OUR SERVICES
Reflow Soldering
Reflow soldering is a process used in the manufacturing of printed circuit boards (PCBs) to attach electronic components to the board’s surface. It involves the controlled application of heat to melt and reflow a solder paste, which creates secure and reliable electrical connections between the components and the PCB.
What Is Reflow Soldering?
Reflow soldering is the core soldering process in SMT (Surface Mount Technology) assembly. Simply put: solder paste is first printed onto the PCB pads, components are placed on top, and then the entire board is sent through a reflow oven. A precisely controlled heating profile melts the solder paste, allowing it to wet the pads and component leads; upon cooling, solder joints are formed. It is called “reflow” because the solder paste completes the soldering through a melt-then-solidify cycle.
Basic Process Flow
The SMT assembly line follows a fixed sequence, with reflow soldering as the step that actually forms the solder joints:
Solder Paste Printing — Solder paste is stenciled onto the PCB pads; paste volume and alignment set the foundation for joint quality.
Component Placement — A pick-and-place machine mounts SMT components onto the pasted pads with high speed and precision.
Reflow Soldering — The entire board passes through the reflow oven; the controlled profile melts the paste, wets pads and leads, and forms metallurgical bonds.
Cooling — The board exits the oven and cools at a controlled rate, solidifying the joints and locking in their grain structure.
Inspection — AOI and X-Ray verify joint quality, checking for defects such as bridging, tombstoning, voids, and shift.
Reflow Soldering vs. Wave Soldering
| Reflow Soldering | Wave Soldering | |
|---|---|---|
| Application | SMT surface-mount components | THT through-hole components |
| Heating method | Whole board passes through oven; solder paste melts | Bottom-side wave contact; barrel filling |
| Typical scenarios | Chip resistors, ICs, BGA, QFN | Connectors, electrolytic capacitors, pin headers |
For mixed-technology boards (SMT + THT), the usual sequence is reflow soldering for SMT parts first, followed by wave soldering or selective soldering for THT parts.
The Four Temperature Zones of Reflow Soldering (Using Lead-Free SAC305 as an Example)
| Stage | Temperature Range | Function |
|---|---|---|
| Preheat | Room temp → ~150°C | Gradual ramp-up; activates flux; prevents thermal shock |
| Soak | 150–180°C | Fully activates flux; volatiles escape; evens out board temperature |
| Reflow | Peak 235–245°C; time above liquidus (217°C), i.e., TAL, 60–90s | Solder paste melts, wets pads and leads, forms metallurgical bond |
| Cooling | Peak → room temp; controlled cooling rate | Solder joint solidifies; controls grain structure; prevents thermal stress |
This “temperature–time” curve is the reflow profile, and it is the core of reflow soldering quality.
Key Quality Indicators of Reflow Soldering
Peak temperature and TAL: Determine whether the solder joint is fully melted.
Ramp-up/ramp-down rate: Prevents thermal shock, tombstoning, and cracking.
Solder wetting and fillet height: Indicators of good soldering.
BGA void rate: Inspected by X-ray; benchmarked against IPC Class 2/3.
IMC layer thickness: Measured by cross-sectioning; typically 1–4 μm is optimal.
Tombstoning, bridging, cold joints, shift: Common defects detected by AOI.
Types of Reflow Ovens
Forced-convection hot-air oven: The most mainstream type; hot-air circulation provides excellent temperature uniformity.
Infrared oven: Primarily radiant heat; now rarely used on its own.
Vapor phase soldering (VPS): Heat transfer via boiling liquid vapor; oxidation-free; used in high-end/military applications.
Vacuum reflow: Vacuum is drawn during the reflow stage, significantly reducing BGA void rate.
Laser reflow: Localized heating; used for precision rework or heat-sensitive components.
Reflow Soldering Capability
1. Temperature Profile Visualization (the Core)
Measured profile chart: Time on the X-axis, temperature on the Y-axis; annotate the preheat/soak/reflow/cooling zones and overlay the solder paste spec window (e.g., SAC305: peak 235–245°C, TAL 60–90s).
Multi-point thermocouple layout diagram: Simultaneous temperature measurement at different board locations (large pads, small pads, under BGA, board edge) to prove in-oven uniformity.
CPK data: Statistical process capability for peak temperature and TAL across consecutive batches.
Before/after comparison: “Before vs. after optimization” profiles showing how tombstoning/void issues were resolved.
2. Solder Joint Quality Evidence
Macro photos: Joint luster, wetting angle, fillet height, no bridging/tombstoning.
X-Ray images: BGA ball shape, void percentage (e.g., <25% meets IPC-A-610 Class 3).
Cross-sectioning: IMC layer thickness (typically 1–4 μm is optimal), no Kirkendall voids.
Pull/shear test reports: Solder joint mechanical strength data.
3. Oven and Process Control Demonstration
Reflow oven parameter screenshots: Zone set temperatures, conveyor speed, nitrogen concentration (e.g., <1000 ppm O₂).
Real-time oven temperature monitoring: Show the in-oven thermocouple feedback system.
Nitrogen reflow capability: Demonstrate low oxidation and high wettability.
Double-sided reflow process description: How bottom-side heavy components are secured during the second reflow (red glue/dot dispensing).
4. Defect Prevention and Closed-Loop Control
Reflow recommendations in DFM reports: Pad design, stencil aperture, thermal balance layout.
Common defect reference table: Tombstoning, bridging, voids, cold joints — cause + countermeasure + actual photo.
First-article confirmation process: Triple confirmation via first-article X-Ray + AOI + profile.
SPC control charts: Batch trends for peak temperature and TAL.
5. Scenario-Based Presentation
Video: Board flow from oven entrance to exit, paired with profile animation.
Display booth: Large-format profile chart + physical board + X-Ray image displayed side by side.
Customer audit package: Profile report + X-Ray + cross-section + CPK, summarized on a single page.
Comparison samples: Good joints vs. typical defective joints (under microscope).
Need a Reflow Soldering Service?
Looking for expert reflow soldering services? Our skilled technicians and state-of-the-art equipment ensure precise and reliable soldering for your PCBs. Get in touch today to discuss your project and benefit from our top-notch soldering solutions.
Benefits of Choosing Us
“Every board has a profile record” — Measured reflow profiles are archived by batch/product for full traceability.
“Void rate comes with data” — Actual measured BGA void rate (mean + max), benchmarked against IPC Class 2/3.
“Mixed assembly is not a problem” — Mature profiles exist for leaded/lead-free mixed assembly, double-sided reflow, and thick boards with high thermal mass.
“DFM up front” — At the prototype stage, we already recommend stencil apertures and reflow profiles based on component thermal mass and board layout.
“Nitrogen is optional” — Nitrogen or air reflow can be selected based on the customer’s reliability requirements and cost targets.