Reflow Soldering: Everything You Need to Know

Reflow soldering is a crucial process in today's Surface Mount Technology (SMT), enabling the secure mounting of parts ranging from the smallest 0201 passives to complex, high-density Ball Grid Arrays (BGAs). With the ongoing reduction in component sizes, accurate heat management has become a crucial requirement for reliability, which means the process must be error-free.

This engineering guide dissects the thermodynamics, fluid dynamics, and metallurgy behind the process. We will cover the four critical zones of the thermal profile, convection heat transfer mechanisms, and advanced strategies for preventing latent failures like cracked capacitors or thermal pad voids.

For high-reliability soldering, high-precision equipment is essential. JLCPCB utilizes advanced 10-zone convection reflow ovens to maintain strict thermal control. This capability allows us to successfully manage complex PCBA projects for industries like aerospace and medical, consistently achieving a zero-defect standard.

Modern multi-zone convection reflow oven

What Is Reflow Soldering?

Basically, reflow soldering is a method that uses solder paste, a sticky combination of solder alloy powder and flux, to hold electrical components temporarily on their spots, then the whole assembly is put through a controlled heat treatment that is controlled. This heat melts the solder, thereby forming a permanent bond of electromechanical nature.

Unlike wave soldering, where the solder source is a molten bath, reflow relies on the material already deposited on the board. The process is defined by a phase transition:

1. Solid/Viscous State: The paste holds components in place during placement.

2. Liquid State: The alloy reaches its liquidus temperature (T_L), coalescing and wetting the metal surfaces.

3. Solid State: The alloy cools, forming a specific grain structure that determines mechanical strength.

Reflow Soldering vs Wave Soldering vs Selective Soldering

To choose the right assembly path, one must distinguish reflow from its counterparts:

Reflow Soldering: The main method for SMT. One of its best features is the self-alignment - due to the surface tension of the liquid solder, the components that are slightly off will be drawn to the center of their pads. It permits the mounting of both sides and the processing of large quantities at a time.

Wave Soldering: Traditionally used for Through-Hole Technology (THT). The board passes over a turbulent wave of molten solder. While fast for legacy boards, it exerts high thermal shock and is unsuitable for fine-pitch SMT due to bridging risks.

Selective Soldering: A robotic variation of wave soldering that targets specific through-hole pins. It is precise but slower.

Common Solder Alloy Types for Reflow Soldering

The thermal profile used relies entirely on the chemistry of the alloy. The melting point dictates the peak oven temperature.

Showing solder paste reflow phases from deposition to intermetallic formation

Reflow Soldering Process: Four Critical Zones

The "Thermal Profile" is the DNA of a successful solder joint. It is a graph of Temperature (Y-axis) vs. Time (X-axis) representing the journey of the PCB through the oven. A standard lead-free profile is divided into four distinct zones.

Four Critical Zones of Reflow Oven Temperature Profile

#Zone 1: Preheat in Reflow Soldering (The Ramp-Up)

Temperature Range: Ambient (25℃) to 150℃.

Target Ramp Rate: 1℃ to 3℃ per second.

The primary goal of the preheat zone is to elevate the temperature of the entire assembly safely. "Safely" is the operative word. If the temperature rises too fast (e.g., >3℃/sec), the volatile solvents in the flux will expand rapidly and "explode," causing solder splattering or solder balls.

Furthermore, rapid heating creates a massive thermal differential (ΔT) between the ceramic body of a component and its internal metal electrodes. This stress can cause micro-cracks in Multi-Layer Ceramic Capacitors (MLCCs), leading to immediate or latent short circuits.

#Zone 2: Thermal Soak in Reflow Soldering (Equalizer)

Temperature Range: 150℃ to 180℃ (Plateau).

Duration: 60 to 120 seconds.

The soak zone is where the physics of heat transfer becomes critical. A PCB is rarely uniform; it contains massive copper ground planes (high thermal mass) and isolated traces (low thermal mass). Without a soak, the small 0402 resistors would reach reflow temperature seconds before a large BGA.

The soak functions as a thermal halt, permitting the heavy substances to "bridge" the light ones, and thus keeping the temperature difference (ΔT) all over near zero before entering the reflow area.

Flux Activation: During this phase, the organic acids in the flux attack the oxides on the pads and leads, cleaning the metal to ensure wetting.

Risk: In case of excessive soaking, the flux may lose its cleaning effect just before the solder melts, which may consequently cause defects such as Graping (uncoalesced solder powder) or Head-in-Pillow.

