Reflow Profile Basics: Soak Time, Peak, and Delta-T

Achieving a perfect solder joint is less about luck and more about thermal management. In surface mount technology (SMT), the difference between a reliable product and a field failure often comes down to reflow profile basics: soak time, peak, and delta-t.

At APTPCB (APTPCB PCB Factory), we understand that a reflow profile is the thermal "recipe" that a printed circuit board (PCB) undergoes inside the reflow oven. It dictates how fast the board heats up, how long the flux remains active, and the maximum temperature components must endure. Getting this wrong leads to cold solder joints, tombstoning, or damaged components.

This guide covers everything from the fundamental definitions to advanced troubleshooting, ensuring your designs transition smoothly from prototype to mass production with APTPCB.

Key Takeaways

Before diving into the technical metrics, here are the core concepts you must understand to control your assembly quality.

The "Recipe" Analogy: A reflow profile is a graph of temperature versus time. It must align with the solder paste manufacturer's specifications and the thermal limits of your components.
Soak Time Function: This phase equalizes the temperature across the PCB. It allows the flux to activate and remove oxides before the solder melts.
Peak Temperature: This is the highest temperature reached. It must be high enough to form a good intermetallic bond but low enough to prevent component delamination.
Delta-T ($\Delta$T) Criticality: This measures the temperature difference between the hottest and coolest spots on the board. A high Delta-T causes uneven soldering.
Validation is Mandatory: You cannot guess a profile. It requires a thermal profiler with thermocouples attached to the actual PCB assembly.
Voiding Control: Proper profiling is essential for qfn reflow best practices to reduce voids and ensuring long-term reliability.

What reflow profile basics: soak time, peak, and delta-t really means (scope & boundaries)

Building on the key takeaways, we must define the specific zones of a thermal profile to understand how they interact.

A standard SMT reflow profile consists of four distinct zones: Preheat, Soak, Reflow (Peak), and Cooling. While all are important, the interaction between soak, peak, and the resulting Delta-T is where most defects occur.

The Soak Zone

The soak zone is the plateau in the temperature curve, typically between 150°C and 200°C (for lead-free solder). Its primary purpose is thermal equalization. On a complex board, large copper planes heat up slowly, while small resistors heat up fast. The soak time allows the cooler parts to catch up to the hotter parts, reducing Delta-T before the solder melts. It also allows volatile solvents in the solder paste to evaporate gently.

The Peak (Reflow) Zone

This is where the magic happens. The temperature rises above the "liquidus" point of the solder alloy. For standard SAC305 lead-free solder, the melting point is roughly 217°C. The peak temperature usually targets 235°C to 245°C. The duration spent above the melting point is called Time Above Liquidus (TAL).

Delta-T ($\Delta$T)

Delta-T is not a zone but a measurement of uniformity. It is the difference in temperature between the coolest component (often a heavy BGA or connector) and the hottest component (often a small capacitor) at any given moment. Minimizing Delta-T ensures that all joints reflow simultaneously, preventing tombstoning and twisting.

For more on how these phases fit into the broader assembly process, see our guide on SMT and THT assembly.

The Preheat, Soak, Reflow (PEAK) (REFLOW) profile basics: soak time, peak, and delta-t metrics that matter

Understanding the definitions is the first step; now we must quantify them with specific metrics to evaluate quality.

Process engineers use these metrics to determine if a profile is "in spec." Deviating from these ranges is the leading cause of assembly defects.

Metric	Why it matters	Typical Range (Lead-Free)	How to Measure
Ramp Rate (Slope)	Controls how fast the PCB heats up. Too fast causes thermal shock and solder splatter.	1°C to 3°C per second	Thermal Profiler (Slope calculation)
Soak Time	Allows flux activation and thermal equalization. Too long exhausts the flux; too short leaves cold spots.	60 to 120 seconds (150-200°C)	Time duration between two temp points
Peak Temperature	Ensures proper wetting and intermetallic formation. Too high damages parts; too low causes cold joints.	235°C to 250°C	Maximum recorded temp on any thermocouple
Time Above Liquidus (TAL)	Determines the grain structure of the solder joint. Too long creates brittle joints.	45 to 90 seconds	Time duration above 217°C
Delta-T ($\Delta$T)	Indicates thermal uniformity. High Delta-T risks partial reflow.	< 10°C at Peak	Difference between Max and Min probes
Cooling Rate	Affects solder grain structure. Fast cooling creates finer, stronger grain structures.	2°C to 4°C per second	Slope of the cool-down curve

How to choose reflow profile basics: soak time, peak, and delta-t: selection guidance by scenario

Once you know the metrics, you must adapt them to your specific board design, as one profile does not fit all.

The "perfect" profile depends heavily on the thermal mass of the PCB and the sensitivity of the components. Here is how to choose the right approach for different manufacturing scenarios.

Scenario 1: Simple Consumer Electronics (Low Complexity)

Profile Type: Ramp-to-Spike (RTS).
Why: These boards have uniform thermal mass. A linear ramp is faster and puts less thermal stress on the paste.
Trade-off: Higher throughput but less forgiveness for temperature variance.

