Solder Alloys, Flux and Paste
Leaded and lead-free alloys, melting ranges, wire sizes, flux types and activity, solder paste and how much to use.
Solder alloys
Solder is named by its composition in weight percent. The choice sets the melting temperature, the strength, the wetting and the legal status of the product. Alloys marked “lead-free” are required for most products sold in the EU, the UK and many other regions (RoHS) but are not required for hobby work, repair or some exempt uses.
| Alloy | Melting (°C) | Use | Notes |
|---|---|---|---|
| Sn63/Pb37 | 183 (eutectic) | Hand soldering, repair, hobby | Easiest; shiny joints; no pasty range |
| Sn60/Pb40 | 183 to 190 | General hand soldering | Almost as good as 63/37; small pasty range |
| Sn62/Pb36/Ag2 | 179 | Silver-plated parts, ceramics | Stops silver leaching from plated terminals |
| SAC305 (Sn96.5/Ag3/Cu0.5) | 217 to 220 | Industry standard lead-free | Good strength and fatigue; needs more heat |
| SAC0307 (Sn99/Ag0.3/Cu0.7) | 217 to 228 | Low-cost lead-free | Less silver, wider pasty range |
| Sn99.3/Cu0.7 | 227 | Wave soldering, hobby lead-free | Cheap; duller joints; hot |
| Sn96.5/Ag3.5 | 221 | Strong joints, high temperature | Eutectic; used in electronics and plumbing |
| Sn42/Bi58 | 138 | Low-temperature, heat-sensitive parts | Brittle; do not mix with lead |
| Sn5/Pb95 | 308 to 312 | Step soldering, high temperature | First pass before a second, cooler solder |
| Lead-free silver-bearing for plumbing | 221 to 230 | Potable water pipes | Use the plumber’s alloy and flux, not electronics solder |
Melting points are typical: check the data sheet. Never use acid-core (plumbing) solder or flux on electronics: it corrodes the circuit.
Leaded or lead-free
Lead-free solder is a legal and environmental requirement, not a technical improvement for hand soldering. It melts about 35 °C higher, wets less readily and gives duller joints, so the tip runs hotter, the flux is exhausted faster and pads are more at risk. Leaded solder remains the easier choice for a beginner, for repair of older leaded boards and for work not covered by RoHS, but its use needs lead hygiene (see Soldering Safety, Fumes and ESD).
| Property | Sn63/Pb37 | SAC305 |
|---|---|---|
| Melting | 183 °C | 217 to 220 °C |
| Typical tip temperature | 320 to 350 °C | 350 to 380 °C |
| Wetting | Excellent | Good; needs more active flux |
| Joint appearance | Bright, smooth | Duller, grainier (still sound) |
| Tip life | Long | Shorter (more tin attack) |
| Tin whiskers | Very rare | Risk in some finishes (see Joint Quality, Inspection and Failure) |
| Legal in new EU products | No (RoHS), with exemptions | Yes |
Do not mix the two on a board you plan to certify: lead contamination of lead-free joints or the reverse can weaken them. With bismuth solders the mixing is a real hazard (brittle low-melting phase).
Worked example: the extra heat of lead-free
- Tip margin above melting at a typical setting: Sn63 at 350 °C is 167 °C over 183 °C; SAC305 at 380 °C is 161 °C over 219 °C. The lead-free margin is about the same although the tip is 30 °C hotter, so lead-free needs the hotter tip just to match the speed.
- The same joint at the same tip temperature of 350 °C: leaded margin 167 °C, lead-free only 131 °C, i.e. about 78 % of the thermal drive, so a joint that took 2 s with leaded solder takes about 2.5 s with lead-free.
Solder wire: diameter, core and quantity
Hand-solder wire is hollow: its core is filled with flux (typically 1 to 3 % by weight), so the flux arrives with the solder. Choose the diameter to match the joint: 0.5 mm for fine SMD work, 0.8 mm general, 1.0 to 1.5 mm for large pads and wires. A very thin wire is easy to control but slow; a thick wire floods small joints.
Worked example: how much wire is a joint?
- A typical through-hole fillet and barrel fill takes about 2.7 mm³ of solder.
- Wire length = volume ÷ cross-sectional area. For 0.8 mm wire the area is π/4 × 0.8² = 0.50 mm², so the length is 5.4 mm (about 5 mm).
