A lead-free reflow profile is a measured temperature history for the solder joints, components and PCB as an assembly travels through an oven. The objective is not to copy a generic curve. It is to create a repeatable process window that activates the flux, melts every intended joint, stays inside component and material limits, and cools the assembly without unnecessary thermal stress.
This guide explains how to establish that window for a new product. The numerical ranges below are practical starting points, not production specifications. The solder paste supplier's current technical data sheet, the lowest-rated component, the PCB material limits and measured product data always take priority.
Reflow Classification Is Not a Production Recipe#
IPC/JEDEC J-STD-020 defines moisture/reflow sensitivity classification conditions for nonhermetic surface-mount devices. Those conditions help establish what a component can withstand during classification; they are not instructions to drive every production board to that maximum temperature. Texas Instruments explicitly advises keeping the production peak below the component classification temperature and selecting the actual profile from the solder paste recommendation and every component rating.
IPC-7530, meanwhile, addresses temperature profiling for mass-soldering processes. Together, the two documents reinforce an important distinction: component survivability sets a ceiling, while the paste, joint formation and assembly thermal behavior define the useful process window below it.
Inputs That Define the Process Window#
Collect these inputs before changing oven settings:
- Solder alloy and paste: alloy liquidus, flux activation behavior, recommended ramp, soak, time above liquidus and peak range.
- Component limits: package classification temperature, duration limits, moisture sensitivity level and any vendor-specific reflow restrictions.
- PCB construction: layer count, copper distribution, board thickness, laminate, surface finish and large thermal planes.
- Assembly geometry: tiny passives, large BGAs, shields, connectors, heat sinks and parts with very different thermal mass.
- Print and placement: stencil aperture, paste volume, component alignment and pad geometry. A profile cannot repair a fundamentally poor print.
- Oven capability: number and length of zones, conveyor speed, airflow, exhaust balance and air or nitrogen atmosphere.
Review the SMT stencil design and the PCB BOM at the same time. Paste deposition and hidden part substitutions are frequent reasons a previously stable recipe stops working.
Typical Lead-Free Profile Stages#
| Stage | Illustrative starting range | Primary purpose |
|---|---|---|
| Preheat ramp | About 0.5–2°C/s | Warm the assembly without excessive thermal shock or flux spatter. |
| Soak | About 150–200°C for 60–120 s | Reduce temperature differences and allow flux activity to develop. |
| Time above liquidus | Often 45–90 s above the alloy liquidus | Complete wetting and intermetallic formation. |
| Peak | Commonly 235–250°C for SAC processes | Provide sufficient margin above liquidus while protecting parts and board. |
| Cooling | Controlled; respect paste and component limits | Solidify joints consistently without avoidable thermal stress. |
For SAC305, Kester lists a melting range of 217–219°C. That makes roughly 217°C a common reference for time above liquidus, but the exact value must follow the alloy actually printed on the board. A low-temperature alloy or mixed-alloy repair process needs a different window.
Preheat and ramp
The initial ramp warms the board, components, paste and trapped volatiles. Too fast a ramp can increase splatter, solder beading, package stress or warpage. Too slow a ramp can consume useful flux activity before the joints reach reflow. Measure the ramp at the product, not merely from the oven's zone setpoints.
Soak
A soak region can reduce the temperature spread between low-mass and high-mass locations. It may also help minimize thermally driven tombstoning, although land pattern, paste volume, placement and wetting symmetry remain critical. An unnecessarily long or hot soak may exhaust the flux or increase oxidation, so it should be justified by measurement rather than habit.
Reflow, peak and time above liquidus
The coldest target joint must spend enough time above the alloy liquidus to wet properly. At the same moment, the hottest component, pad and laminate location must remain below its permitted limit. The acceptable conveyor speed and zone settings are the settings that satisfy both conditions with reasonable margin.
Insufficient energy can cause incomplete wetting, grainy joints, head-in-pillow defects or open connections. Excessive peak or dwell can damage components, discolor laminate, increase intermetallic growth, degrade flux residues or contribute to warpage. Higher is not automatically safer.
Cooling
Cooling is part of the profile, not dead time after soldering. TI cites a J-STD-020 maximum cooling slope of 6°C/s from peak toward liquidus for classification context. The production target should follow paste and component guidance and stay within the oven's controlled capability. Extremely aggressive cooling can stress packages and boards; overly slow cooling changes joint microstructure and extends heat exposure.
Ramp-to-Peak or Soak Profile?#
A ramp-to-peak profile increases temperature more continuously toward reflow. It can preserve flux activity and shorten thermal exposure when the assembly heats uniformly. A soak profile holds or slowly rises through a defined range before reflow, which can improve thermal equalization on a board with a large delta-T.
Neither shape is universally superior. Start with the paste supplier's preferred window, profile the real board, and choose the shape that puts both the coldest joint and hottest sensitive location inside their limits. Do not select a profile shape solely because it worked on another assembly.
How to Profile the Actual Assembly#
- Choose a representative board. Use the intended stack-up, copper, finish, components, paste and any carriers or pallets.
- Identify hot and cold candidates. Include a tiny passive near a board edge, a center-area joint tied to a ground plane, a large BGA or connector and the most temperature-sensitive package.
