Soldering Flux and Solder Paste Explained
Solder does not stick to oxide, and every metal surface on a board has some. Flux is the chemistry that strips it off at the moment of soldering, and solder paste is flux and powdered alloy combined for reflow. Choosing the wrong one gives dull joints, poor wetting, residue problems and rework. This guide explains the families, how leaded and lead-free change the choice, and what we stock from Qualitek.
In this guide
1.What flux does
Flux is applied to the joint before and during soldering. It does four things: it dissolves the oxide layer on the pad and the component lead, it keeps new oxide from forming while the metal is hot, it lowers the surface tension of the molten solder so it wets and flows, and it carries heat into the joint. Without it, solder balls up and sits on the surface. Flux comes as the core of solder wire, as a liquid for wave and selective soldering, as a gel or paste for rework, in pens for touch-up, and as the vehicle in solder paste.
Fluxes are classified by their base and their activity. The base decides what the residue is like and how it is cleaned; the activity decides how much oxide it can remove and how aggressive the residue is if it is left behind.
2.The three flux families
Rosin flux is the traditional type, refined from pine resin, usually with mild activators (RMA). It wets well on most surfaces and its residue is non-corrosive but sticky and visible, so boards are normally cleaned with isopropyl alcohol. Use it for hand soldering, through-hole work and repairs to older assemblies.
No-clean flux uses a low-solids synthetic resin that leaves a thin, clear, non-corrosive residue designed to stay on the board. It is the standard for volume production, surface mount reflow and wave soldering because it removes a cleaning stage. The trade-off is lower activity, so surfaces need to be reasonably clean to start with.
Water-soluble flux is organic-acid based and highly active, so it wets difficult or oxidised surfaces that defeat other fluxes. Its residue is corrosive and conductive and must be washed off completely in deionised water. Use it where joint reliability is paramount and a wash line exists: medical, aerospace, automotive and military work.
3.Leaded or lead-free changes the choice
Tin-lead solder melts at 183°C; the common lead-free alloy SAC305 melts at about 217°C, and process temperatures are 30 to 40°C higher. More heat means faster oxidation of the joint and faster exhaustion of the flux, so lead-free fluxes carry more activity and a wider working temperature window. Using a leaded flux in a lead-free process gives grainy, dull joints with poor wetting. The reverse, a lead-free flux on tin-lead, usually works but leaves more residue than needed.
Qualitek supplies both: 392 is a no-clean flux for leaded processes, 392-35 its lead-free counterpart, and 737N a water-soluble flux for lead-free. If you are mid-way through converting a line, read Leaded vs Lead-Free Solder: What's the Difference? first.
4.Picking a flux for the job
| Application | Flux type | Why |
|---|---|---|
| Hand soldering and PCB repair | Rosin, or no-clean for convenience | Rosin cleans best and is forgiving; no-clean saves the wash |
| Surface mount reflow | No-clean (in the paste) | Minimal, inert residue; no cleaning stage |
| Wave and selective soldering | No-clean liquid flux, lead-free grade where needed | Sprayed or foamed, low solids, consistent residue |
| High-reliability assemblies | Water-soluble | Highest activity; residue fully removed in the wash |
| Oxidised or aged components and pads | Activated rosin or water-soluble | Needs more activity to wet tarnished surfaces |
| Lead-free hand soldering | Lead-free no-clean or activated rosin | Survives the higher tip temperature |
5.Flux versus solder paste
Flux contains no metal. It prepares the surface and is used alongside solder wire or preforms, or on its own for rework and reflow of parts that already carry solder, such as BGAs.
Solder paste is powdered solder alloy, typically 88 to 90% by weight, suspended in a flux vehicle that holds it in a printable, tacky paste. It is printed through a stencil or dispensed from a syringe onto the pads, components are placed into it, and the board goes through a reflow oven. The flux cleans the surfaces as the temperature rises and the powder melts into the joint. Paste is how surface mount assembly is done; it is also used for rework with a hot-air station, and dispensing pastes such as Qualitek 619D are formulated for syringe application rather than printing.
6.Alloys, powder types and the Qualitek range
Paste is specified by alloy, flux type and powder size. SAC305 (96.5% tin, 3% silver, 0.5% copper) is the standard lead-free alloy for RoHS production. Sn62/Pb36/Ag2 is the silver-bearing leaded alloy favoured for ceramic terminations and reliability work, and Sn63/Pb37 the eutectic tin-lead. Powder Type 3 (25 to 45 micron) suits most stencil printing; Type 4 (20 to 38 micron) prints finer apertures for 0402 and fine-pitch work; Type 5 is finer still for micro-BGA and dispensing.
From the Qualitek range we stock:
- 862 lead-free no-clean, SAC305, Type 3 and 4 and 825 lead-free no-clean, SAC305, Type 4
- 798LF water-soluble lead-free, SAC305 and 769LF water-soluble lead-free, SAC305, Type 3 and 4
- 818 no-clean leaded, Sn62/Pb36/Ag2 and 775-2 water-soluble leaded, Sn62/Pb36/Ag2
- 619D no-clean dispensing paste for syringe work
Cored wire follows the same logic: lead-free wire in no-clean (NC600, NC601) and water-soluble (WS700) cores, plus AIM Glow Core SN100.
7.Handling, thinners and cleaning
Liquid flux loses solvent to evaporation in an open wave or spray tank, which raises its solids content and changes how it behaves. Check specific gravity against the datasheet and bring it back with the matching thinner: Qualitek 300A or 700A depending on the flux. Never dilute with a generic solvent.
Solder paste is stored refrigerated and brought to room temperature, still sealed, before opening. Stir gently, keep stencil life within the datasheet figure, and do not return used paste to the jar.
Residue. Rosin residue comes off with isopropyl alcohol; water-soluble residue needs deionised water, ideally warm, and a dry; no-clean residue stays unless the assembly needs coating or probing. Residue that is left where it should not be is one of the causes covered in 10 Common Soldering Problems and How to Fix Them. Flux also builds up on the iron tip, so pair this guide with How to Clean & Maintain Soldering Tips.