- That is why a joint takes a short, single “touch” of wire: feeding 20 mm leaves a blob.
| Wire diameter | Area | Length for 2.7 mm³ |
|---|---|---|
| 0.5 mm | 0.20 mm² | 13.8 mm |
| 0.8 mm | 0.50 mm² | 5.4 mm |
| 1.0 mm | 0.79 mm² | 3.4 mm |
| 1.5 mm | 1.77 mm² | 1.5 mm |
The solder is fed to the joint, not to the iron tip, and the flux in the core is used up on contact: never reuse the same inch of wire across an oxidized surface.
Flux: types, activity and cleaning
The flux is chosen for the oxide it has to clear and for what it leaves behind. The IPC J-STD-004 classification names it by base (RO rosin, RE resin, OR organic, IN inorganic) and activity (0 low, 1 moderate, 2 high), with L, M, H for halide content; for example ROL0 is a low-activity rosin with no halide.
| Flux type | Activity | Residue | Cleaning | Use |
|---|---|---|---|---|
| Rosin (R) | Mild | Amber, non-corrosive, sticky | Optional; alcohol removes it | Hobby work, wire |
| RMA (mildly activated) | Moderate | Small amount, benign | Optional | Electronics repair |
| RA (activated) | High | Corrosive if left | Must be cleaned | Oxidized parts |
| No-clean | Moderate | Small, benign, hard | Not required | Production, paste |
| Water-soluble (organic) | High | Corrosive if not washed | Wash in warm water or DI water | Lead-free, tough oxides |
| Acid (plumbing) | Very high | Highly corrosive | Not for electronics | Pipes, sheet metal only |
- Residue is not harmless in every case: no-clean flux is benign when fully heated, but a flux that never reached temperature stays active and can corrode or leak current in damp conditions. Clean boards that go into high-impedance, high-voltage, conformal-coated or humid service.
- Cleaning: isopropyl alcohol (IPA) 90 % or higher and a stiff brush for rosin; for water-soluble flux, warm water and a soft brush, then dry. Use gloves and ventilation; IPA is flammable.
- Extra flux: a pen or gel flux on a joint that is hard to wet, on rework and on drag soldering fine-pitch parts is almost always the cure for dull or bridged joints.
Solder paste
Solder paste is a mixture of tiny solder spheres (the powder) in a flux medium. It is printed or dispensed on the pads, the parts are placed in it and the board is reflowed (see Reflow Profiles, Ovens and Hot Plates) so that all the joints form at once. It sticks the parts in place until the solder melts.
| Powder type | Sphere size | Typical use |
|---|---|---|
| Type 3 | 25 to 45 µm | Pitch 0.65 mm and larger; general |
| Type 4 | 20 to 38 µm | Pitch 0.4 to 0.5 mm; 0402 parts |
| Type 5 | 15 to 25 µm | Pitch 0.3 mm and under; 01005, micro BGAs |
| Syringe / jar | any | Syringes dispense by hand; jars for printing through a stencil |
Worked example: how much of the paste is metal?
- A typical paste is 88.5 % metal by weight. SAC305 powder has a density of about 7.4 g/cm³ and the flux medium about 1.0 g/cm³.
- Volume of metal per 100 g of paste = 88.5 ÷ 7.4 = 12.0 cm³; volume of flux = 11.5 ÷ 1.0 = 11.5 cm³.
- Metal by volume = 12.0 ÷ (12.0 + 11.5) = 51 %: about half the printed paste volume becomes solder, the rest evaporates or is left as flux residue. That is the reason a printed paste deposit is roughly twice the size of the final joint.
- The paste density is therefore about 4.3 g/cm³.
Stencils and the paste volume
A stencil is a thin steel foil (usually 0.10 to 0.15 mm) with apertures at each pad. Paste is wiped across it, leaving a precise deposit on every pad. Two rules govern whether the paste releases cleanly from the aperture: the area ratio (aperture area ÷ wall area) should be at least 0.66, and the aspect ratio (aperture width ÷ foil thickness) at least 1.5.
Area ratio = (L × W) ÷ (2 × (L + W) × T)
| Part | Aperture | Foil | Area ratio | Paste volume | Metal in it |
|---|---|---|---|---|---|
| 0402 | 0.50 × 0.55 mm | 0.10 mm | 1.31 | 0.03 mm³ | 0.01 mm³ |
| 0603 | 0.80 × 0.90 mm | 0.12 mm | 1.76 | 0.09 mm³ | 0.04 mm³ |
| 0805 | 1.00 × 1.30 mm | 0.12 mm | 2.36 | 0.16 mm³ | 0.08 mm³ |
| 1206 | 1.20 × 1.70 mm | 0.12 mm | 2.93 | 0.24 mm³ | 0.12 mm³ |
| QFP 0.5 mm pitch | 0.25 × 1.50 mm | 0.12 mm | 0.89 | 0.05 mm³ | 0.02 mm³ |
Worked example: paste for an 0805 resistor pad
- Aperture 1.00 × 1.30 mm, foil 0.12 mm: volume = 1.00 × 1.30 × 0.12 = 0.16 mm³ of paste.