Scenario 2: High-Reliability Industrial/Server (High Complexity)

Profile Type: Ramp-Soak-Spike (RSS).
Why: These boards often contain heavy copper layers and large BGAs mixed with small passives. A distinct soak zone is required to minimize Delta-T.
Trade-off: Longer cycle time, but essential for yield.

Scenario 3: QFN and Bottom-Terminated Components

Focus: qfn reflow best practices to reduce voids.
Adjustment: Extended soak times allow volatile gases to escape from under the component body before the solder creates a seal.
Risk: If the ramp is too fast, gas gets trapped, creating voids.

Scenario 4: Fine-Pitch BGA Assembly

Focus: bga voiding control: stencil, reflow, and x-ray criteria.
Adjustment: Careful control of Peak Temperature and TAL. The BGA ball and the paste must melt together perfectly.
Validation: Requires X-Ray Inspection to verify wetting and void percentage.

Scenario 5: Flexible Circuits (FPC)

Focus: Material sensitivity.
Adjustment: Flex materials (Polyimide) absorb heat differently than FR4 and may require carrier pallets. The profile must account for the thermal mass of the pallet.
Link: Learn more about Flex PCB capabilities.

Scenario 6: Double-Sided Assembly

Focus: Component retention.
Adjustment: The second pass (side B) must not remelt the heavy components on side A to the point where they fall off. The profile is often slightly cooler or uses a different support fixture.

The Preheat, Soak, Reflow (PEAK) (REFLOW) profile basics: soak time, peak, and delta-t implementation checkpoints

Reflow profile basics: soak time, peak, and delta-t implementation checkpoints

Selecting the profile is theoretical; implementing it on the factory floor requires a rigorous step-by-step process.

At APTPCB, we follow a strict protocol to ensure the theoretical profile matches reality.

Paste Data Review: Obtain the datasheet for the specific solder paste (e.g., SAC305, SnPb). Note the activation temp and melting point.
Component Audit: Identify the most thermally sensitive component (e.g., plastic connectors) and the most thermally massive component (e.g., shielding cans, BGAs).
Thermocouple Attachment: Attach 3-6 thermocouples to a "Golden Board."
- Location 1: Leading edge of the PCB.
- Location 2: Center of a large BGA (drill through the back if necessary).
- Location 3: Body of a sensitive component.
- Location 4: Small passive component (fastest heating).
Oven Setup: Input the initial zone temperatures and conveyor speed based on the selected scenario (RTS or RSS).
Run Profiler: Send the Golden Board through the oven.
Analyze Delta-T: Check the temperature difference at the soak and peak. If Delta-T > 10°C, adjust the soak zone duration or conveyor speed.
Verify TAL: Ensure the coolest spot stays above liquidus for at least 45 seconds.
Verify Peak: Ensure the hottest spot does not exceed component specs (usually 260°C).
Lock Recipe: Save the oven settings as the master program for that specific assembly part number.
First Article Inspection (FAI): Run a production board and inspect using AOI Inspection and X-Ray.

The Preheat, Soak, Reflow (PEAK) (REFLOW) profile basics: soak time, peak, and delta-t common mistakes

Even with a defined process, errors can occur. Recognizing these common mistakes helps in rapid troubleshooting.

1. The "Graping" Effect

Symptom: Solder particles look like a cluster of grapes rather than a smooth joint.
Cause: The soak time was too long or the temperature was too high, causing the flux to exhaust (dry out) before the reflow phase. The solder powder oxidizes and fails to coalesce.
Fix: Reduce soak time or switch to a paste with higher activity.

2. Tombstoning (Manhattan Effect)

Symptom: A small component stands up on one end.
Cause: Uneven heating (high Delta-T) between the two pads. One pad melts first and pulls the component upright.
Fix: Increase soak time to equalize temperatures across the pads before the solder melts.

3. Solder Beading / Solder Balls

Symptom: Small balls of solder appear alongside chip resistors or capacitors.
Cause: excessively fast ramp rate causes the solvent in the paste to boil and "explode," ejecting solder.
Fix: Reduce the initial ramp rate (Preheat slope).

4. Voiding in BGAs/QFNs

Symptom: Large air pockets seen under X-ray.
Cause: Insufficient TAL or Peak temperature prevents gas from escaping.
Fix: Optimize the profile for bga voiding control: stencil, reflow, and x-ray criteria. Slightly increasing TAL can help gas escape.

5. Board Delamination

Symptom: Blisters or separation of PCB layers.
Cause: Peak temperature exceeded the material's Tg or decomposition temperature, or moisture was trapped in the board.
Fix: Bake the PCBs before reflow to remove moisture, or lower the peak temperature.

6. Cold Solder Joints

Symptom: Dull, grainy joints with poor electrical connection.
Cause: Peak temperature was too low, or TAL was too short. The solder never fully wetted the pad.
Fix: Increase peak zone temperature or slow down the conveyor.

