip.

1. Preparation: switch off the phone and take out the battery if it is removable. Put the motherboard in the PCB holder. Apply a small amount of flux around the edges of the IC.
2. Shielding: cover plastic parts and nearby chips with Kapton tape so they do not melt.
3. Heating: hold the hot air wand about 1 to 2 cm above the chip. Move it in circles. Never hold it in one spot, or the board will burn.
4. The nudge: after about 30 seconds, gently touch the side of the chip with the tweezers. If it moves easily, the solder has melted.
5. The lift: lift the chip straight up. Do not slide it, or you may bridge (connect) the tiny pads underneath.

How do we prepare the board for the new IC?

Preparing the board means cleaning the pads until they are flat and shiny.

1. Apply a little flux on the old pads.
2. Place the solder wick on the bumps of old solder.
3. Heat the wick with the soldering iron and wipe gently across the pads.
4. Repeat until the pads are flat and shiny.

• Why it matters: if the pads are not flat, the new chip will sit crooked and will not work.

How do we fit the new IC?

The new IC is fitted by aligning it correctly on the clean pads and heating it until the solder melts and holds it.

Alignment

• Marker: look for a tiny dot or arrow in one corner of the chip.
• Match: line up that dot with the mark on the motherboard.
• Risk of error: a chip placed the wrong way round can cause a short circuit when the phone is switched on.

Soldering the chip in place

1. Apply a thin layer of flux on the clean pads.
2. Place the new chip on the pads with the marks matched.
3. Heat with the hot air wand, moving in circles, as you did for removal.
4. Stop when the chip settles onto the board.
5. Leave the board to cool without touching the chip.

What are the safety rules for IC replacement?

Safe IC replacement means protecting your lungs, your skin, the board and the new chip.

• Fumes: soldering smoke harms the lungs. Work in a room with a fan or an open window.
• Burns: the nozzle and the board are very hot. Keep your fingers away and use tweezers.
• Patience: if the chip is not moving, wait. Do not force it.
• Cooling: let the board cool naturally for about 2 minutes before testing it.
• No blowing: do not blow on the board with your mouth, as moisture can cause rust.

What mistakes damage the board?

Most damage comes from wrong heat, wrong movement or wrong placement.

Evaluation

1. What is the purpose of flux in advanced soldering?
2. Why must we move the hot air wand in circles instead of holding it still?
3. Which tool do we use to soak up extra solder from the board?
4. Why is the dot or mark on the corner of an IC important?
5. Why should you lift the chip straight up and not slide it?

Assignment

Write the five steps for removing an IC in the correct order. Add one sentence to explain why each step is done.