#Zone 3: Reflow (Time Above Liquidus)

Peak Temperature: 235℃ - 250℃ (for SAC305).

Time Above Liquidus (TAL): 45 to 90 seconds.

Only during this period, the solder is in its liquid state. The surface tension of the liquid solder controls the situation and, hence, the components are automatically aligned.

Metallurgy: The molten solder interacts with the copper pad, leading to the creation of a layer of Intermetallic Compound (IMC), which is commonly Cu6Sn5. This layer acts as the joint's "glue".

The Goldilocks Window:

● Too Short: The solder doesn't wet fully, creating a Cold Solder Joint (dull, grainy, weak).

● Too Long: The IMC layer grows too thick (>5μm), becoming brittle and prone to fracture under mechanical shock.

Expert Tip: For BGAs reflowed in this zone, optical inspection is impossible. JLCPCB utilizes X-Ray Inspection (AXI) to peer through the package and verify that the balls have collapsed and wetted properly.

Learn about JLCPCB Quality Control.

#Zone 4: Cooling

Cooling Rate: 2℃ to 4℃ per second.

Cooling must be aggressive but controlled. Rapid cooling freezes the solder structure quickly, creating a fine-grain structure that is mechanically strong and fatigue-resistant. Slow cooling allows large, coarse grains to grow, which are weaker. However, cooling too fast (shock cooling) can warp the FR-4 substrate or crack ceramic components due to CTE (Coefficient of Thermal Expansion) mismatches.

Choosing the Right Reflow Oven for Reflow Soldering

There are different types of heat. The consistency of the solder joint depends upon how heat is transferred to the board.

Convection Reflow Oven (Industry Standard)

Modern high-volume assembly, including JLCPCB’s production lines, utilizes Forced Convection Ovens.

● Mechanism: Air or nitrogen that has been heated is passed through nozzles onto the PCB.

● Advantage: The transfer of heat is effective and, to a great extent, not influenced by the color of the components. "Shadowing" effects, where tall components prevent heat from reaching smaller neighboring ones, are minimized.

A reflow oven showing fans circulating hot air through heaters and onto the PCB conveyor belt below.

Vapor Phase Reflow Oven

VPS uses the latent heat of condensation of a specialized perfluorinated liquid (like Galden). The board is lowered into a vapor blanket.

● Advantage: The temperature is physically clamped to the boiling point of the fluid (e.g., 230℃), making overheating impossible. It provides perfect heating uniformity for extremely heavy copper boards.

● Disadvantage: High cost and complexity; typically reserved for low-volume aerospace or prototyping.

Nitrogen Atmosphere in Reflow Oven

Advanced reflow ovens often replace air with Nitrogen (N₂).

● The Chemistry: Oxygen promotes oxidation. By flooding the tunnel with N₂ and keeping Oxygen levels <50 ppm, oxidation is halted.

● Benefit: Notably, the wetting angle of the solder is improved dramatically; this is especially true for surfaces such as OSP that are difficult to solder and for components with fine pitches.

Nitrogen reflow soldering

Thermal Profiling in Reflow Soldering: The Foundation of Quality

A common misconception is that setting the oven to "250℃" means the board reaches 250℃. It does not. The board temperature is a function of conveyor speed, air velocity, and the board's own thermal mass.

Process Window Engineering

Engineers must establish a Process Window - the overlap between:

1. Component Limits: The maximum temperature the most sensitive component (e.g., a plastic connector or electrolytic cap) can tolerate (usually 260℃).

2. Solder Requirements: To melt the alloy and establish an effective bond, the appropriate minimum temperature of 217℃ from 15℃ stands necessary.

Thermocouple Placement for Accurate Profiling

To develop a legitimate profile, technicians put K-type thermocouples on a "Golden Board" (which is an expendable copy of the production board).  Placements are strategic:

● The Cold Spot: Usually a ground pin of a large BGA or a heavy inductor. This ensures the heaviest part gets enough heat.

● The Hot Spot: Usually a small component near the edge of the board or a thin PCB area. This ensures the lightest part doesn't burn.

● Sensitive Components: Any temperature-critical sensors.

If the "Cold Spot" doesn't reach the liquidus, you get cold joints. If the "Hot Spot" exceeds 260℃, you get damaged components. The art of profiling is balancing the oven zones to keep all points inside the window.

Showing a graph with different thermocouples all staying within a green "Safe Zone" box, bounded by Min Solder Temp and Max Component Temp.

Mixed Technology Reflow Soldering: Challenges and Process Solutions

One of the toughest challenges in PCBA is Mixed Technology: placing a tiny 0201 capacitor next to a massive screw terminal or shield.