The Preheat, Soak, Reflow (PEAK) (REFLOW) profile basics: soak time, peak, and delta-t FAQ

Here are answers to specific questions regarding the impact of profiling on manufacturing logistics and costs.

Q: How does optimizing the reflow profile affect the total assembly cost? A: While profiling takes engineering time, it reduces the "Cost of Poor Quality." A bad profile leads to rework, scrap, and field failures. Investing in a robust profile upfront lowers the overall unit cost by maximizing First Pass Yield (FPY).

Q: Does the reflow profile impact the lead time for my order? A: For new products (NPI), profiling adds a few hours to the initial setup. However, for repeat orders, the saved recipe allows for immediate production. It does not significantly impact standard lead times.

Q: How do different PCB materials affect the required soak time? A: Materials with high thermal conductivity (like Metal Core PCBs) dissipate heat rapidly. They require a more aggressive heat input or longer soak times compared to standard FR4 to reach the same reflow temperature.

Q: What testing methods are used to validate the profile? A: The primary method is a thermal profiler (like KIC or DATAPAQ) that travels through the oven. Secondary validation involves cross-sectioning joints (destructive) or X-ray inspection (non-destructive) to verify wetting and voiding.

Q: What are the acceptance criteria for a "good" profile? A: The profile must fall within the "process window" defined by the solder paste manufacturer (e.g., Alpha, Indium) and IPC J-STD-020 standards. Key criteria include TAL of 45-90s, Peak Temp of 235-250°C, and a Ramp Rate < 3°C/s.

Q: Can I use the same profile for leaded and lead-free assembly? A: Absolutely not. Leaded solder (SnPb) melts at ~183°C, while Lead-Free (SAC305) melts at ~217°C. Using a leaded profile for lead-free boards will result in no reflow (cold joints). Using a lead-free profile for leaded boards may overheat components.

Q: How does soak time influence flux activity? A: Flux cleans oxides. If the soak is too hot or too long, the flux activates and burns off before the solder melts, leaving the metal unprotected against re-oxidation. This leads to "head-in-pillow" defects on BGAs.

Q: Why is Delta-T higher on larger boards? A: Larger boards have more variation in copper density and component mass. The physical distance between the edge (heated by convection and radiation) and the center also contributes to thermal lag, increasing Delta-T.

Resources for reflow profile basics: soak time, peak, and delta-t

To further understand the ecosystem of PCB assembly, explore these related APTPCB resources:

SMT vs. THT Assembly: Understand where reflow fits in the broader assembly picture.
BGA & QFN Assembly: Specific challenges for bottom-terminated components.
X-Ray Inspection: How we verify hidden solder joints after reflow.
DFM Guidelines: Design tips to make your board easier to profile and assemble.

The Preheat, Soak, Reflow (PEAK) (REFLOW) profile basics: soak time, peak, and delta-t glossary

A quick reference for the technical terms used in thermal profiling.

Term	Definition
Liquidus	The temperature at which the solder alloy becomes completely liquid (approx. 217°C for SAC305).
Solidus	The temperature at which the solder alloy is completely solid.
Eutectic	An alloy where the liquidus and solidus temperatures are the same (it melts/freezes instantly, e.g., Sn63Pb37).
TAL (Time Above Liquidus)	The duration the solder joint remains in a liquid state. Critical for wetting.
Delta-T ($\Delta$T)	The maximum temperature difference between any two points on the PCB at a specific time.
Soak Zone	The part of the profile where temperature is held relatively steady to equalize the board.
Ramp Rate	The speed at which the temperature changes, measured in degrees per second (°C/s).
Flux	A chemical agent in solder paste that removes oxides and promotes wetting.
Wetting	The ability of molten solder to spread across and bond to the metal pad.
Intermetallic Layer	The bond formed between the solder and the copper pad; essential for electrical connection.
Thermocouple	A sensor used to measure temperature at specific points on the PCB during profiling.
Reflow Oven	A machine with multiple heating zones used to melt solder paste.
Voiding	Air or gas trapped inside a solder joint, weakening it.
Tombstoning	A defect where a component stands vertically on one pad due to uneven wetting forces.

Conclusion (next steps)

Mastering reflow profile basics: soak time, peak, and delta-t is the bridge between a functional design and a reliable product. It requires a balance of chemistry, physics, and precise equipment control. A well-tuned profile minimizes voids, prevents thermal shock, and ensures that every joint—from the smallest resistor to the largest BGA—is electrically and mechanically sound.

At APTPCB, we treat profiling as a critical science, not an afterthought. Whether you are prototyping a complex IoT device or scaling up automotive electronics, our engineering team validates every thermal recipe before production begins.

Ready to move to manufacturing? When submitting your data for a DFM review or quote, please provide:

Gerber Files: Including paste layers.
BOM (Bill of Materials): To identify thermal mass of components.
Assembly Drawings: Indicating any special component orientation.
PCB Stackup: To estimate thermal conductivity.
Special Requirements: E.g., specific solder paste brands or IPC Class 3 requirements.