Soldering, PCB Fabrication and Circuit Assembly

Soldering — the technique that decides everything else

A solder joint is an alloy bond, not glue. The solder must wet both the component lead and the pad, which requires both to be hot enough before the solder arrives.

The correct sequence — and the order is the whole technique:

1. Clean the pad and lead. Oxide prevents wetting.
2. Tin the iron tip with a little solder for thermal contact.
3. Heat the joint — touch the tip to both the pad and the lead together, for about one to two seconds.
4. Feed solder into the JOINT, not onto the iron. The heated metal melts the solder, which is what proves it is hot enough.
5. Remove the solder, then the iron.
6. Do not move the joint while it cools, about a second.

The one thing beginners get wrong: melting solder on the iron and carrying it to the joint. The flux burns off in transit and the pad is cold, so the solder sits on top without bonding — a dry joint.

A good joint is shiny, smooth, and concave where it flows onto the pad and up the lead, like a small volcano.

A dry joint is dull, grainy, and convex — a ball sitting on the pad. It may conduct today and fail next week, which makes it far worse than an obvious failure.

Common faults: dry joint (insufficient heat); solder bridge between adjacent pads (too much solder); lifted pad (too much heat, too long); cold joint from movement while cooling.

Desoldering: heat the joint until the solder is fully molten, then apply the desoldering pump and release. Desoldering braid works by wicking — place it on the joint and heat through it.

Safety. The tip is at 350 °C. Return the iron to its stand every time. Solder fumes are flux, not lead, but still irritant, so work in ventilated space. Wash hands afterwards if using leaded solder. Never flick molten solder off the iron.

Printed circuit boards

Types:

• Single-sided — copper on one face. Cheapest; simple circuits.
• Double-sided — copper both faces, connected by plated through holes (PTH).
• Multilayer — four or more layers.

Plated through holes are the connection between layers, and they are made by electroless plating — the process in Chemistry Module 2, which is the only method able to deposit copper on the non-conducting wall of a drilled hole.

Fabricating a single-sided board — the experiment:

1. Design the layout. Track width suits current; keep power tracks wider. Mirror the artwork correctly for the transfer method used.
2. Transfer the pattern — toner transfer from a laser print, or photoresist and UV exposure.
3. Etch in ferric chloride, which dissolves the unprotected copper: $$\text{FeCl}_3 + \text{Cu} \rightarrow \text{FeCl}_2 + \text{CuCl}$$ Agitate gently; warm etchant works faster. Ferric chloride stains permanently and attacks metal — wear gloves, keep it off clothing and sinks, and dispose of it as directed rather than down the drain.
4. Remove the resist, clean the board.
5. Drill the holes, typically 0.8–1.0 mm.
6. Inspect against the layout for broken tracks and unintended bridges before populating.
7. Populate and solder, lowest components first so the board can lie flat.

Breadboard versus PCB

Breadboard — solderless, for rapid prototyping and testing. The strips run in rows of five, with the two centre columns separated by the channel so a DIP package straddles it. Power rails run along the edges.

Limitations, and they matter: contacts are springy and unreliable after repeated use, stray capacitance makes it unsuitable above a few megahertz, and it cannot carry much current. A circuit that works on breadboard and fails on PCB, or vice versa, is usually a contact problem, not a design problem.

General-purpose (vero/dot) PCB — a permanent build with soldered joints and wire links. Sturdier and the natural next step after the breadboard prototype works.

Interconnection methods

• Soldering — permanent, reliable, low resistance.
• Crimping — a mechanical cold weld between terminal and conductor. Use the correct crimping tool, not pliers; a crimp made with pliers looks acceptable and fails under vibration.
• Wire wrapping — tight turns of solid wire on a square post; the corners bite in and gas-tight joints form. Used for prototyping.
• Connectors — pluggable, for anything that must be serviced.

The assembly experiment

The syllabus asks you to assemble a working circuit on general-purpose PCB and demonstrate it — any two, one option being a fixed-voltage power supply: transformer, rectifier diodes, capacitor filter, regulator.

That circuit is the block diagram from Electrical and Electronics Module 3, built in hardware. Measure at each stage as you go:

• After the transformer: AC, RMS, check against the turns ratio.
• After the rectifier, no filter: pulsating DC. On the CRO you should see the full-wave shape at 100 Hz, twice the supply frequency.
• After the capacitor: nearly steady DC with small ripple. Use AC coupling on the CRO to see the ripple clearly.
• After the regulator: steady DC, ripple almost gone.

Measuring at each stage is the point of the experiment. It converts four blocks in a textbook into four waveforms you have seen.

The record

Per experiment: aim; apparatus; circuit diagram in BIS/IEEE symbols; component list with values and ratings; procedure; observations with readings and units; waveforms sketched to scale on graph paper; result; and precautions.

Write it up the same day.