What is solder paste?
Solder paste is a mixture of tiny solder alloy spheres suspended in flux, printed onto PCB pads before surface-mount components are placed. During reflow the solder melts and forms the joints, while the flux removes oxides and helps wetting. Paste is specified by alloy, powder size and flux type.
Solder paste explained
Solder paste is the material that makes SMT assembly possible. By weight it is mostly metal — typically around 85–90% — in the form of small spherical solder particles. The rest is flux: a blend of activators, resins or rosins, solvents and thickeners that gives the paste its sticky, printable consistency.
Paste is applied through a stencil, a thin metal foil with apertures that match the pads. A squeegee pushes the paste across the stencil and through the apertures, leaving a controlled deposit on each pad. The volume of each deposit depends on the aperture size and the stencil thickness. Many production lines then measure the deposits with solder paste inspection (SPI) before placement.
After printing, components are placed into the wet paste, which holds them in position by its tackiness. The board then goes through reflow soldering: the flux activates and cleans the metal surfaces, the solder melts and wets the pads and leads, and the joints form as the board cools.
Pastes are classified by three main properties. The alloy sets the melting point and joint properties; SAC305 (tin, 3% silver, 0.5% copper) is the most common lead-free alloy. The powder type, defined in IPC J-STD-005, sets the particle size. The flux type, classified in IPC J-STD-004, determines activity and whether residues are left on the board (no-clean) or must be washed off (water-soluble).
Solder paste is perishable. The flux and metal can react over time, and the solvents evaporate once the paste is exposed to air, so storage and handling follow the paste manufacturer's data sheet.
When solder paste matters
Solder paste matters on every SMT assembly, but it becomes critical on fine-pitch parts, BGAs and QFNs and small passives such as 0201s. Industry experience generally attributes a large share of SMT defects to the printing step, so paste choice and deposit volume have a direct effect on yield.
Too much paste causes bridges and solder balls; too little causes opens, weak joints and head-in-pillow on BGAs. Small apertures need a finer powder so that enough particles pass through cleanly. Thermal pads under QFNs often need a windowpane aperture pattern to limit voiding.
The flux type matters for reliability and later processes. No-clean residues are usually left on the board, but they may need to be removed before conformal coating or on high-impedance circuits. Water-soluble fluxes must be washed off completely. Mixing lead-free and tin-lead materials, or using an alloy that does not match the component finishes, can lead to unreliable joints.
Solder paste types and alloys compared
Finer powders print better through small apertures but oxidise faster. SAC305 melts at around 217–220°C, while Sn63/Pb37 melts at 183°C, so the reflow profile changes with the alloy. Tin-lead paste is easier to process at lower temperatures, but RoHS restricts lead in most products. Powder type and alloy are chosen together with the stencil design, because aperture size limits which powders print reliably.
Solder paste in your quote request
In your quote request, state whether the assembly must be lead-free or tin-lead, and note any required alloy or flux type, for example no-clean or water-soluble. If boards will be coated or used in high-impedance circuits, say whether flux residues must be removed. Mention any cleanliness requirement your product must meet. Our free DFM check flags pad and aperture-related issues, such as unbalanced pads on small passives, that often show up as paste and reflow defects.
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PCB assembly →FAQ about solder paste
What is solder paste used for?
Solder paste is used to attach surface-mount components to a PCB. It is printed onto the pads through a stencil, holds the components in place after placement and then melts in the reflow oven to form the solder joints. The flux in the paste cleans the metal surfaces so the molten solder can wet the pads and component terminations.
Is solder paste the same as flux?
No. Flux is one ingredient of solder paste. Solder paste is a mixture of solder alloy powder and flux, with the metal making up most of the weight. Flux on its own contains no solder; it only cleans oxides and improves wetting. Liquid or gel flux is used separately for rework, wave soldering and some hand-soldering tasks.
How should solder paste be stored?
Most solder paste is stored refrigerated, typically between about 0°C and 10°C, in sealed containers and used within the shelf life stated by the manufacturer. Before use it must reach room temperature in the closed container, usually over several hours, to prevent condensation. Paste should not be frozen, and opened paste has a limited working life on the stencil.
What do solder paste type numbers mean?
The type number describes the particle size of the solder powder, as defined in IPC J-STD-005. A higher number means finer particles: Type 3 is roughly 25–45 µm, Type 4 roughly 20–38 µm and Type 5 roughly 15–25 µm. Finer powders release better from small stencil apertures, so they are chosen for fine-pitch components and small passives.
Related terms
- Reflow soldering
Reflow soldering is the process used to solder surface-mount components to a PCB. Solder paste is printed onto the pads, components are placed on the paste, and the board passes through an oven that follows a controlled temperature profile. The paste melts, wets the pads and leads, and forms solder joints as it cools.
- SMT
SMT (Surface Mount Technology) is the dominant PCB assembly method where components are placed directly onto the surface of the board and soldered using reflow ovens. It enables smaller, denser, double-sided boards and supports nearly all modern component packages including BGAs, QFNs, and 0402 passives.
- Stencil
A stencil is a thin laser-cut steel sheet with apertures matching the SMT pads on the PCB, used to apply solder paste precisely before component placement. Typical thickness is 100-150 µm. Stencil aperture design affects solder paste volume and is critical for fine-pitch BGAs and QFNs to prevent insufficient solder or bridging.
- BGA and QFN
BGA and QFN packages are surface-mount IC packages whose solder joints sit underneath the component body. A BGA (ball grid array) connects through an array of solder balls on its underside, while a QFN (quad flat no-leads) has flat pads along its edges and usually a large exposed thermal pad, so joints cannot be fully inspected visually.
- PCBA
PCBA (Printed Circuit Board Assembly) is the process of populating a bare PCB with electronic components — resistors, capacitors, ICs, connectors, and other parts — to produce a functional electronic assembly ready for testing or integration into a finished product.
- RoHS compliance
RoHS compliance means the PCB and all its components meet the EU's Restriction of Hazardous Substances Directive (2011/65/EU), which limits 10 specified substances including lead, mercury, cadmium, and four phthalates. RoHS compliance is mandatory for electronic equipment placed on the EU market.