Soldering Materials 101

What are the key materials needed to complete a soldering task?

You might think that solder is the only thing being consumed. But several other items are essential to completing the job and get used up in the process.

We will also introduce some tools you may never have known were needed. We will start with solder and flux. Then we'll progress to the less well known items like desoldering braids, flux cleaning supplies, soldering tape, and replacement soldering tips.

Solder is the basic material of a soldered joint. You will most commonly encounter it as a spool of metallic wire that's easy to bend and manipulate. The composition of most solder consists of a metal alloy (a mixture of metals that can't be separated mechanically) with a low melting point encased around a core of flux, a chemical compound which helps the solder to wet and flow as desired.

The solder alloy, when molten, will adhere to the items being joined if they are clean and heated above the solder melting point. Even though nearly all solder intended for electronic use has a core of flux material, you still can and should add flux to improve the result when needed.

There are two main alloy types of solder, leaded and lead-free. Leaded solder wets and flows more easily, requiring less heat, and is generally considered to be easier to work with. However, lead isn't good for humans or the environment, so lead-free solder was created as a safer alternative.

Lead-free solder has largely replaced leaded solder in electronic manufacturing applications, especially since laws mandating the reduction of hazardous substances went into effect in the early 2000's. However, there are still industries and applications in which leaded solder is used, as the bonds lead-free solder creates are weaker to external forces. For the purposes of soldering consumer electronics, lead-free solder is preferable.

Lead-free solder is frequently an alloy of tin (symbol Sn), silver (Ag) and copper (Cu). A common blend is known as SAC or 305. This contains 3% silver. It melts at a higher temperature (about 215-220°C) than leaded solder (183°C) and is a bit more difficult to work with, mainly because it melts over a temperature range and doesn't wet surfaces as readily. There are also alloys of just tin and copper which melt at a slightly higher temperature (227°C), these have even poorer wetting ability.

Leaded solder actually contains more tin than lead. The easiest to use is solder containing 63% tin and 37% lead usually known as 63-37 solder. This is the eutectic mixture (meaning it has the lowest possible melting point) which melts like a pure metal at a fixed temperature.

The lower melting temperature (183°C) and superior wetting properties make 63-37 easier to use. This is the solder alloy of choice for performing repairs. The lower melting temperature is less risky to nearby components. The downside is the lead content which is poisonous.

One other common alloy is 60-40 which melts at a slightly higher temperature and not quite as smoothly.

Other alloys used in electronic soldering include Bismuth (Bi) based alloys and Indium (In) alloys. Both of these types are for low temperature applications. The numbers indicate the percentages of each metal in the alloy.

Generally you would encounter these alloys in soldering Surface Mount Devices (SMD). Bismuth-Tin (58Bi42Sn) and Bismuth-Tin-Silver (57Bi42Sn1Ag) melt at 138°-140°C. They can be used for low temperature soldering. Sometimes they are melted into existing joints to lower the melting point for easier repair.

Solders containing Indium metal are also available with very low melting points such as an Indium-Tin alloy (52In48Sn) melting at 118°C.

Most solder alloys are available in wire form with a flux core. Alternatively, they are also available as pastes and as spheres.

The most common form is a wire with flux as a central core. The wire is available in a wide variety of diameters ranging from 0.010" (0.25mm) to 0.125" (3.125mm). The size of the joint and the heat source being used, determine the appropriate diameter.

Made up of small solder spheres in a flux base, many solder pastes can be applied with a syringe. It is usually used for soldering surface mount devices.

These are tiny spheres of uniformly sized solder that are used for special purposes such as renewing the solder on chips with ball grid array (BGA) connections.

While the solder is the material that makes the joint, flux is essential to making the solder flow and stick.

Soldering flux is a chemical compound, typically composed of rosin (derived from pine sap) and a reactive component known as an activator. It's main purpose is to remove oxidation, which helps it bond to other surfaces and reduces the surface tension of molten solder.

Because solder will not stick to metal with a layer of oxide, and because oxide layers form more readily when hot, flux serves to both clean and protect the surfaces being joined. The flux must be heated to its activation temperature, around 140°C, to clean this oxide layer and coat the joint to keep it from re-oxidizing.

Flux also promotes wetting, allowing solder to flow more readily into the small gaps between components. This creates bonds with more surface area, which are stronger and more conductive as a result. If you've ever had the issue of your solder melting into a more pasty consistency, adding more flux is likely the solution to get your solder to a more liquid state.

Rosin is the most common flux base you will encounter. There are also synthetic resin fluxes that allow other flux properties to be tailored.

Common activators are Abietic acid (naturally occurring in pine sap), halides, and amines, which can be either acidic or basic. Their main purpose is to undergo a chemical reaction with the oxidized metal that forms on the surface of the components that are exposed to air. These activators will either reduce metal oxides back into pure metal, or form soluble metal chlorides or metal salts, which can be dissolved or absorbed by the flux. The flux then serves as a protective barrier between the metal and air to prevent further oxidation before the solder can create a bond.

Tacky/gel fluxes are easy to apply and stay put and are commonly used for soldering surface mount components. They are a good all around flux form for making soldered connections.

