Even with the same iron, the feel of soldering changes completely with the solder and flux. Practicing with materials that spread smoothly and make thin joints builds skill fast, while old solder or an unsuitable alloy gives cold joints no matter how much you practice. This article covers the types of solder alloy, the thickness, the role of flux and cleaning, and storage and usage habits.
The three elements of soldering
Soldering is a combination of solder (an alloy), flux, and heat. The iron supplies the heat (How to Choose a Soldering Iron), the solder fills the joint, and the flux strips the oxide film from metal surfaces so the solder spreads well.
Alloy types
| Type | Typical composition | Melting point | Characteristics |
|---|---|---|---|
| Leaded (eutectic solder) | Sn63/Pb37 | 183°C | Easy to work, smooth joints. Contains lead |
| Leaded | Sn60/Pb40 | 183–190°C | General electronics work, sets over a slight range |
| Lead-free (SAC) | Sn96.5/Ag3.0/Cu0.5 (SAC305) | 217–220°C | The most widely used lead-free alloy |
| Lead-free (Sn-Cu) | Sn99.3/Cu0.7 | 227°C | Cheap, needs a high working temperature |
- Leaded solder melts well at low temperatures and the joint surface is shiny, so it is easy for beginners to judge. But it contains lead, so you have to be careful in handling and disposal.
- Lead-free solder is used in products that follow RoHS rules (limits on hazardous substances in electronics), needs a higher working temperature, and a matte surface is normal.
- If you make products that you sell or that are subject to regulation, you must use lead-free, but for personal hobby use it is a matter of choice. For work that young people will touch, or where ventilation is hard, lead-free is safer.
- Mixing leaded and lead-free on one board can weaken the joints, so if possible stick to one type.
Thickness
| Diameter | Use |
|---|---|
| 0.3–0.5 mm | Surface mount (SMD), very small pads |
| 0.6–0.8 mm | Perfboard, general through-hole parts (most used) |
| 1.0 mm+ | Large terminals, thick wire, connectors |
Thick solder puts out a lot at once and bridges easily, and solder that is too thin is slow on large pads. For a first purchase, 0.8 mm with built-in flux is a safe choice.
The role and types of flux
When heated, flux dissolves and strips the oxide film on metal surfaces and keeps it from oxidizing again until soldering is done. Without flux, solder just balls up and will not stick.
- Built-in flux (rosin core): 2 to 3% is inside the solder wire. The most common.
- Rosin flux: pine resin based, safe and effective. It gives off a mild smoke when heated that can irritate the airways, so ventilation is needed.
- No-clean flux: little residue and does not have to be wiped off, but wiping makes it cleaner.
- Water-soluble flux: strongly active but must be washed off with water. Leftover causes corrosion.
- Flux pen or paste: applied additionally on pads where solder does not stick well, when using solder wick, or when replacing parts. When using outside flux, check that it is a non-corrosive electronics type.
Cleaning flux residue
It is good to wipe off the brown residue left on the board after soldering.
- Dip a brush (such as a toothbrush) in isopropyl alcohol (IPA, 90% or more) and scrub.
- Wipe with a cotton swab or dry cloth and let it dry completely.
- If you used water-soluble flux, wash with water and dry thoroughly.
Often it makes no difference to operation if you do not clean, but in high-impedance circuits or humid environments the residue can create leakage current.
Storage and handling
- Storage: keep solder reels in a dry place, and if the surface oxidizes and clouds, wipe it before use. Old solder whose flux has dried out will not solder, so replace it with new.
- Wash your hands: especially after touching leaded solder, always wash your hands before eating or drinking. The amount of lead that vaporizes at soldering temperature is very small, and the main risk is the path of lead on hands reaching the mouth.
- Do not eat or drink at the bench.
- Disposal: follow local separate-collection guidance for solder scraps and used solder wick, not ordinary trash.
- Children and pets: keep solder reels and small pieces out of reach.
Ventilation and respiratory protection are covered in detail in Soldering Safety.
Buying tips
- A 100 g reel is enough for a beginner, and 500 g is economical for someone who solders a lot.
- Choose genuine products that state the alloy composition and flux content. Very cheap products with no manufacturer marking may have low purity and solder poorly.
- Keeping a flux pen and solder wick separately makes corrections easier.
Summary
For perfboard, 0.6–0.8 mm flux-core solder is the safe choice; leaded (Sn63/Pb37) is easy but needs care in handling lead, and lead-free (SAC305) is safer but needs a higher temperature. Use extra flux when needed and wipe with IPA after soldering. If a fault appears after soldering, see Desoldering and Rework.