How to make a soft solder joint

The process of connecting metals using a tin-based filler metal, known as soft solder, is called soft soldering. The melting point of a tin/lead alloy is about 200°C. This temperature isn't high enough to melt the materials you are joining but it is high enough to melt the solder to create a strong, watertight connection.

The only condition here is that the solder joint cannot be directly subjected to mechanical strain or high pressure. To achieve this, a fitting, such as using a capillary fitting or Yorkshire fitting, can be used to create a larger contact surface.

Steps

1. Where to use a soft solder joint

2. Check the quality of your tools

3. Prepare your components

4. Choose the right filler metal

5. Make your soft solder joint

6. Remedy any soldering issues

If you have a blow torch or gas torch to hand, you can use it to soft solder copper pipe and assemble capillary joints. However, bear in mind that if you only have lead solder, you must not:

• solder outdoor connections that may be exposed to frost;

• solder potable water supply pipe;

• solder gas pipe.

For any other type of pipe assembly, it's important to place supports such as wall mounted pipe clips at regular intervals and at any point where the pipe changes direction to absorb the shock of water hammer. This phenomenon is caused by the fluid in the pipe stopping or changing direction suddenly and can cause your soldered joints to rupture.

Remember that it is a legal requirement to use lead-free solder to join pipes used to supply water for cooking, drinking and bathing. It's worth noting that tin solder can also be used in car bodywork repairs to smooth out surfaces.

• It's important to check that your gas torch is suitable for soft soldering. A maximum temperature of 350°C is sufficient to melt lead and even lead-free solder (which has a slightly higher melting point).

• Flux, whether you use a paste or liquid, plays an essential role in the success of your solder joints. Check that the flux you have is suitable for your application.

• It's best to use steel wool rather than emery cloth to clean pipe as emery cloth can leave behind deposits.

• It is important never to heat copper excessively as it can become porous.

Soft soldering can be used to join components that lay on top or or slot into each other meaning capillary action is used to connect them. Start by cleaning the male and female parts and coating them with flux. Once the parts are aligned and brought up to the right temperature, the solder can be deposited around the joint. The solder will soften and, thanks to the flux, it should be drawn into the gap between the parts.

However, if you are not confident in your skills, you can opt to tin the contact surfaces before connecting. Take the parts you want to connect and clean the materials. You can then apply your flux and coat the contact surfaces with a fine layer of tin. You can even allow these parts to cool before connection which means you can prepare several components for assembly later on.

If you have a lot of components to prepare, you can dip the surfaces you need to solder in a solder pot first.

The use of tin/lead solder is heavily restricted under RoHS directive 27 01 2003. With this in mind, an increasing number of lead-free equivalents is now available.

Binary alloys

Some examples of binary alloys include:

• tin/lead;

• tin/copper;

• tin/silver.

Ternary alloys

Ternary alloys include:

• Tin/copper/silver.

Some solder wire is flux cored meaning you don't have to think about finding the right match. For a greater amount of filler metal, you can use solder bars which are more flexible and can also be used to fill solder pots.

To create a solder joint, proceed as follows,

• Remove oxidisation from the components you want to solder using steel wool or emery cloth and degrease if necessary.

• Use flux to cover the parts you have just cleaned.

• Use a torch to heat the surfaces to the correct temperature taking care not to overheat.

• Apply a thin layer of solder along the joint.

• Repeat the process for any parts you need to join.

• Hold the components together and heat at the same time.

• If the solder melts correctly you can join or cover the components.

• Wipe a damp cloth over your joints to eliminate excess tin.

• Check that the solder is spread over the whole of the joint.

Here are three problems commonly encountered while soldering along with their respective solutions:

1. If a leak appears, drain the pipe and heat it to desolder the joint. Start the process over again.

2. If the solder does not soften correctly over your components and instead forms droplets, this means that you have not adequately prepared the joint. Add more flux in this case.

3. If the solder is not penetrating the gap, heat the joint a little more. Check that the part you are soldering is open or the air contained inside the pipe will try to escape via the joint and interfere with the connection. Do not leave any water inside the pipe. The steam produced will stop your solder from adhering correctly.

1. Choose the correct tin alloy depending on how the components will be used.

2. Be sure to check if there is any possible health risks of the materials. Do not use lead in any areas used for eating or drinking as once lead is in the body it can be stored for many years.

3. The biggest risk when soldering is burns. It is therefore essential to wear suitable protective gear.

4. It is strongly recommended to wear welding glasses and a mask designed to protect you from acid gases which are released as flux is heated.

Required skills

As long as you follow all advice and instructions, it should be fairly straightforward to create a good solder joints. You should get satisfying results by following precautions and safety recommendations and by using the right products. The more you practise your technique, the more you'll master the techniques of soft soldering which can come in handy for a number of common repair jobs.

Time required

5 minutes

Number of people required

One person

Tools and equipment

• Soldering torch, gas torch

• Gas bottles

• Gas lighter

• Workbench with vice

• Wire brush

• Steel wool

• Flux

• Solder

• Damp cloth

Safety and workwear

• Welder's apron

• Safety shoes

• Welding gloves

• Safety helmet

• Safety goggles

• Dust mask

• Welding curtains

Please note: this is a non-exhaustive list; be sure to match your personal protective equipment to the job at hand.

• Flame welding accessories buying guide

• Soldering iron buying guide