Reflow Soldering in SMT

Introduction: Reflow soldering is a process used in electronics manufacturing to permanently attach surface-mount components to a printed circuit board (PCB) using solder paste. This method offers high reliability, scalability, and precision, making it essential for modern SMT (Surface-Mount Technology) assembly.

 

What Is Reflow Soldering?

Reflow soldering uses a reflow oven or reflow soldering machine to provide a controlled heating environment that melts solder paste and forms strong, conductive joints between component terminations and PCB pads. As technology has evolved, several heating types have been developed:

• Gas-phase (Vapor Phase) Reflow Soldering
• Infrared Reflow Soldering
• Far-Infrared Reflow Soldering
• Infrared Heating with Hot Air
• Full Hot Air Reflow Soldering
• Nitrogen-Filled Reflow Ovens (for oxidation-sensitive applications)

Among these, far-infrared, infrared hot-air, and all-hot-air reflow soldering are the most widely used in practical SMT production.

 

How Reflow Soldering Works

• Solder Paste Application: A mixture of powdered solder and flux is applied to the PCB pads, usually through stencil printing, ensuring precise volume and alignment.
• Component Placement: Surface-mount components are positioned on the solder paste; its tackiness holds them in place.
• Heating in a Reflow Oven: The PCB moves through multiple heating zones. The solder paste melts (“reflows”) and wets both pads and component leads, forming metallurgical bonds.
• Cooling: The assembly is cooled under controlled conditions to solidify the joints without inducing thermal shock.

Infrared Reflow Welding

First Generation: Hot plate reflow soldering furnace.
Second Generation: Infrared reflow soldering furnace – 80% of thermal energy is transferred as infrared electromagnetic waves (wavelength 0.7 µm to 1 mm). Near-infrared is 0.72–1.5 µm, mid-infrared 1.5–5.6 µm, and far-infrared 5.6–1000 µm. Heating occurs when the infrared wavelength resonates with molecular vibration frequencies, causing rapid heating. Typical working range is 1–8 µm.

Advantages:

• Quick flux activation and wetting
• Fast heating with minimal temperature gradient
• Flexible temperature curve control
• Low cost and high efficiency

Disadvantages:

• Poor penetration depth and shadow effect (uneven heating)

Countermeasure: Increase hot-air circulation during reflow.

 

Infrared Hot-Air Reflow

The third-generation system combines infrared heating with forced hot-air convection. Wind speed is maintained at 1.0–1.8 m/s to balance uniform heating without displacing components or accelerating oxidation.

Hot Air Generation Methods

Axial fan: prone to laminar flow and uneven temperature boundaries
Tangential fan: mounted outside the heater to produce vortex flow for precise zone control

Structural Components

Heaters: tube or plate types, aluminum or stainless steel
Conveyor systems: heat-resistant PTFE-coated fiberglass belts, stainless steel mesh, or chain rails for inline production

Control Systems

Forced convection temperature regulation for stable, repeatable profiles.

 

Vapor Phase Reflow (VPS)

Originally used for thick-film IC soldering in the U.S., VPS offers extremely uniform heating and rapid thermal transfer. The process immerses the assembly in a saturated vapor medium (such as FC-70), which condenses on the PCB, releasing latent heat.

Advantages:

• Uniform heating regardless of component geometry
• Constant welding temperature without complex controls
• Low-oxygen environment reduces oxidation
• High thermal efficiency

Applications and Best Practices

Reflow soldering is essential in high-reliability applications such as aerospace, medical devices, automotive electronics, wearables, and high-density consumer products. To ensure quality:

• Use fresh, well-stored solder paste
• Optimize stencil design for fine-pitch components
• Prebake moisture-sensitive components or substrates
• Match thermal profiles to PCB mass and alloy type
• Apply nitrogen atmosphere for oxidation-sensitive assemblies

 

Why Choose NextPCB for Reflow Soldering

At NextPCB, we specialize in precision reflow soldering for prototypes and volume production. Our advanced ovens support nitrogen reflow, infrared hot-air, and specialized thermal profiles to suit diverse PCB designs, including rigid-flex and high-density interconnects. With in-line SPI, AOI, and X-ray inspection, we deliver consistent, high-yield assemblies for your most demanding projects.

Looking to achieve flawless solder joints with precision reflow soldering technology? Partner with NextPCB for expert PCB assembly services, advanced reflow oven processes, and strict quality control from prototype to high-volume production.

Request your free consultation and quote today to ensure optimal performance and reliability in every board.

 

> Recommend reading: Common Components on a PCBA and What Matters in Assembly

> Recommend reading: Reflow Soldering in SMT

> Recommend reading: PCB Assembly - The Most Comprehensive Guide | NextPCB