Reflow Profiles, Ovens and Hot Plates
The reflow profile and its numbers, toaster ovens and hot plates, moisture-sensitive parts and cooling.
What reflow is
Reflow soldering melts solder paste that has been printed on the pads, with the parts already placed in it, by heating the whole board along a controlled temperature curve. All the joints form together; surface tension pulls the parts into line; and the board is soldered in a few minutes. It is the method used for nearly all surface-mount assemblies (see Surface-Mount Soldering), from a toaster oven in a workshop to a ten-zone oven in a factory.
The reflow profile and its numbers
The profile is the temperature of the board against time. A standard lead-free profile has four zones: preheat (a steady ramp to about 150 °C), soak (a slow rise to 180 °C that lets the flux work and the board temperature equalize), reflow (a quick rise above the liquidus to a peak) and cool-down.
| Parameter | Lead-free (SAC305) | Leaded (Sn63) | Why |
|---|---|---|---|
| Ramp to soak | 1 to 3 °C/s | 1 to 3 °C/s | Too fast cracks ceramics and splatters paste |
| Soak | 150 to 180 °C, 60 to 120 s | 100 to 150 °C, 60 to 120 s | Flux activates; temperature evens out |
| Time above liquidus | 45 to 90 s above 217 °C | 30 to 90 s above 183 °C | Wetting; too long gives brittle intermetallics |
| Peak temperature | 235 to 250 °C | 205 to 225 °C | 20 to 30 °C over liquidus for wetting |
| Cool-down | 2 to 4 °C/s | 2 to 4 °C/s | Fine grain structure; stress |
Worked example: build a profile and check it
- Ramp 1.5 °C/s from 25 °C to 150 °C takes (150 − 25) ÷ 1.5 = 83 s. Soak to 180 °C over 90 s ends at 173 s. Ramp 1 °C/s to 245 °C takes 65 s (reaching 245 °C at 238 s); hold 25 s; cool 3 °C/s takes (245 − 25) ÷ 3 = 73 s. Total 337 s (5.6 min).
- Time above 217 °C: the time up the ramp from 217 to 245 °C is (245 − 217) ÷ 1 = 28 s, plus the hold of 25 s, plus the cooling from 245 to 217 which takes 28 ÷ 3 = 9.3 s. Total 62 s: inside the 45 to 90 s window.
- The same shape with a faster profile (ramp 3 °C/s, 2 °C/s to the peak, hold 15 s) gives only 36 s above liquidus: close to the lower limit; thick or large parts may not reach liquidus together.
- For comparison a leaded profile with a peak of 215 °C and a hold of 20 s stays above 183 °C for 63 s.
Measure the profile on a real board with a thermocouple taped or soldered to a joint near a large part, because the board temperature lags the oven. Follow the paste maker’s profile if it differs from this typical one.
Toaster ovens
A small convection toaster oven with a thermocouple controller can reflow boards well. A basic oven heats too slowly or unevenly, and its thermostat swings by ±10 to 20 °C, so use a controller (a PID reflow controller with a thermocouple) that switches the element to follow the profile.
Worked example: can the oven follow the profile?
- Treat the oven interior and elements as 120 J/K (typical small oven). With half of the 1 500 W reaching the air and board, heating from 25 to 245 °C takes 26 400 J ÷ 750 W = 35 s: a ramp of 6.3 °C/s on average, enough for the 1 to 3 °C/s profile.
- The board itself ( 100 × 80 × 1.6 mm FR-4, 24 g at 1.85 g/cm³ and c = 1.1 J/g/K) needs 5 731 J to reach 245 °C, small beside the oven’s own heat capacity.
- The issue is the cool-down: an oven cools at only about 1 °C/s with the door shut. Open the door at the end of the hold (or have a fan) to reach 2 to 4 °C/s.
- Place the board on a rack in the middle, away from the elements, and use a wire rack or tray lined with foil; keep paper and plastic out.
- Do not use the oven afterwards for food (flux and lead).
- Run the profile once on a spare board to learn the oven’s lag, and then adjust.
Hot plates
A hot plate or skillet heats the board from below. It gives fine control for small boards and one-sided assemblies and lets you watch the paste melt, but heats the underside first and cannot reflow both sides. An aluminum plate on a hotplate stabilizes the temperature.
Worked example: an aluminum plate on a hot plate
- A 100 × 100 × 6 mm aluminum plate has a mass of 162 g and a heat capacity of 146 J/K (c = 0.90 J/g/K).
- Heating it from 25 to 260 °C stores 34 263 J. A 600 W hot plate with 60 % efficiency delivers 360 W, so the heat-up takes 95 s (1.6 min): a mean ramp of 2.5 °C/s, in the right range.
- The plate’s large heat capacity also smooths the control: the temperature changes slowly and can be steered by hand, but it cools slowly too. Lift the board off the plate with tweezers or a spatula for fast cooling.
Hot air and pre-heating
For a single board or a rework, hot air (see Irons, Stations, Tips and Hot Air) with a pre-heater is a flexible method. Warm the whole board from below to 100 to 150 °C, then use the hot air over the part. This reduces the thermal shock and the gradient across the board. On large or multilayer boards without a pre-heater the board warps and parts crack.