Keep in mind that too much flux is nearly always better than too little. You can clean up excess flux, but you can't readily get a good-quality solder joint with too little flux.

Fluxes come in paste, tacky (gel), and liquid forms. They are also found as cores in solder wire. The solids content usually inversely corresponds to the flowability. Paste (highest solids), then gel/tacky, then liquid (lowest solids).

Some pastes can be applied with a stiff brush.

Tacky fluxes are often applied with a syringe and can be used to hold small components in position for soldering (hence tacky).

Liquid fluxes are applied with a soft brush or a flux pen and are easy to apply to multiple connections at once.

Flux has several characteristics that are noted in descriptive type designators. Among these:

• Flux Base or Vehicle (what the main ingredient is)
  • Rosin (R) Very good general purpose material. Mainly used for rework/repair.
  • Resin (RE or RES) Alternative to Rosin.
  • Water Soluble - Usually more aggressive and almost always needs cleaning after application.
    • Inorganic (IN)
    • Organic (OR)
• Activation is the degree to which a flux aggressively removes oxide; too much isn't necessarily better, as corrosion usually increases with activation. The activator does the cleaning. Some vehicles, like rosin, have built-in acidic activators.
  • Low/None (L)
  • Medium/Mildly Activated (M)
  • High/Activated (H)
• Halide - This helps with oxide removal but adds to the toxicity of the smoke and vapors as well as requiring cleaning. Fewer fluxes contain halides now.
  • No Halide (0) at the end of the designator.
  • Halide (1) at the end of the designator.

A flux pen with no-clean flux

You may see flux represented as no-clean. This isn't another type of flux, but has to do with the residue the flux leaves after the soldering process. No-clean means that the residue left behind by the flux is unlikely to cause issues with the components on a PC board.

Corrosion is the main item evaluated in clean/ no-clean designations. Flux residue may still be conductive. Frequently, the amount of flux base in the no-clean is reduced, so less potentially conductive residue is left.

When printed circuit boards are tightly packed with components and may be nearly impossible to clean, no-clean is an advantage. Sometimes you are making a repair and the no-clean is a great help, too. Still, if you can, you are generally better off cleaning the flux residue off the board, no-clean or otherwise.

The main consumable is copper desoldering braid. It is made of very fine copper wires braided together to make a ribbon like shape. Usually it has a dry flux already applied. When held against a joint and heated, the solder from the joint will wick into the braid and allow components to be removed. You may be able to improve performance by putting some additional flux on the joint itself and then using the braid.

You should always have some desoldering braid on hand. It comes in several widths and may even have a flux already in the braid. Mistakes happen and desoldering braid is a convenient way to undo them.

While not strictly a consumable, as an aside, a solder sucker / vacuum desolderer is handy for removing larger components which may be attached by larger amounts of solder.

When you solder, especially on printed circuit boards, you should clean up the flux and flux residue (even from "no-clean" fluxes). You can use several things to do this. For dissolving the flux:

• 91-99% Isopropyl Alcohol is right up there as an almost universal flux solvent. It's the most commonly used since it is readily available.
• Flux remover is probably the ultimate choice. If you use it, make sure to use a flux remover that will work with your current flux.
• For really stubborn cleaning, you can use acetone, but beware. Acetone can dissolve many different plastics which you didn't want dissolved. It will often remove markings on the circuit board and on the chips themselves, which can lead to problems. It's best to test it first to see if the board will swell or otherwise be damaged.
• Distilled or deionized water is great for any flux that's water soluble; in most cases, other cleaners won't work or will work poorly on those fluxes.

When cleaning, you can use cotton swabs (the kind on long wood sticks, not the regular kind for ear cleaning) or lint-free wipes. Don't skimp.

This isn't how to use your soldering iron more effectively. The tips of your soldering iron are replaceable unless the iron is inexpensive. The replaceable tips serve two purposes.

1. They allow you to change tips so you can use a tip shape best suited to the job if other shapes are available.
2. They allow you to replace worn-out tips.

Most soldering irons have an iron-plated copper tip. Tips wear out mainly because overheating leads to oxidation that damages the iron plating. Once the plating is damaged, the tip must be discarded. Sadly, the whole unit must be discarded if your iron doesn't have replaceable tips.

If possible, you should have two to three kinds of tape on hand.

• Painter's masking tape is very useful for low heat jobs like holding through hole components in place or light duty heat shielding.
• Polyimide tape (often called Kapton™) for heat shielding when you want to heat a certain area only. The usual upper limit is 260°C but it can work even to 400°C.
• Aluminum foil tape can also work as a heat shield and it's thickness makes it durable.

The main function of these tapes is protection.

• They help you avoid melting things you don't want melted, like plastic connectors or other components
• They can protect heat sensitive components like integrated circuits.

Soldering Glossary of Terms

How to Solder and Desolder Connections

Soldering Iron Not Melting Solder

Soldering Iron Not Heating

Soldering Best Practices

Soldering Iron Tips

More Information on flux-good read

More information on lead-free solder - Good stuff

More information on leaded solder