Fundamentals of soldering on printed circuit boards

Soldering   |   7 min read

Soldering is a joining technique that involves melting solder to bind metals together. Solder on a printed circuit board (PCB) is similar to connective tissue. It acts as a conductive adhesive that adheres components to the substrate and maintains circuit board continuity.

Equipment and materials for soldering

The right tool and materials are needed for soldering. The quality of these items used will in turn determine the quality of the end-product the soldered component is used in. You’ll need:

1. Soldering iron
2. Solder wire
3. Solder flux

Soldering Iron

Also known as a soldering gun, the soldering iron is the most important equipment for soldering. The handle, element, and bit (or tip) are the 3 major components of a soldering iron.

Soldering irons have the same heating element as electric heaters. Heat is produced when electricity flows through the element. This heats the piece of iron, which then passes the heat to the soldering junction.

Instead of standalone soldering irons, soldering and desoldering rework stations are used in repair shops, factories, and laboratories. These systems offer greater efficiencies and can perform more complex tasks than individual, single-function equipment.

Solder Wire

An alloy containing 60% tin and 40% lead is the most common form of solder wire used in electronics manufacturing. It has a melting point of 190 degree Celsius and solidifies as it cools.

This alloy comes in a variety of gauges in wire form. The use of thinner gauges over thicker gauges is preferred. 18 or 22 gauge solder wire is a suitable choice for general application.

Solder Flux

Another substance known as flux, often known as solder paste, is used to facilitate the soldering process.

Flux rids the oxide coating on the surface of solderable metals and increases the wetting ability of the solder.

Solder wire nowadays has flux in the centre core. The melting point of flux is lower than that of solder wire. It does away with the requirement for a separate flux.

Soldering requires a soldering iron, solder wire, and flux, among other things. Other soldering accessories include a soldering gun stand, a cutter, a desoldering pump, and so on.

Types of solders

There are so many different types of solder on the market nowadays that choosing the appropriate one for your project might be challenging. The 4 main types of solders are:

1. Lead alloy solders
2. Lead-free solders
3. Flux core solders
4. Silver alloy solders

Lead alloy solders

The electronics revolution began with lead alloy solder. A 60/40 (tin/lead) combination with a melting point of 180-190 degree Celsius is the most common.

Tin, often known as soft solder, is chosen for its lower melting point, whereas lead is utilised to prevent tin whisker formation. The tensile and shear strengths improve as the tin content increases.

Lead-free solders

When the European Union began to ban the use of lead in consumer electronics, lead-free solder became popular.

Tin whiskers can be reduced by employing improved annealing processes, adding nickel to the mix, and applying conformal coatings.

Lead-free solders have a greater melting point than traditional solders.

Flux core solders

Flux core solder comes in the shape of wires twisted around a cylindrical object. A reducing agent is found in its core.

Flux is released during soldering, and it eliminates the oxidised layer that has developed on the metal’s surface – the metal surface becomes clean and ready for soldering as a result. The flux also improves the wetting qualities of the solder.

Rosin is used as a flux while soldering electrical components, whereas acid cores are used as a flux for metal joining and piping.

Silver alloy solders

Silver alloy solders are used for high-temperature, high-reliability interconnect applications. They can be either lead-free or lead-based.

Silver was only added to lead-based alloy solders in the beginning to avoid a phenomenon known as silver migration. Silver from the silver coating seeps into the solder. When this solder is applied to the metals before soldering, the junctions become brittle and susceptible to breaking.

Silver alloy solders are available in a variety of silver, lead, and other alloy ratios. The cost of these solders is determined by the alloy ratio.

Types of soldering techniques

The soldering techniques can be divided into 2 categories:

1. Soft soldering
2. Hard soldering

Soft soldering

Soft soldering is a common process for electronics, as well as plumbing.

This soldering method is usually used to create electrical connections and adhere electronic components to PCBs. However, the strength of the bonding created by this technique is not as high as hard soldering.

A eutectic tin/lead alloy is used to soft solder metals together. The heat source is usually an electric or gas-powered soldering iron.

Soft soldering is most commonly used to connect connectors to cables, place jumper wires on parts or printed circuit boards, solder wires from transformer coils, and make repairs.

Hard soldering

Hard soldering or silver soldering bonds 2 pieces of metals (i.e. base metals) together, not by melting the solder directly, but by heating the base metals to a temperature that the solder holding the metals together instantly melts. When cooled, an immensely tight joint is formed – through “capillary effect”.

Temperature required to melt the solder material, usually silver or brass, is higher than soft solder and requires a blowtorch. This method is used for joining pieces of brass, copper, silver, or gold.

Brazing is a form of hard soldering.

How to solder on PCB (a step-by-step guide)

Step 1: Prepare the equipment and materials needed.

• A soldering station or a 20-40 W soldering iron and stand
• Solder lead 60/40 (22 gauge preferably)
• Soldering flux
• Pitch bending tool
• Damp sponge
• Steel wool or fine sandpaper (to clean connections before soldering)
• Nippers (or cutter)

If you’re dismantling soldered components on a PCB, you’ll also need a solder braid/wick or desoldering pump to remove melted solder.

Step 2: Switch on the soldering iron and wait for it to heat up.

Step 3: Clean the solder tip by wiping the heated bit on the moist sponge.

Step 4: Tin the solder bit by melting a little solder on it.

Tinning aids in the flow of heat from the solder tip to the joint.

Step 5: Ensure the copper pad on the PCB is clean so that solder will adhere to it.

If it is not, clean it with steel wool.

Step 6: Solder a component to the PCB.

To solder an electronic component in place on the PCB, put the hot soldering tip on both the PCB copper pad and component lead to heat them up. Solder that comes into contact with them would melt and flow into the middle of the pad and fill up the gap around the lead. When done, remove the solder material, followed by the tip. Soldering a connection merely takes a few seconds.

Step 7: After soldering the components, snip off excess component leads with a pair of nippers because leads coming into contact with one another can cause short circuits.

Step 8: Clean off excess flux that’s spread onto the PCB with a cleaning chemical, like isopropanol.

Soldering quick tips

Here’s some common-sense advice that sometimes even soldering veterans forget:

1. There are many different types of bits and solder materials. Use the right tools and materials for the job.
2. To ensure a good, strong joint is formed, do not move the component or touch the solder before the solder cools or blow-cool the joining. There should be an adequate amount of solder – not too little, not too much.
3. Take great care when heating the connection. Overheating can cause damage to the electronic component or PCB. To reduce the probability of damaging the component, attach a metal crocodile clip to the lead of the component to reduce heat flow from the connection to the component.
4. To increase the lifespan of the soldering tip and ensure optimal performance, ensure the bit is properly tinned to prevent oxidisation. Melted solder flows smoothly on a tinned tip.
5. Take care of your own safety. Work in a well-ventilated environment as fumes from solder can irritate and even be harmful. Wear safety goggles to keep your eyes protected. Heated solder iron should be placed on a proper holder and not left lying around to prevent burn injuries or even fires.