- Pre-heat to the soak temperature and hold; add the hot air only for the reflow step.
- Watch for the paste turning shiny and the part sinking and aligning; then stop heating at once.
- Do not exceed the part’s rated temperature (many plastic connectors tolerate 260 °C for only 10 s).
Moisture-sensitive parts
Plastic IC packages absorb moisture from the air. In a reflow oven the water flashes to steam, and the package can crack or delaminate (the “popcorn” effect). The standard J-STD-020/033 rates parts by their moisture sensitivity level (MSL), which gives the floor life after the dry bag is opened.
| MSL | Floor life | Conditions |
|---|---|---|
| 1 | Unlimited | any |
| 2 | 1 year | ≤ 30 °C / 60 % |
| 2a | 4 weeks | ≤ 30 °C / 60 % |
| 3 | 168 hours (7 days) | ≤ 30 °C / 60 % |
| 4 | 72 hours | ≤ 30 °C / 60 % |
| 5 | 48 hours | ≤ 30 °C / 60 % |
| 5a | 24 hours | ≤ 30 °C / 60 % |
| 6 | Bake first; use within the time on the label | ≤ 30 °C / 60 % |
A part past its floor life must be baked to drive the moisture out, per the label (the typical schedules below are for plastic packages in high-temperature trays; reels and low-temperature trays need a gentle bake):
| Bake | Time | Notes |
|---|---|---|
| 125 °C | 4 to 24 h | High-temperature trays only |
| 90 °C, < 5 % RH | 1 to 3 days | Parts in plastic trays at most 90 °C |
| 40 °C, < 5 % RH | 5 to 14 days | Reels and low-temperature trays |
- Keep sensitive parts in their dry bags or in a dry cabinet (under 10 % RH) and note the time out.
- Do not reflow a board in a damp room or after it has stood out for days: bake the bare board too (typically 120 °C for a few hours for FR-4).
- BGAs and large QFNs are the most vulnerable parts.
Cooling, residue and checks
- Cool at 2 to 4 °C/s to a fine grain; too slow gives a coarse, weaker structure and thicker intermetallics, too fast risks cracking ceramics.
- Residue: no-clean flux can stay; other fluxes must be cleaned (see Solder Alloys, Flux and Paste).
- Inspect every board under magnification (see Joint Quality, Inspection and Failure): bright, concave fillets, no bridges, no tombstones, all pins wetted.
- Record the profile; a thermocouple log is your quality record.
Worked example: why the time above liquidus matters
- The intermetallic layer grows with the square root of time (see How Soldering Works). At 62 s above liquidus the layer is the reference; at 90 s it is about √(90 ÷ 62) = 1.20 times thicker, and the flux has been exhausted.
- Below about 45 s, large parts do not reach liquidus together (a wide board has a spread of 10 to 20 °C), which leaves open joints. The window is therefore 45 to 90 s.
Troubleshooting
| Symptom | Likely cause | What to do |
|---|---|---|
| Solder balls and splatter | Ramp too fast, moisture, paste exposed too long | Slow the ramp; bake; fresh paste |
| Cold, unmelted paste | Peak too low or too short | Raise peak to 235 to 250 °C; lengthen time above liquidus |
| Dull, grainy joints | Cooling too slow, joint disturbed | Faster cool-down; do not move the board |
| Tombstones | Uneven heating or paste | Slower ramp; check pads (see Surface-Mount Soldering) |
| Voids in joints | Flux trapped; fast ramp; old paste | Longer soak; fresh paste; vacuum oven |
| Cracked or popcorned ICs | Moisture; too hot or too fast | Bake parts; check MSL; lower peak |
| Warped board | Heating or cooling too quickly, unsupported | Support the board; slower ramps |
| Parts shifted | Oven airflow too high, vibration | Lower the fan; handle gently |
Related
- Soldering: the overview of this guide.
- Solder Alloys, Flux and Paste: paste and stencils.
- Surface-Mount Soldering: parts and defects.
- Rework, Desoldering and Repair: rework by hot air.
FAQ
What peak temperature do I need for lead-free reflow?
About 235 to 250 °C at the joint, with 45 to 90 s above 217 °C; check the paste data sheet and the part’s maximum rating.
Can I reflow in an ordinary toaster oven?
Yes, if it is a small convection oven with a thermocouple controller and you open the door to cool it. Never use it for food again.
Hot plate or oven?
A hot plate suits single-sided boards and watching the process; an oven suits double-sided boards and more parts.
What does MSL mean?
Moisture sensitivity level: how long a plastic package can stay out of its dry bag before baking is needed. MSL 3 is 168 hours.
How do I measure the profile?
Tape or solder a thermocouple to a joint on a spare or the real board, near a large part, and log it through the cycle.
References
- IPC/JEDEC J-STD-020, Moisture/Reflow Sensitivity Classification; J-STD-033, Handling, Packing, Shipping and Use of Moisture-Sensitive Devices.
- IPC-7530, Guidelines for Temperature Profiling for Mass Soldering; solder paste data sheets (profile recommendations).
- Lee, N.-C., Reflow Soldering Processes and Troubleshooting, Newnes.
Last reviewed: October 2026. The profile and oven figures are typical or illustrative: always follow the solder paste and part data sheets.