Micro Soldering Techniques – SS3 Computer Hardware & GSM Repair Lesson Note (First Term)

Micro soldering is precision soldering on the very small parts of a phone motherboard. It needs steady hands, the right temperature and the right tools, such as a hot air rework station, a micro soldering iron, a microscope and flux. The two main skills are replacing integrated circuits (ICs) and fixing broken connections with jumper wires.

Behavioural objectives

By the end of the lesson, students should be able to:

• name the main tools used in micro soldering and state what each one does;
• describe the steps for removing an IC from a motherboard;
• describe the steps for reballing and installing an IC;
• explain how a jumper wire is used to repair a broken track;
• list common mistakes in micro soldering and how to avoid them.

What is micro soldering?

As mobile phones get thinner and more powerful, the parts inside get smaller. We have moved past basic soldering to micro soldering. It needs steady hands, the right temperature, and an understanding of how microscopic “legs” (pins) connect to the motherboard.

This lesson covers two important skills for a professional technician: replacing integrated circuits (ICs) and jumpering broken connections.

Which tools are needed for micro soldering?

A regular “big” soldering iron cannot do this work. You need precision tools:

• Hot air rework station: uses controlled hot air to melt the solder under a chip without touching it.
• Micro soldering iron (T12 or JBC type): has a very fine, needle-like tip for tiny spots.
• Microscope: your eyes cannot see the gaps between IC pins. A magnification of 7x to 45x is standard.
• Flux: the “magic liquid”. It cleans the metal, prevents oxidation, and helps the solder flow where it should go.
• Solder wick (desoldering braid): a copper braid used to “soak up” old solder from the board.

How is an IC replaced?

Many ICs in phones are fitted as a BGA (Ball Grid Array). This is a “chip” with many tiny balls of solder underneath it.

How do you remove an IC?

1. Protection: cover nearby plastic parts (like connectors) with Kapton tape or heat shields so they do not melt.
2. Apply flux: put a little flux around the edges of the chip.
3. Heating: use the hot air gun (usually set between 330°C and 380°C) in a circular motion.
4. The “nudge” test: once the solder melts, gently touch the side of the chip with tweezers. If it wiggles and returns to its spot, the solder is molten.
5. Lifting: lift the chip vertically with tweezers. Do not drag it, or you will tear the copper pads off the board.

How do you reball and install an IC?

1. Cleaning: use the soldering iron and wick to make the pads on the motherboard perfectly flat.
2. Reballing: use a stencil and solder paste to create new, uniform balls on the bottom of the new IC.
3. Alignment: look for the directional dot (a tiny mark on one corner of the IC). It must match the mark on the motherboard.
4. Soldering: apply heat until the chip “settles” into place. Surface tension pulls it into the centre.

How do you repair a broken track with a jumper wire?

Sometimes a track (the copper path inside the board) breaks because of water damage or a heavy fall. We cannot replace the track inside the board, so we run a “jumper wire” outside it. This is like a bypass.

1. Identify the break: use your schematic to find where the signal starts and where it ends.
2. Scratch the surface: if the pad is gone, use a surgical blade to gently scratch the green mask until you see the shiny copper path.
3. Tin the wire: use 0.02 mm enamelled copper wire. Its thin enamel coating stops it causing a short circuit if it touches other parts. You must “tin” the ends with solder first.
4. Make the connection: solder one end to the source and the other to the destination.
5. Apply UV green oil: once the wire is in place, cover it with UV mask (green oil) and shine a UV light on it. This hardens the oil and “glues” the wire down so it does not move.

What mistakes should you avoid?

• Too much heat: if you heat the board for too long, the layers can separate (delamination) and the board is ruined.
• Static electricity: always wear an ESD (anti-static) wrist strap. A tiny spark from your finger can kill a CPU before you even start.
• Too much solder: in micro soldering, less is more. Too much solder causes “bridges”, where two pins touch and cause a short.

Practise on scrap boards before attempting a repair on a customer’s device.

Evaluation

1. What is the purpose of flux in micro soldering?
2. Why must we pay attention to the dot on the corner of an IC?
3. Describe what UV green oil is used for during a jumper repair.
4. A plastic battery connector is close to the IC you want to remove. What should you do before using the hot air gun?

Assignment

1. List, in order, the five steps for removing an IC from a motherboard.
2. State three common mistakes in micro soldering and say how each one can be avoided.