Mixed SMT Assembly and Thermal Mass Imbalance

Mixed Technology (or Mixed Assembly) is defined as the integration of components with extreme variances in thermal mass on a single PCB. This typically involves placing microscopic passives (like 0201 or 01005 chips) adjacent to high-mass components such as:

● Large BGAs: Which often have integrated metal heat spreaders (lids).

● Shielded Inductors: Which contain dense ferrite or iron cores that act as heat sinks.

● Through-Hole Connectors: Which are increasingly reflowed using "Pin-in-Paste" techniques rather than wave soldering.

While standard SMT lines optimize for throughput speed, mixed technology lines must optimize for thermal equilibrium. The fundamental engineering challenge is that a 0201 resistor heats up almost instantly, while a large shielded inductor lags by 20-30 seconds. If the profile isn't tuned perfectly, the resistor will overheat and oxidize before the inductor's solder paste even reaches liquidus.

The Thermal Lag Problem and Execution

The 0201 has almost zero thermal mass; it heats up instantly. Conversely, a heavy BGA or shielded inductor acts as a heat sink, lagging behind significantly. If you ramp up too fast (Linear Profile), the 0201 flows while the terminal is still solid. If you simply raise the temperature, the 0201 might "cook," exhausting its flux before the heavy part is ready.

How it is Performed (Ramp–Soak–Reflow): Mixed SMT soldering is performed by implementing a specific "Extended Soak" strategy. Process engineers configure the oven's Zone 2 to maintain a steady temperature plateau (typically 150℃  - 175℃ ) for an extended window of 90 to 120 seconds.

● The Mechanism: The heavier components are still continuing to absorb heat in this "thermal pause" while the small components have already risen in temperature and are just waiting (stopped).

● The Result: The thermal differential (ΔT) collapses. When the board finally enters the Reflow Zone, both heavy and light components cross the liquidus line simultaneously, ensuring uniform wetting without thermal damage.

This graph clearly shows the difference in thermal profile required to handle different component types.

Pin-in-Paste (Intrusive Reflow) for Through-Hole Components

Engineers often want to eliminate the manual or wave soldering step for through-hole connectors.

● The Trick: Solder paste is printed over the through-holes. The component pin is inserted through the paste. During reflow, the paste melts and is wicked down into the hole by capillary action.

● Requirement: The paste volume has to specifically measure the filling of a hole up to IPC standards (Annular ring volume + Hole volume - Pin volume).

Need help with a complex assembly? JLCPCB’s engineering team reviews your Gerber files for "Pin-in-Paste" feasibility and stencil design optimization. Get a Quote for Mixed Assembly

Mixed Assembled PCB: BGA next to a through-hole header and 0402 resistors

Common Reflow Soldering Defects and Ways to Fix Them

Even with perfect equipment, physics can be unforgiving. Identifying the root cause is key to prevention.

#1 Tombstoning (The Manhattan Effect)

Symptom: A small passive component (0402/0603) stands vertically on one end, looking like a tombstone.

Root Cause: Torque imbalance. The solder on one pad melted and wetted before the other. The surface tension pulled the component upright.

Fix:

● Design: Ensure pads have symmetric thermal connections. Don't connect one pad to a massive ground plane without thermal relief spokes.

● Process: Slow down the ramp rate into reflow to ensure both pads reach the liquidus simultaneously.

#2 Solder Balling

Symptom: Tiny spheres of solder scattered around the joint or on the soldermask.

Root Cause:

● Explosive Outgassing: Ramp rate too fast (>3°C/s) trapped solvents.

● Oxidation: Solder powder oxidized (paste mishandling).

Fix: Check the preheat slope and ensure the paste was stored/thawed correctly.

#3 Voiding (Gas Pockets)

Symptom: Empty cavities inside the solder joint, visible only via X-ray. Critical for QFN thermal pads.

Root Cause: Flux volatiles trapped during the liquid phase.

Fix:

● Increase the soak time to allow volatiles to escape before reflow.

● Use a "Vacuum Reflow" process for critical power electronics (removes voids by pressure differential).

#4 Head-in-Pillow (HiP)

Symptom: A BGA ball rests on the pad but hasn't coalesced with the paste. It looks like a head resting on a pillow.

Root Cause:

1. Warpage: The BGA warped (smiled/frowned) during heating, lifting the ball off the paste.

2. Flux Exhaustion: The flux dried out before the ball touched the paste.

Fix: Verify moisture sensitivity handling (MSL) and optimize the soak profile to be shorter/cooler.