- Attach fine thermocouples securely. Measure joint temperatures at the solder interface where possible. When validating a package temperature limit, follow the component supplier's specified measurement location; TI describes a thin thermocouple attached at the top center of the package for its devices.
- Run the board through a calibrated profiler. Record actual traces and compare each channel with the paste and component limits.
- Adjust one controlled variable at a time. Conveyor speed changes the time in every zone, while zone-temperature changes can affect only part of the curve but may interact through airflow and board thermal lag.
- Repeat and confirm. One acceptable run does not demonstrate process repeatability. Confirm the recipe on multiple assemblies and retain the profile as part of the manufacturing record.
Manage Delta-T Across the Board#
Delta-T is the temperature difference between the hottest and coldest monitored locations at a meaningful point in the cycle. Large copper planes, shields, thick connectors and center-of-board BGAs can lag behind exposed small components. If the cold joint barely reaches the paste window while a small package approaches its limit, the process has little manufacturing margin.
Possible responses include a more gradual thermal balance, adjusted conveyor speed, selective zone changes, improved airflow, a carrier redesign or an assembly/layout change. In design review, balanced copper and thermal-relief choices can make the PCB assembly process more robust before the oven recipe is created.
Diagnosing Common Reflow Defects#
| Symptom | Profile-related checks | Other checks |
|---|---|---|
| Poor wetting or opens | Cold-joint peak, time above liquidus, flux activation and oxidation exposure | Finish condition, contamination, paste age and print volume |
| Solder balls or beading | Initial ramp, solvent release and soak duration | Stencil aperture, paste slump, mask geometry and placement pressure |
| Tombstoning | Side-to-side heating imbalance and wetting timing | Pad geometry, paste symmetry, placement and component termination |
| Head-in-pillow | Peak, time above liquidus and package/board warpage through reflow | Paste transfer, sphere oxidation, package coplanarity and support |
| Voiding | Ramp/soak behavior and time available for volatiles to escape | Aperture design, paste chemistry, pad design and via configuration |
| Discoloration or damage | Excessive peak, dwell or repeated reflow exposure | Component rating, laminate capability and moisture handling |
Treat this table as a diagnostic map, not proof of root cause. Reflow defects usually involve interactions among design, material, printing, placement and thermal history.
Moisture, Nitrogen and Repeat Reflow#
A good temperature profile does not reset component floor life. Moisture-sensitive devices must be stored, tracked and, when appropriate, baked according to J-STD-033 and the component manufacturer's instructions. Heating an overexposed package can create delamination or cracking even when the measured curve is otherwise within limits.
Nitrogen can reduce oxidation and widen the wetting margin for some assemblies, but it is not a substitute for clean surfaces, suitable flux or a controlled profile. Qualify the process in the atmosphere that production will actually use. Also consider double-sided assemblies: parts on the first side may experience a second thermal cycle, and selective repair adds further exposure.
Production Setup Checklist#
- Lock the alloy, paste product and storage/handling rules.
- Confirm component peak, duration, MSL and repeated-reflow limits.
- Define a starting curve from the current paste data sheet.
- Instrument representative hot and cold locations on the real assembly.
- Record ramp, soak, liquidus crossing, TAL, peak, cooling and board delta-T.
- Inspect first articles using acceptance criteria appropriate to the product.
- Run functional tests and, where risk warrants, X-ray or cross-section hidden joints.
- Freeze the approved recipe with oven, line, conveyor, atmosphere and profiler details.
- Reprofile after meaningful changes to paste, BOM, component supplier, layout, stack-up, carrier, oven maintenance or line transfer.
Official References#
- IPC document revision table for the current revisions of IPC-7530 and J-STD-020.
- Texas Instruments: Customer Board Assembly Reflow Profile for Lead Free Soldering.
- Texas Instruments: Reflow Profile for Lead-Free Soldering.
- Kester SAC305 alloy technical data.
Frequently Asked Questions#
What is a typical peak temperature for lead-free reflow?
Many SAC processes begin development in the 235–250°C range, but there is no universal production target. Use the paste supplier's window and stay below every component and PCB material limit with measured margin.
What is the liquidus temperature of SAC305?
Kester lists SAC305 as melting at 217–219°C. Confirm the specification for the exact alloy and paste used on the assembly.
How long should a board stay above liquidus?
About 45–90 seconds is a common starting range for some lead-free pastes, but the approved value must come from the paste technical data sheet and measured joint results.
Where should thermocouples be placed?
Place them at predicted hot and cold solder joints, high-thermal-mass areas and temperature-sensitive packages. Attach them securely with minimal added thermal mass and follow the component vendor's location guidance when verifying a package limit.
Can reflow settings fix tombstoning?
They can reduce a thermal imbalance, but tombstoning also depends on pad geometry, copper balance, paste volume, placement and termination wetting. Diagnose all contributors before changing the recipe.
When should an assembly be reprofiled?
Reprofile after significant changes to the PCB, component population, paste, supplier, carrier, oven, conveyor, atmosphere or production line, and whenever trend data indicates the existing margin has shifted.