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Soldering Guide

Soldering is an essential part of installing and maintaining an efficient electrical system in an AEG. MOSFETs, low resistance connectors and wiring are crucial components, so it is important to be able to install them, and to repair poor solder joints often found in stock guns.

Essential tools:

  • Soldering Iron - Most are adequate for airsoft but the hotter, the better. A large tip is also useful for maximum heat transfer.
  • Solder Wire - Lead free solder is excellent and does less damage to the environment. Most solder comes with a ‘core’ of flux in the middle of the wire, which is preferable to non-flux solder.
  • Flux - This removes impurities and prevents oxidisation, which makes it impossible for the solder to properly stick to the joint. Adding extra flux to the solder joint makes it much easier to get a perfect result. A flux dispensing pen is clean and convenient.
  • Vice + third hand - Firmly held parts are much easier and faster to solder and allow you to work more precisely. A ball and socket vice and reverse action tweezers work well together.
  • Wet sponge - Use this to clean excess solder from the iron before and after each joint is made. The iron must look bright and shiny before solder can stick to it.
  • Tip tinner/cleaner - After a while the tip will become heavily oxidised. Tip tinner/cleaner is the best way to restore it.
  • Heat shrink tubing - Once the solder joint is made, it must be insulated. An assortment of different sizes of heat shrink is an essential part of a good workshop. A heat gun or a lighter are used to shrink the tubing once it is in place.
  • Solder Sucker - For removing large amounts of unwanted solder.
  • Desoldering wick - For removing small amounts of solder, or where a solder sucker is unsuitable.
  • Clippers/wire strippers - For removing insulation to expose wires for soldering. Clippers can be useful for removing excess solder from a joint.
  • Pliers - For bending wires etc into shape.

Soldering is easy to learn but can be frustrating to master. There are two key principles which, if kept in mind will ensure good results:Maximum heat transfer and flux.

 

Maximum Heat Transfer

High, localised temperature.

Hot Iron - Too much heat dissipation can damage nearby parts and lower the temperature of the solder joint, so a hotter iron is better because it allows you to get in and out quickly, making a good join and keeping the temperature of surrounding components lower. A cooler iron must be held on the joint for longer, which causes the whole circuit to heat up more. Also heating the flux for too long will cause it to burn off and become ineffective, which could ruin the joint.

If there are nearby parts which are very sensitive to heat, you can clamp some metal close to the joint to increase heat dissipation away from the sensitive parts. A pair of pliers with an elastic band tied around the handles works well.

Good physical connection - All parts to be soldered must be hot enough. If only one part is hot enough, the solder will only stick to that part. So a good physical connection across the joint to be soldered is essential to ensure maximum heat transfer.

 

Flux

Clean, oxide free soldering.

The high temperature required to melt solder causes the solder and the metal joint to react with oxygen in the air, forming a thin layer of oxide on the surface. Solder cannot bond to the oxide, so flux is used to prevent oxidisation and other impurities.

Flux is so essential in soldering that most solder wire comes with a core of flux running through the middle. This means that when you add fresh solder, flux is applied at the same time. Adding extra flux to the joint before soldering helps a lot and makes for a much easier solder joint. Flux will quickly boil and burn at the high temperatures that soldering requires, so it is important to get in and out quickly to make sure the flux remains effective.

The soldering iron tip will also develop a layer of oxide, and this must be cleaned before and during any work. Tip tinner/cleaner is the best way to clean a heavily contaminated iron.

The fumes given off by burning flux are not particularly good for your health (estimated to be on par with cigarette smoke), so good ventilation, a mask and/or a solder fume extractor are recommended if you need to do a lot of soldering.

 

Step by step Guide

Most problems you run into can be solved by applying those two key principles. Keeping them in mind, this is a step-by-step guide on my preferred soldering technique. As you do more soldering you may develop your own technique, which is perfectly valid.

Before soldering, you should plan everything out carefully to make sure you are soldering the correct parts. Soldering can be undone, but it is time consuming and tedious.

  1. If you are soldering wire, use clippers or a wire stripper to remove insulation from area to be soldered.
  2. If you are soldering wire, you may need to put the heat shrink tubing over the wire before soldering. It is irritating to have to undo a solder joint because you forgot to put the heat shrink on first.
  3. Apply flux to both parts to be soldered. Be liberal, but try to avoid getting flux on anything but the solder joint. It’s sticky and attracts dirt. It can be cleaned off with isopropyl alchohol.
  4. Wet the sponge.
  5. Turn on the soldering iron.
  6. Firmly fix everything in place. I prefer to use a vice to hold one part (the larger part if there is one), and reverse action tweezers with a base to hold the other part. Make sure both parts won’t move under pressure from the soldering iron.
  7. Clean the soldering iron tip. Once the iron is up to temperature, wipe it on the wet sponge. If the tip is clean and shiny, it is ready for soldering. If it is dark and dull, it must be cleaned with tip tinner/cleaner. Firmly rub the iron in the tinner/cleaner until it is coated in solder, then wipe it on the wet sponge. Repeat this until the tip is clean and shiny.
  8. Tin the soldering iron. Take your solder wire and melt a small amount on the tip of the soldering iron. The solder should coat it easily and not form a bead. If it does form a bead as it melts, the tip is not properly clean. Repeat step 5.
  9. Apply the soldering iron to the joint. With your solder wire in the other hand, apply heat and pressure to the center of the joint so as to evenly heat both parts. Watch closely because as soon as the solder flows from the soldering iron to the joint, you must add more solder to the joint. Touching it to the liquid solder melts it much faster than touching it to solid metal. Adding more solder at this stage allows the fresh flux in the solder wire to mix into the whole joint. Once the joint is covered, remove the solder wire first and then quickly remove the soldering iron.
  10. Clean the soldering iron on the wet sponge.
  11. Inspect the solder joint. If the joint looks dull and/or some parts of the joint have not been covered in solder you have a ‘cold’ joint which must be redone. Apply flux to the joint, and repeat step 7. If there is now a lot of solder on the joint, use a solder sucker or desoldering wick to remove the excess first. If the joint is shiny and the solder appears to have made a good bond with the parts, you are done! Apply heat shrink to the joint if possible, and enjoy your perfect solder joint.
  12. Turn off the soldering iron! It can damage the iron to leave it on longer than necessary.