4 Soldering Defects: (1) Cold Joint (grainy), (2) Tombstone (vertical resistor), (3) Solder Balls, (4) X-Ray image showing voids (white spots) in a BGA.

Quality Control and Inspection in Reflow Soldering

You cannot improve what you cannot measure. Post-reflow inspection is the gatekeeper of quality.

1. AOI (Automated Optical Inspection): Cameras scan the board using different angles of light. They check for component presence, polarity, skew, and visible solder fillets.

2. AXI (Automated X-Ray Inspection): Mandatory for BGAs, QFNs, and LGAs. The X-ray technology used here will go through the protective layers and show the percentage of voiding and bridging, as well as the presence of HiP defects, if any, underneath.

3. Electrical Testing (ICT/Flying Probe): Checks if the physical connection conducts electricity.

DFM Best Practices for Reflow Soldering

Certain golden rules constitute success criteria for a reflow process:

1. Solder Paste Handling: Paste is perishable. Keep it refrigerated (2-10℃).

Crucial: Allow it to thaw naturally for 4 hours before opening the jar. Opening a cold jar induces condensation, adding water to the paste → immediate Solder Balling.

2. Stencil Design: For fine-pitch components, the stencil aperture should be slightly smaller than the pad (e.g., 10% reduction) to prevent bridging.

3. Thermal Reliefs: Always use thermal spokes when connecting pads to ground planes. This is the #1 way to prevent cold joints and tombstoning.

Conclusion

Reflow soldering is a triumph of material science and process engineering. It allows us to manufacture reliable electronics at a scale and density previously impossible. However, it requires respect for the "Process Window." From the thixotropic properties of the paste to the grain structure formed during cooling, every variable matters.

The point that, for designers, is pretty obvious is that DFM starts layout-wise. Thus, besides the schematic itself, one must take into consideration symmetric pads, proper thermal reliefs, and component spacing as well.

Ready to scale your PCBA production? JLCPCB combines 10-zone reflow ovens, automated X-ray inspection, and rigorous DFM checks to deliver high-reliability assemblies, from prototype to mass production.

Get an Instant PCBA Quote.

FAQs about Reflow Soldering

Q1: What is the optimal Time Above Liquidus (TAL) for SAC305?

In the case of standard SAC305 lead-free solder, the industry became accustomed to a range of 60 to 90 seconds above 217℃ as being the "sweet spot". If the TAL is kept short (<45s), you may end up with problems such as poor wetting or cold joints. On the other hand, if it is too long (>120s), the components may get overheated, causing an IMC layer that is brittle and thus more susceptible to drop-shock failure to be formed.

Q2: Can you reflow through-hole components?

Yes, using the Pin-in-Paste (PiP) or Intrusive Reflow technique. This works best for components capable of withstanding $260^\circ\text{C}$ (high-temp plastics) and requires careful stencil calculation to ensure there is enough paste volume to fill the hole.

Q3: Why are my 0402 resistors standing up (Tombstoning)?

This is usually a thermal imbalance. If one pad is connected to a large copper trace and the other to a thin trace, the thin side heats up faster. The solder melts, wets the component, and pulls it upright before the heavy side melts. Fix this by adding thermal relief spokes to the heavy connection.

Q4: Is Nitrogen reflow necessary?

For standard boards, air reflow is sufficient. However, Nitrogen (N₂) is highly recommended for:

1. OSP (Organic Solderability Preservative) surface finishes.

2. Ultra-fine pitch components (0.3mm pitch BGAs).

3. Mission-critical aerospace/medical boards. Nitrogen prevents oxidation during the soak phase, significantly improving wetting.

Q5: How many times can a PCB undergo reflow?

Standard practice allows for three thermal cycles:

1. Reflow Side 1 (Top).

2. Reflow Side 2 (Bottom).

3. One rework cycle (if needed). Exceeding this degrades the FR-4 material (risk of delamination) and can crack components. Always track the cumulative thermal exposure of your boards.

Q6: What's the difference between an air and a nitrogen atmosphere reflow?

Nitrogen atmosphere reflow (<50 ppm oxygen) significantly reduces oxidation during the high-temperature reflow process. Benefits include improved wetting (especially for lead-free SAC alloys), brighter solder joints, reduced dross formation, and potentially allowing slightly lower peak temperatures. The cost includes nitrogen generation/supply infrastructure and increased operating expenses.

Air reflow is adequate for many applications with proper profile optimization. Nitrogen becomes more important for lead-free soldering, OSP surface finish boards, fine-pitch components, and aerospace/military applications requiring maximum reliability.