What is 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.
Reflow soldering explained
Reflow soldering is the core process of SMT assembly. A stencil is used to print solder paste — a mixture of small solder particles and flux — onto the pads. Pick-and-place machines then position the components on the wet paste, which holds them in place. The board is conveyed through a reflow oven with several heating zones and a cooling zone.
The temperature profile is usually described in four stages. In preheat, the board is warmed gradually to limit thermal shock. In the soak or thermal-soak stage, temperatures across the board even out and the flux activates to remove oxides. In the reflow stage, the temperature rises above the liquidus of the solder alloy so the solder melts and wets the surfaces; the time above liquidus is controlled. In the cooling stage, the joints solidify, and the cooling rate affects joint structure.
Common lead-free alloys, such as SAC305, melt at around 217–220°C, so peak temperatures are higher than for traditional tin-lead solder, which melts at 183°C. The profile must suit the paste, the alloy, the board thickness and the components; manufacturers check it with thermocouples on a representative board.
Most reflow is convection-based, with forced hot air. Vapour-phase reflow uses a boiling heat-transfer fluid to give very even heating, which can help on heavy or complex boards. Ovens may use a nitrogen atmosphere to reduce oxidation. Double-sided boards are reflowed twice, so components on the first side must tolerate a second pass.
When reflow soldering matters
Reflow soldering affects almost every SMT assembly, so it matters most when a design pushes the process: large boards with very different thermal masses, fine-pitch parts, BGAs and QFNs, double-sided assembly, or heat-sensitive components. A profile that suits small passives may underheat a large BGA or connector, while a profile that is hot enough for those may overheat sensitive parts.
Typical reflow defects include tombstoning, where a small chip component stands up on one end; solder bridging between pads; solder balls; voids under BGAs and thermal pads; head-in-pillow on BGAs; and insufficient wetting. Many of these trace back to design choices such as unbalanced pad sizes, uneven copper around pads, poor stencil aperture design or components placed in thermal shadows.
Moisture-sensitive components must be stored and baked according to their moisture sensitivity level, otherwise trapped moisture can crack packages during reflow. Board materials with a low Tg can also warp at lead-free temperatures.
Reflow vs wave vs selective soldering
Reflow is the standard for SMT. Wave soldering suits boards with many through-hole joints. Selective soldering is used when through-hole parts must be soldered on a board that already carries SMT parts on the bottom side.
Reflow soldering in your quote request
In your quote request, include the BOM and pick-and-place file and note any components with special thermal limits, moisture sensitivity or a required reflow profile from the datasheet. If your design is double-sided, state which parts must go on which side. Mention if you need lead-free or tin-lead assembly. Our free DFM check flags footprint and pad issues that often cause reflow defects, such as unbalanced pads on small passives.
Need boards assembled? Send your Gerber files and BOM for a free turnkey quote from the right assembly partner.
PCB assembly →FAQ about reflow soldering
At what temperature does solder reflow?
Solder reflows when it is heated above its melting point. Traditional tin-lead solder (Sn63/Pb37) melts at 183°C, while common lead-free alloys such as SAC305 melt at around 217–220°C. The oven peak is set somewhat above the melting point to ensure complete wetting, and the exact profile depends on the paste manufacturer's recommendations and the board.
Which is better, reflow or wave soldering?
Neither is better in general; they suit different components. Reflow soldering is the standard for surface-mount components and gives precise control of each joint through printed solder paste. Wave soldering is efficient for boards with many through-hole components. Many mixed-technology boards use reflow for the SMT parts and wave or selective soldering for the through-hole parts.
What causes tombstoning in reflow soldering?
Tombstoning happens when the solder on one end of a small chip component melts and wets before the other, so surface tension pulls the component upright. Common causes are unequal pad sizes, uneven copper or trace connections that make one pad heat more slowly, uneven paste deposits and placement offset. Balanced pad design and thermal relief reduce the risk.
Related terms
- 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.
- Wave soldering
Wave soldering is a bulk soldering process in which an assembled PCB is passed over a standing wave of molten solder, soldering all exposed through-hole joints on the underside in one pass. It is mainly used for through-hole components and can also solder certain glued SMT components on the bottom side of the board.
- Selective soldering
Selective soldering is an automated through-hole soldering process that solders specific joints one at a time using a precision nozzle of molten solder. Unlike wave soldering, it requires no fixtures and only applies flux to targeted joints. Ideal for mixed-technology boards with SMT on both sides and few THT components.
- 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.