Solder paste lets you solder tiny surface-mount parts that are too small to do well with a soldering iron and wire. You apply the grey paste to the board’s pads, place the components on top, then heat the board so the paste melts and forms every joint at once. This guide explains what solder paste is, how to use it step by step, whether you can use it with a soldering iron, and how to store it safely.
Two things called “soldering paste”
Solder paste is tiny solder powder mixed with flux, used to reflow surface-mount (SMD) parts. Flux paste (also loosely called soldering paste) is just flux, used to help a soldering iron and solder wire flow. This guide covers both, starting with solder paste.
What Is Soldering Paste?
Solder paste is a grey, putty-like mixture of microscopic solder balls suspended in flux. When heated, the flux cleans the metal and the solder powder melts and joins together into a normal solder joint. It is the standard way surface-mount components are attached, both in factories and on the hobby bench.
Because the term is used loosely, it helps to separate the two products:
| Product | What it is | Used for |
|---|---|---|
| Solder paste | Solder powder + flux | Reflowing SMD parts onto pads |
| Flux paste (soldering paste) | Flux only, in paste form | Helping an iron and wire wet and flow |
Both come in a tub or a syringe. Solder paste actually deposits metal; flux paste does not, it only improves wetting. Knowing which you have matters, so check the label.
What Is Solder Paste Used For?
Solder paste is used wherever components are too small or too numerous to hand-solder pin by pin:
- Surface-mount assembly, attaching SMD resistors, capacitors, ICs and connectors to a PCB.
- Fine-pitch and BGA parts, chips with many tiny pads or hidden pads underneath, which cannot be soldered with an iron at all.
- Small-batch and prototype boards, where applying paste and reflowing is far faster than soldering every joint by hand.
- Rework, replacing a surface-mount chip using fresh paste and hot air.
For normal through-hole parts and wires, you do not need solder paste, ordinary solder wire and an iron are simpler.
How to Use Solder Paste (Step by Step)
The standard method applies paste to the pads, places the parts, then heats the whole board so every joint forms at once. This is called reflow.
- Apply the paste to the pads. Use the syringe to put a small dot on each pad, or spread it through a stencil for many pads at once. Aim for a thin, even amount, too much causes bridges.
- Place the components. Set each part onto its pads with tweezers. They do not need to be perfect, surface tension pulls them into place as the paste melts.
- Heat the board (reflow). Warm the board with a hot air rework station, a hotplate or a reflow oven. The paste turns shiny and liquid as the solder melts, then the parts settle.
- Let it cool, then inspect. Do not move the board while the solder is still molten. Once cool, check each joint for bridges or gaps with a magnifier.
Heat gently and evenly
With hot air, keep the nozzle moving and pre-warm the whole area so parts do not blow around or overheat. Leaded paste melts around 183°C and lead-free around 217°C, but you heat a little above that to let the joints reflow fully.
Can You Use Solder Paste With a Soldering Iron?
Yes, for small jobs, though solder paste is designed for reflow heating rather than an iron. To use it with a soldering iron:
- Apply a little paste to the pad or joint from the syringe.
- Place the part, holding it in position with tweezers.
- Touch the iron to the pad, not the paste directly, so the heat flows in and melts the paste into a joint. Add a touch of solder wire if a joint looks starved.
This works for a handful of SMD joints. For more than a few parts, or for fine-pitch and BGA chips, hot air reflow is faster and far more reliable. A controlled iron helps, see how to choose a soldering station.
Using Flux Paste and Tinning Paste
The other “soldering paste” is flux paste, sometimes sold as tinning or soldering paste. It contains no solder powder, just flux, and it is used alongside an iron and solder wire to improve wetting:
- Reviving difficult joints, a dab of flux paste helps solder flow on oxidised pads and old joints.
- Drag-soldering rows of pins, flux paste lets solder run cleanly along fine-pitch legs.
- Tinning tips and wires, some tinning pastes help coat a tip or wire with a fresh layer of solder.
Use a small amount, apply it to the joint, then solder as normal with your iron and wire. Clean off any sticky residue afterwards. For electronics, only use rosin or no-clean flux paste, never an acid or plumbing paste.
Storage and Safety
- Keep solder paste cool. Most solder paste is stored in a fridge (around 2 to 10°C) and has a limited shelf life. Let it return to room temperature before use, and stir gently if it has separated.
- Mind the lead. Leaded paste contains lead, so wash your hands after use and keep it away from food. Lead-free paste avoids this but still needs care.
- Work in ventilation. The flux gives off fumes when heated. Solder in a ventilated space, ideally with a fume extractor.
- Avoid skin and eye contact with the paste, and follow the safety information on the product.
Watch: How to Use Solder Paste
A short demonstration of applying solder paste and reflowing surface-mount parts:
Frequently Asked Questions
How do you use solder paste?
Apply a small amount to the board’s pads with a syringe or stencil, place the surface-mount components on top, then heat the board with hot air, a hotplate or a reflow oven until the paste melts and forms the joints. Let it cool before moving it.
What is solder paste used for?
Soldering surface-mount (SMD) components onto circuit boards, especially fine-pitch and BGA chips that cannot be soldered well with an iron. It is also used for rework and small-batch assembly.
Can you use solder paste with a soldering iron?
Yes, for a few small SMD joints: apply the paste, place the part, then heat the pad with the iron so the paste melts. For more than a handful of parts, or fine-pitch chips, hot air reflow is faster and more reliable.
What is the difference between solder paste and flux paste?
Solder paste contains solder powder plus flux and actually forms joints when heated. Flux paste contains only flux and adds no solder, it just helps an iron and solder wire wet and flow.
Does solder paste need a stencil?
No. A stencil speeds up applying paste to many pads at once, but for a few parts you can apply it straight from the syringe, one dot per pad.
How do you store solder paste?
Keep it cool, usually refrigerated at around 2 to 10°C, and use it within its shelf life. Bring it back to room temperature before use and stir gently if it has separated.