- Metal = 0.16 × 0.51 = 0.08 mm³ of solder per pad, assuming perfect release. With a typical 75 % transfer efficiency the joint gets about 0.06 mm³.
- Area ratio = 1.30 ÷ (2 × 2.30 × 0.12) = 2.36: good.
- A 0.5 mm pitch QFP with a 0.25 × 1.50 mm aperture on a 0.12 mm foil has an area ratio of 0.89: still above 0.66, but a 0.15 mm foil would drop it to 0.71, so fine pitch needs a thin (0.10 to 0.12 mm) foil.
Worked example: how many boards from one jar?
- A board with 380 pads averaging 0.10 mm³ of paste each uses 38 mm³ = 0.04 cm³ of paste, i.e. 0.16 g at 4.3 g/cm³.
- Add 15 % for waste on the stencil: 0.19 g per board. A 500 g jar covers about 2 684 boards; a 10 g syringe covers about 54 boards.
Stencil use in prototyping and ordering stencils are covered in PCB Prototyping and the pads themselves in PCB Design and Layout.
Storage and handling
- Paste: keep it refrigerated at 0 to 10 °C, let it reach room temperature unopened (a few hours) before use so that moisture does not condense into it, and stir gently for a minute. Check the jar’s shelf life and print life; never return used paste to the jar.
- Wire: keep it clean and dry in its reel; oxidized or fingerprinted wire wets poorly.
- Flux pens and gels: cap them, because the solvent evaporates and leaves a sticky, ineffective residue.
- Safety data: keep the sheets for the flux and the solder (see Soldering Safety, Fumes and ESD).
Troubleshooting
| Symptom | Likely cause | What to do |
|---|---|---|
| Wire spits and splatters | Too hot, or wet flux core meeting a hot iron | Lower the tip temperature; feed solder to the joint, not the iron |
| Joints are dull | Lead-free solder, or disturbed joint | Normal for lead-free if smooth; reheat with fresh flux if rough |
| Solder balls around the pads (paste) | Paste slumped, moist or reflow ramped too fast | Dry the board, check the ramp, new paste |
| Paste will not release from the stencil | Area ratio too low, paste dried out | Thinner foil, bigger aperture, fresh paste |
| Flux residue is sticky or white | Flux not fully heated or not cleaned | Reheat or clean with IPA |
| Joint corrodes later | Active flux left on the board, acid flux used | Clean thoroughly; never use plumbing flux on electronics |
Related
- Soldering: the overview of this guide.
- How Soldering Works: wetting and the pasty range.
- Surface-Mount Soldering and Reflow Profiles, Ovens and Hot Plates: where paste is used.
- PCB Prototyping: ordering stencils.
FAQ
Which solder should a beginner use?
Leaded Sn63/Pb37 or Sn60/Pb40 rosin-core wire of 0.8 mm is the easiest, if it is legal for your use and you follow lead hygiene. Otherwise use a good SAC305 or Sn99.3/Cu0.7 with a flux pen.
What is the best flux?
A no-clean or RMA flux for general work; an extra flux pen helps with oxidized or lead-free work. Avoid acid flux on electronics.
Do I need to clean flux off?
Not for no-clean flux that has been fully heated and for normal low-voltage work. Clean rosin and all active fluxes from high-impedance, high-voltage, coated or humid-service boards.
How much solder does a joint use?
About 2.7 mm³ for a through-hole joint: roughly 5 mm of 0.8 mm wire. An 0805 pad uses about 0.08 mm³ of solder (0.16 mm³ of paste).
How long does solder paste last?
Typically 6 months refrigerated and a day or two out of the fridge once opened; check the label. Paste that is dry or has separated should be discarded.
References
- IPC J-STD-004, Requirements for Soldering Fluxes; J-STD-005 (solder paste); J-STD-006 (electronic grade solder alloys).
- IPC-7525, Stencil Design Guidelines (area and aspect ratios).
- Directive 2011/65/EU (RoHS); Klein Wassink, Soldering in Electronics.
Last reviewed: October 2026. Alloy melting points, flux classes and stencil rules are typical: follow the manufacturer’s data sheets.